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Chinese Journal of Lasers

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 Start Pub.:1974 • Monthly
 Name:Chinese Journal of Lasers
 Managed by:Chinese Academy of Sciences (CAS)
 Sponsored by:Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences and Chinese Optical Society (COS)
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 Organizer:Chinese Laser Press
 Editor-in-Chief:Ruxin Li
 ISSN:0258-7025
 CN:CN 31-1339/TN
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 Address:No.390 Qinghe Rd., Jiading, Shanghai, China
 Zip Code:201800
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  Topics(2021.1)More +
     Reviews   Measurement and metrology
     Spectroscopy   Beam transmission and control
     Fiber optics and optical communication   Laser devices and laser physics
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        Measurement and metrology

    Sun Yuan, Lin Xiuyun, Xiong Jinxin, Ren Guojing,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Lidar scanning obtain point cloud date can not only directly measure the three-dimensional (3D) model of the object, but also reveal the intensity of the object. The laser intensity data reflects a variety of the characteristics of the target surface, which can be applied for registration of different measuring stations and filtering of point cloud data. It can also be used to extract and classify the target object by using the intensity data or combining the intensity data with the point cloud RGB data, so as to provide a basis information for feature extraction and leaf area calculation and biomass estimation. However, the influence factors such as the angle and distance will impact the laser intensity data of the same features. These deviations reduce the accuracy of point cloud registration, classification, and extraction, which is not conducive to the full use of point cloud information. Therefore, it is needed to establish a correction model to make the intensity data accurately reflects the feature information for rapid extraction.

    Methods Based on lidar ranging equation, first, a pre experiment processing is designed to predict the influence of scanning background, illumination change, and leaf inclination on the intensity data of the research objects. Second, the polynomial models are analyzed to fit the intensity data corrected equation, and the standard value of each material is defined through indoor experiment. Through the model, the angle, which is not easy to measure, is converted into height. After that, the intensity correction models of seven different materials (ginkgo leaves front and back, ginkgo branch, white paper, soapberry leaves front and back, and soapberry branch), with six different distances (2 m, 3 m, 4 m, 5 m, 6 m, and 7 m) and six different heights (0 m, 0.5 m, 1 m, 1.5 m, 2 m, and 2.5 m) is going to be established. Third, the coordinate transformation method of outdoor standing trees (two species: ginkgo and soapberry) is designed, and the correction model is used to obtain the corrected data of the reflectance of branches and leaves for each species. Finally, a threshold method and a random forest method are selected for intensity classification, and the classification result is analyzed to achieve the purpose of using point cloud intensity data for ground feature classification.

    Results and discussions 1) In the comprehensive analysis of various correction models, quadratic polynomial usually has the characteristics of simple calculation way and good simulation effect. For the seven kinds of materials, the data range after correction is less than 0.1, and the reflection intensity of each material is more stable than raw data, and is not affected by the distance and height. 2) Before correction, the peak value of reflected intensity of point cloud with leaves is smaller than that of yellow leaves and leaves off. The reflected intensity of leaves is less than that of branches. The reflected intensity of ginkgo ranges from 0.163 to 0.506 and that of soapberry ranges from 0.182 to 0.505. After correction, the range of intensity data for all materials decreases by an order of magnitude and fluctuates within a very small range of value. Among the correction results, the minimum range appears only 0.005 in the data of front of gingko leaves. While in the data of back of gingko leaves the range is also smaller, only 0.007. 3) For standing tree experiment, reflectance for single target and all return point cloud are all drawn. Experimental results show that the curves of gingko reflected intensity are smoother. The reflected intensity interval of ginkgo point clouds is 35, that is, the reflected intensity is 0.175 with a largest number of clouds. After the correction of the leaf model and the branch model, the reflection intensity of the most standing point clouds is 0.035 and 0.065. For soapberry, the intensity interval is 18, and the reflected intensity is 0.09 with a largest number of clouds. After the correction of the leaf model and the branch model, the reflection intensity of the most standing point clouds are 0.05 and 0.065. 4) Different classification methods are used to separate leaves and branches from the corrected intensity of the two tree species, the threshold classification is more applicable. The highest accuracy of the threshold classification with row data is 37.406%. After correction, the classification accuracy of ginkgo leaf model could reach 75.780% which increases by 83%. With the addition of RGB information, the classification accuracy of the random forest model with corrected data is improved from 85.645% to 91.504% which increases by 6.8%. The accuracy of leaf model correction for both species are 91.504% and 84.323%, respectively.

    Conclusions The calibration model method established in this paper can accurately correct the laser intensity data of the natural diffuse reflection target object. The selected natural standing-tree verifies the experiment successfully. Branches and leaves can be distinguished by the reflection intensity after calibration, which provides the possibility to further use point cloud data for branch and leaf separation and tree species identification.

    Chinese Journal of Lasers, 2021, 48(1): 0104001      

    Li Guicun, Fang Yami, Zhang Hao, Yu Haobiao, Liu Zongming, Song Ting, Sun Jun,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective The synthetic wavelength method based on a femtosecond optical frequency comb has been widely used in high-precision long distance ranging systems owing to the capability to measure absolute distances, traceability to the length standard and simple setup. However, optical power variations will cause phase shift variations in photodetection. This phenomenon, which is frequently referred to as the power-to-phase conversion (PPC) effect, will eventually lead to distance measurement errors, thus deteriorating the precision of the ranging system. The conventional methods, such as phase bridge measurement and impulse response measurement, usually focus on reducing the PPC effect and generating ultralow phase noise, ultrahigh stable microwave signals by intermode beating from a femtosecond frequency comb. However, it still lacks a comprehensive research about the influence of PPC on the ranging errors of a femtosecond laser ranging system and the corresponding correction technique. In the present study, a polynomial fitting correction method is proposed to improve the precision of the ranging system. Combined with the phase ranging method, a correction look up table (LUT) is formed by referencing to a length standard and adopting the least-square based polynomial fitting. We believe that our work can extend the femtosecond laser based high-precision ranging technique to be applied to outdoors, complicated industrial environments or even non-cooperative targets, which significantly broadens its application area.

    Methods In this study, a repetition-rate-locked femtosecond laser is used as the laser source. First, the Michelson-like interferometer is established, and optimal experimental parameters are determined by studying the PPC effect of different synthetic radiofrequency (RF) signals consisting of the repetition rate and its high-order harmonics via intermode beating of the femtosecond laser after photodetection. Then, using the fourth order harmonic as the fine ruler, phase shifts are extracted and investigated as a function of the optical power based on fast Fourier transform (FFT). Combined with the phase ranging method, a correction LUT is formed by adopting the least-square based polynomial fitting with different degrees. Finally, comparisons of corrected and uncorrected ranging results are made to verify the effectiveness of the proposed correction method. By comparing the distance errors after corrections with polynomial fitting of different degrees, we have determined that using the 4th degree correction method can obtain the best correction performance. In addition, by comparing corresponding residual errors of corrected and uncorrected distances versus linear stage displacements after applying linear fit in the measurement range, the proposed correction method is further proven to be very effective in improving the precision of the femtosecond laser ranging system.

    Results and Discussions A custom-made, 200MHz repetition-rate-locked all-polarization-maintaining femtosecond fiber laser referenced to a highly stable frequency standard is used in the ranging system. With the increase of incident optical power, we have investigated the RF power of different beat notes and identified three operation zones for the applied photodiodes (FGA015, Thorlabs), i.e. the linear regime for low optical power, the saturation regime and above saturation regime for high optical power (Fig.2). The latter two are usually classified as the non-linear regime. The results demonstrate all the beat notes are under the linear regime if the optical power is lower than 2.3mW. Besides, we have also investigated PPC coefficients of different beat notes in detail (Fig.3). The overall results show that the PPC effect remains at a relatively low level for all beat notes when the optical power is less than 2mW. Considering the low PPC effect and high signal-to-noise ratio for high precision distance measurement, the incident optical power is chosen to be 2mW. Then, using the 800MHz (fourth order harmonic) RF signal as the fine ruler, we have formed a correction LUT under different RF power levels (from -19.55dBm to -10.87dBm) using polynomial fitting of 2nd degree, 3rd degree, and 4th degree by referencing the calculated distance results to a length standard of 10mm (Fig.4). By comparing the distance errors after corrections with polynomial fitting of different degrees, we have found the 4th degree correction method can achieve a higher precision. Specifically, the errors are reduced to ±0.05mm for the 4th degree polynomial correction method while the range error correction results of the 2nd and 3rd degree polynomial correction are both -0.15 mm to 0.1 mm (Fig.5). In addition, we have tested the effectiveness and feasibility of our correction method by comparing corrected and uncorrected distance results at different incremental displacements of a high precision linear stage. After linear fit of corrected and uncorrected distances versus linear stage displacement in the range of 110mm, the residual errors can be significantly reduced from ±0.25mm to ±0.08mm (Fig.6), and the linear correlation coefficient is increased from 0.999989 (uncorrected) to 0.999998 (corrected).

    Conclusions Aiming at the error caused by the power-to-phase conversion (PPC) in high-precision absolute distance measurements using the femtosecond laser synthetic wavelength method, a 4th degree polynomial fitting correction method is proposed to improve the precision of the ranging system. In this study, a Michelson-like interferometer is established, and synthetic radiofrequency beat signals are achieved via intermode beating of the femtosecond laser after photodetection. Then, phase differences are extracted and investigated as a function of the optical power based on FFT. Combined with the phase ranging method, a correction LUT is formed under different optical power levels by adopting the least-square based polynomial fitting method. Experimental results show that using the fourth order harmonic, the measurement distance error has a slope of 2.7mm/mW with the optical power ranging from 1mW to 3mW without correction, while the residual error range can be significantly reduced from ±0.25mm to ±0.08mm in 110mm measurement range. This verifies the effectiveness of the proposed correction method. Meanwhile, we have qualitatively investigated the transport of electron-hole carriers in the intrinsic part of the junction, indicating the generality of our correction method for other semiconductor photodiodes such as avalanche photodiodes (APDs) and positive-intrinsic-negative (PIN) photodiodes. Consequently, our present work demonstrates that the femtosecond laser based high-precision ranging technique can be potentially extended to outdoors, complicated industrial environments where the optical power variations are large, which will significantly promote its real industrial applications, such as advanced large-volume manufacturing, assembly and precise shape, and dimensional measurement of workpieces.

    Chinese Journal of Lasers, 2021, 48(1): 0104002      


        Spectroscopy

    Yu Dan, Sun Yan, Feng Zhishu, Dai Yuyin, Wang Qiuyun, Chen Anmin, Jin Mingxing,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Laser-induced breakdown spectroscopy (LIBS) is a novel atomic-emission spectroscopic technique that has been widely used for the elemental analysis of materials. Investigators in the field of LIBS research are actively working to improve the sensitivity of this technique, and many methods have been proposed to enhance its spectral intensity. With the development of chirped-pulse-amplification technology, femtosecond pulse lasers have been introduced into the study of LIBS. Compared with nanosecond lasers, femtosecond lasers have many advantages for LIBS, but it is important to improve their spectral intensity. In general, the output laser beam of a femtosecond laser system is linearly polarized. In a linearly polarized laser field, electrons undergo alternating acceleration and deceleration in each optical period of the laser pulse. However, femtosecond lasers with circular polarization can accelerate electrons continuously, so they attain higher energies. The energies of electrons after irradiation with a circularly polarized femtosecond laser beam are different from those of electrons after irradiation with a linearly polarized femtosecond laser beam. This makes the optical signal from a plasma induced by a circularly polarized femtosecond laser beam different from that induced by a linearly polarized femtosecond laser beam. It is therefore necessary to compare the plasma emission induced by a femtosecond laser under both linear and circular polarizations.

    Methods We focused a femtosecond laser beam onto the surface of a brass sample to produce plasmas, and we analyzed the resulting plasma spectra. To compare the effects of linear and circular polarizations on the spectral-emission intensity, we adjusted the polarization of the femtosecond laser beam from linear polarization to circular polarization by using a quarter-wave plate. The sample was placed on a three-dimensional translation table to avoid over-ablation. We collected the light emission from the laser-induced plasma and focused the collected light into an optical fiber, which transmitted it to a spectrometer. The light dispersed by the spectrometer was detected with an intensified charge-coupled device (ICCD). The ICCD was synchronized using an electrical signal from the synchronization-and-delay generator of the femtosecond laser system. Each spectral datum was an average of 50 laser shots. The whole experiment was carried out in air.

    Results and Discussions We first compared the time-integrated spectra from femtosecond-laser-induced brass plasmas obtained under circular and linear polarizations. The spectral intensities of Zn (I) and Cu (I) obtained with a circularly polarized laser beam were higher than those obtained with a linearly polarized laser beam, and the spectral intensity increased by about 15%. An important difference between linear polarization and circular polarization in the femtosecond laser field is that the kinetic energies of electrons subject to differently polarized laser fields are different. Under linear polarization, electrons undergo alternating acceleration and deceleration in each optical period of the pulse, so they attain low kinetic energy. In contrast, electrons are always accelerated under circular polarization, so they attain high kinetic energy. Second, we measured the time-resolved spectra of femtosecond LIBS. The time-resolved peak intensities of Cu (I) at 510.55nm and Zn (I) at 472.21nm under circularly polarized laser are higher than that under linearly polarized laser, and the atomic lines in circularly polarized laser-induced plasmas persist longer duration. We thus find that circularly polarized femtosecond pulsed laser irradiation can produce stronger plasmas, which emitting stronger time-resolved spectra during the process of plasma decay. Laser polarization thus plays an important role in femtosecond LIBS. Finally, we calculated the time-resolved electron temperature and density under irradiation with circular and linear polarizations based on the Boltzmann plot and Stark broadening. The changes in the electron temperature and density are similar to the changes in the spectral intensity. Electrons under circularly polarized laser collide with atoms or ions, leading to higher electron temperature and density than those under linearly polarized laser. The higher-energy electrons transfer more energy to the lattice to produce stronger plasmas.

    Conclusions We produced plasmas on the surfaces of brass samples by using focused femtosecond pulse laser, and we measured the spectral lines of Zn (I) and Cu (I) emitted from the plasmas under linear and circular polarizations. The results show that the spectral intensities under circularly polarized femtosecond laser were higher than those under linearly polarized femtosecond laser, and the spectral intensities increased by about 15%. We also measured the time-resolved spectra of linearly and circularly polarized femtosecond-laser-induced brass plasmas. For the same laser energy, atomic lines in the plasma produced by the circularly polarized laser persisted longer duration than those produced by the linearly polarized laser. Compared with linear polarization, the electron temperature and density obtained with circular polarization were also higher. This is because a circularly polarized laser can accelerate the electrons continuously, so the kinetic energy of electron produced by a circularly polarized femtosecond laser are higher than that produced by a linearly polarized femtosecond laser. Electrons with higher kinetic energy collide in the plasma to produce higher electron temperature and density, which thus emit a higher spectral intensity. Therefore, we can improve the signal intensity of femtosecond LIBS by adjusting a femtosecond laser from linear polarization to circular polarization. We expect this work to be useful for the study of femtosecond LIBS.

    Chinese Journal of Lasers, 2021, 48(1): 0111001      

    Li Zehao, Yang Chunyong, Tang Zihao, Peng Miaomiao, Ni Wenjun, Guo Lianbo, Hou Jin, Chen Shaoping,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective In a trace gas detection system based on photoacoustic spectroscopy technology, the photoacoustic cell is a key performance component of the system. More and more high-performance photoacoustic cells have been designed, and many of them have a variety of specially-shaped photoacoustic cells. However, heterogeneous photoacoustic cells with high design properties represented by topology optimization often mean greater processing costs, while traditional photoacoustic cells are often limited by shape optimization parameters and low design freedom. Therefore, research on the photoacoustic cell must be thorough and in-depth. In this paper, a curved beam waist photoacoustic cell with a hyperbolic generatrix is proposed and designed. This scheme introduces innovative generatrix eccentricity parameter, realizes three-dimensional optimization, and effectively fills the gap between traditional photoacoustic cells and highly-designed specially-shaped photoacoustic cells.

    Methods Based on the finite element method, we use COMSOL software to construct a two-dimensional model of a curved beam waist photoacoustic cell with a generatrix eccentricity of 7.14. To ensure the accuracy of the solution, this paper considers the influence of thermal viscosity loss on the simulation model. The first eight acoustic modal values of the structure and the visual mode shape are analyzed with respect to thermal viscosity loss. On this basis, the amplitude-frequency response curve of the photoacoustic cell was analyzed and the quality factor Q was calculated via Lorentz fitting, and the rationality of the microphone placement position was verified. To study the designability of the photoacoustic cell, we optimized the parameters of the photoacoustic cell. When eccentricity trended toward infinity, the effects of the length of the resonant cavity of the curved beam waist photoacoustic cell and the eccentricity of the generatrix on the resonance frequency and the sound pressure amplitude were analyzed in detail. To alleviate the design distress caused by too few parameters, generatrix eccentricity was introduced as the third optimization parameter, and its influences on the resonance frequency and sound pressure amplitude were analyzed in detail. Finally, further study of the influence of the variation of generatrix eccentricity on the resonance peak of the photoacoustic cell was analyzed.

    Results and discusssions Simulation results show that the quality factor of the photoacoustic cell in the preliminary design is as high as 83.3, indicating that the photoacoustic cell is a high-quality photoacoustic cell. Analyzing the optimized photoacoustic cell parameters, it was found that the length of the resonant cavity is more sensitive to the resonant frequency, while the semiminor axis length of the generatrix is more sensitive to the sound pressure amplitude. Further analysis of the simulation results shows that when the semiminor axis length of the generatrix is large, it is very limited to adjust the sound pressure amplitude by changing the length of the resonant cavity, while adjusting the semiminor axis length of the generatrix hardly changes the resonance frequency. Therefore, only relying on these two parameters to optimize the design will be limiting and provide challenges meeting the design requirements of special photoacoustic cells. After the introduction of the generatrix eccentricity parameter, by changing and adjusting the generatrix eccentricity, the resonance frequency can be adjusted within a certain range without affecting the sound pressure amplitude. In the evolution graph of the frequency domain response curve with the eccentricity of the generatrix, it can be seen that two resonance peaks are excited when the eccentricity trends toward infinity, and four resonance peaks are excited when the eccentricity of the generatrix is five. Among the four resonance peaks with an eccentricity of 5, the first and second resonance peaks are stronger, and the quality factors reach 75.9 and 128.9, respectively.

    Conclusions The quality factor Q of the designed photoacoustic cell is higher than 50, so this kind of curved beam waist photoacoustic cell can be designed with a higher quality factor, which can effectively improve the sensitivity of the trace gas detection system. After adding the optimized parameter of generatrix eccentricity, three-dimensional optimization was realized, which has a higher degree of designability than traditional cylindrical photoacoustic cells. The amplitude-frequency response characteristics of the photoacoustic cell show that a small eccentricity can excite multiple resonance peaks, and a resonance peak with a high quality factor can be obtained by adjusting the generatrix eccentricity. This shows that multi-peak resonance can be achieved by adjusting the eccentricity of the generatrix, so that the same photoacoustic cell has two or more effective working frequencies, which greatly improves the designability of the photoacoustic cell. This curved beam waist photoacoustic cell design has important theoretical and engineering application value for the multi-scene adaptability of novel photoacoustic trace gas detection technology.

    Chinese Journal of Lasers, 2021, 48(1): 0111002      


        Beam transmission and control

    Zhang Junxin, Mei Haiping, Shen Liujing, Huang Yinbo, Luo Fu, Wu Xiaoyan,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective When laser propagates in the atmosphere, it is inevitable that a random change of the refractive index of the atmospheric turbulence causes laser spot broken and distorted, which seriously damages the laser beam quality. Under the conditions of weak- and strong-fluctuating media, the turbulence model has relatively rich theoretical research results. In view of the complexity of the turbulence itself, the current theory of light transmission in random media is not enough to perfectly present the far-field transmission characteristics of the laser spot in the strong-fluctuation region. As for the high-altitude laser transmission engineering applications, the spatial distribution of laser energy on the target surface and its temporal variation characteristics are vital for photoelectric system evaluation. However, the research of long-range high-altitude laser atmospheric transmission faces the following problems: first, there are few transmission tests in high-altitude areas; second, short-exposure laser spot data at high frame-rate sampling are rare; third, two-dimensional imaging methods to obtain more comprehensive laser statistical parameters are still lacking. In this study, we use a high-speed camera to perform laser spot tests at different distances in high-altitude areas. Using the laser short-exposure image data at sampling rate of 4000 frame·s -1, the change characteristics of short-exposure far-field laser spots in the turbulent atmosphere are qualitatively described, and the optical turbulence parameters such as ideal point scintillation indexes, aperture average factors, and inner scales are calculated. Our aim is to use the external field laser transmission test data to understand the spatial laser energy distribution on the target surface as well as its temporal variation characteristics under high-altitude conditions and to simultaneously determine its impact on the laser photoelectric system.

    Methods The experiment is performed at an altitude of 3200 m in Gaomeigu, far from the underlying surface. The transmitting terminal is a 532 nm solid-state laser, while the receiving system is consisted of a high-speed CCD camera and a diffuse reflection screen. The camera is placed obliquely in front of the screen at ~9m, the transmitter is located in the balcony in the third floor of the Gaomeigu radar station at ~10m above the ground, and the receiver is located on the open ground at distances of 1, 2, 3, 4, and 4.7km from the transmitting terminal. The selected geographical location can ensure that most of the transmission paths are longer than 15m from the underlying surface and the maximum height is larger than 100m. In addition, the test is also performed in the canyon, far away from the underlying surface. The straight-line distance between two ends is ~2km, and the maximum link distance from the underlying surface is ~300m. The sampling rate of laser spot images is unified to 4000 frame·s -1, each image is 320pixel×240pixel, and the image accuracy is 8bit. For the acquired two-dimensional data of the laser spot, using the imaging method, the scintillation index under different receiving apertures is calculated, and subsequently polynomial fitting is used to obtain the ideal point scintillation index. With the former results, the refractive index structural parameters such as the path, aperture average factor, and inner scale can also be derived. Moreover, this method based on the calculation of the gray value of the light spot can also display the correlation coefficient between the power spectrum and its scaling rate as well as the spatial light intensity distribution.

    Results and Discussion At a distance of 1km, with the playback of a short-exposure spot image at a sampling rate of up to 4000 frame·s -1, the broken laser spot shows a clear mesh-like morphological structure, and a few sharp bright spots are scattered. There are irregular shapes and different size holes inside the spot (Fig. 3). As time elapses, these flowing mesh-like structures and scattered bright spots undergo deformation and reconstruction. There is a certain unclear relationship between size of the reticulated texture and the scale of the turbulence. Using the light spot gray value data, the aperture average factor of different receiving radii on the target surface is obtained. The variation of the laser spot aperture average factor with the receiving aperture shows that the aperture average factor is not only a function of the aperture but also is related to the turbulence state (Fig. 6). The daily variation of Cn2 in high-altitude areas is relatively stable, showing a different trend from the “Mexican hat” trend near the ground at low altitudes (Fig. 7). The power spectral distributions of laser scintillation under different transmission distances show that the cutoff frequency of the power spectrum of a single pixel is generally higher than the average cutoff frequency of the circular domain, which indicates that the aperture smoothing effect of light intensity scintillation still exists in the frequency domain. The scaling rate obtained from the power spectrum shows a positive correlation trend with the ideal scintillation index (Figs. 9 and 11). Taking the spot centroid as the point of origin, the normalized spatial correlation coefficient of the light intensity fluctuations at the distance from the origin point shows that the spatial correlation radius increases with the increase of the scintillation index (Fig. 12). It is a completely different concept from the coherence length.

    Conclusion The main purpose of designing this experiment is to find the transmission conditions far away from the underlying surface in high-altitude areas and to obtain the characteristics of the laser spot transmitted in high-altitude free atmospheric turbulence. Judging from the fact that there is no obvious “Mexican hat” structure in the daily variation trend of most transmission paths, the underlying surface on the transmission path has little influence, but there are still big differences between the real test conditions and high-altitude “free” atmosphere. In the future, we hope that we will have the opportunity to conduct high frame-rate laser spot image testing on higher and farther platforms to promote the understanding of laser transmission in free turbulent atmospheres.

    Chinese Journal of Lasers, 2021, 48(1): 0105001      

    Xiao Yan, Zhang Jing, Wang Pengxiang,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Non-diffractive beam has been an important research topic in optics. In addition to Bessel beam which transmits along a straight line, Airy beam propagating along the quadratic parabola is also a special non-diffractive beam, and this is called self-acceleration property. And it also shows strong recovery force against disturbance during transmission, namely self-healing property. Airy-Gaussian beam, which can be adjusted between Airy beam and Gaussian beam via the distribution factor, is derived from Airy beam. Based on the nonlinear Schr?dinger equation, the transmission in different nonlinear media including the Kerr media, saturated media, and photorefractive media has been studied in recent years. Since Longhi introduced fractional Schr?dinger equation into the field of optics in 2015, it had aroused the interest of research groups at home and abroad. The study on Gaussian beam and Airy beam in the framework of fractional Schr?dinger equation has emerged in the endless stream, but there are few studies on Airy-Gaussian beam. In this paper, taking the fractional Schr?dinger equation as the theoretical model, the evolution process is simulated numerically by the split-step Fourier method to study the propagation and interaction of Airy-Gaussian beams with linear potential. The transmission of Airy-Gaussian beams under linear potential is explored to obtain some meaningful results about beam control. Our results provide theoretical reference for the applications in optical switching and optical logic devices as well as for the adjustment of beams in optical systems.

    Methods The split-step Fourier method is a numerical method for solving nonlinear Schr?dinger equation, considering the diffraction effect and nonlinear effect separately over a small distance. In this paper, based on the fractional Schr?dinger equation, the propagation of single Airy-Gaussian beam and the interaction of dual Airy-Gaussian beams are simulated by the split-step Fourier method. When it comes to exploring the transmission of single Airy-Gaussian beam, the Lévy index, distribution factor, and linear coefficient are analyzed. As for the interaction of dual Airy-Gaussian beams, the effect of changing the Lévy index, beam interval as well as relative phase is observed.

    Resultsand Discussion In the research process, it is found that Airy-Gaussian beam will split into two sub beams when there is no potential, the splitting phenomenon gradually disappears when there is a linear potential, and it evolves periodically, in which the energy of the main lobe hardly changes with the transmission distance and the same is to side lobes. The evolution performs differently when parameter values vary. The period is associated with linear coefficient β, performing that it becomes smaller when β is larger. Besides, the symbol of β can make a difference to the distribution space of the beam. That is when β>0, the beam is only distributed on the left side of the central axis, otherwise on the right side. It is worth mentioning that there is a transition process from splitting to non-splitting when β is closed to 0, in which one of the sub beams gradually approaches to the other, and they finally converge into a beam. When β>0, the incident beam deflects to the left, and vice versa to the right (Fig. 1). With the increase of Lévy index α from 1 to 2, the oscillation amplitude increases, and the evolution path changes from an approximate broken line to a curve. Furthermore, such oscillation is not symmetrical about the central axis, but extends in the half space along the central axis to the x axis (Fig. 2). The energy exchange occurs due to the competition when the energy difference is large, which happens when relative phase δ=±π/2. However, there is almost no competition in the case of in-phase or out-phase because the energy of two beams increases or decreases at the same time (Fig. 3). The intensity distribution changes from symmetry to asymmetry and then to symmetry with the increase of transmission distance in a period (Fig.4). When α exceeds a certain value, the energy exchange will be destroyed. In addition, when α becomes large, two beams do not propagate at equal intervals. At the beginning of any period, the interval between two beams is the largest and that at the half period is the smallest (Fig. 5).

    Conclusion In this paper, a fractional Schr?dinger equation is used as the theoretical model to explore the propagation and interaction of Airy-Gaussian beams under linear potential by using the split-step Fourier method. Generally speaking, the linear potential will destroy the beam splitting, performing that when β increases from 0 to a very small range, and the beam will no longer split, but shows periodic evolution, which is not symmetrical about the central axis but distributed on one side of the central axis. And this evolution is affected by many parameters. Transverse oscillation amplitude and Lévy index α are positively correlated, performing that with the increase of α, the amplitude of transverse oscillation increases correspondingly due to the increment of diffraction effect, and the periodic evolution changes from an approximate broken line to a curve. Its period is affected by linear coefficient β, and it decreases with the increase of β. Moreover, the sign of β can change the deflection direction and distribution space of the beam. When β>0, the incident beam which only distributes on the left side of the central axis deflects to the left, and vice versa. In the process of the interaction, the energy of two beams will exchange periodically when δ=±π/2, but not in the in-phase and out-phase. In addition, this phenomenon will disappear when α approaches to 2. Furthermore, when α is large, two beams do not propagate at equal intervals, with the largest interval at the beginning of the period and the smallest at the half. These conclusions provide a theoretical reference for the application of beams in optical switching and optical logic devices as well as beam control.

    Chinese Journal of Lasers, 2021, 48(1): 0105002      


        Fiber optics and optical communication

    Zhang Huiying, Yu Haiyue, Chen Lingling,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Existing indoor positioning technologies have disadvantages of low positioning accuracy and low positioning speed. The GPS positioning technology is more mature with high positioning accuracy, but it cannot be used in indoor positioning due to its inability to obstacles. Compared with traditional indoor positioning technologies, the visible light positioning technology has the advantages of being free from electromagnetic interference and high safety, and it is widely used. However, visible light positioning is affected by the arrangement of LED light sources, and the receiving signal strength indication (RSSI) positioning algorithm is widely used. In the implementation process, the least squares will be affected by the ranging error, resulting in a decrease in positioning accuracy. Therefore, we conduct research on indoor visible light positioning technology, aiming to propose a more reasonable light source arrangement and a more effective positioning algorithm, improve the positioning accuracy, and shorten the positioning time. Ultimately, the accurate and rapid positioning will be realized in large indoor spaces.

    Methods The optical power distributions obtained by the four-light source arrangement method and the grid-type arrangement method are compared in the 100m×100 m×10m space of a large indoor place, and the light arrangement scheme suitable for large indoor place is established. The improved flower pollination algorithm is adopted, and the initial population distribution becomes more uniform with the addition of reverse learning strategy. The population diversity is improved, and the local optimal problem is solved. The adaptive moving factor is conducive to the global extensive search and the convergence rate is improved. Compared with the RSSI positioning algorithm (least square), the RSSI positioning algorithm based on FPA, and the RSSI positioning algorithm based on PSO, a nine-light source grid arrangement scheme combined with the improved RSSI intelligent pollination positioning algorithm can achieve higher precision and faster positioning in large indoor locations.

    Results and Discussions By comparing the LED source arrangement scheme and various positioning algorithms for visible light positioning, the following results can be obtained: 1) LED source arrangement. The illumination for large indoor spaces has been ensured with grid-type arrangement of nine light sources, the quality of optical communication has been improved, and the flatness of the communication plane power has been ensured. 2) Positioning accuracy. Compared with the RSSI positioning algorithm based on FPA, the improved positioning algorithm in this paper reduces the positioning error by 2.0220 m and improves the positioning accuracy by 99.74%. Compared with the PSO-based RSSI positioning algorithm, the positioning error is reduced by 0.0481 m and the positioning accuracy is improved by 90.32%. The positioning accuracy has significantly improved by the improved algorithm, and an average error of about ±1cm and good stability are obtained.3) Convergence rate of algorithm. In this paper, the improved positioning algorithm rapidly converges to the minimum fitness value after 60 iterations. The RSSI positioning algorithm based on PSO and the RSSI positioning algorithm based on FPA reach the minimum fitness value after 80 and 200 iterations, and the improved positioning algorithm significantly converges faster.4) Positioning time. In the same error range, the positioning time of the improved positioning algorithm is 2--4s, and the positioning time of the PSO-based RSSI positioning algorithm is 4--5.5 s. The improved positioning algorithm greatly shortens positioning time, and it is suitable for rapid positioning in large indoor places. As a result, the proposed nine-light source grid-type arrangement combined with the improved RSSI intelligent flower pollination positioning method significantly improves the positioning accuracy, the positioning error is basically stable in millimeters, and the positioning speed is faster, which can meet the requirements of lighting and communication in large sites.

    Conclusions In this paper, the visible light location technology is proposed to achieve high accuracy of large space indoor locations. The performance of grid configuration model of nine light sources in 100 m×100 m×10 m space is analyzed. An improved RSSI localization algorithm for FPA is proposed. In this paper, a visible light location scheme for the grid-type arrangement of light sources combined with the adaptive pollination signal strength based on reverse learning strategy is proposed, and it has the advantages of high positioning accuracy, fast positioning speed, good stability, and so on. The proposed scheme has a good practicability and broad application prospect.

    Chinese Journal of Lasers, 2021, 48(1): 0106001      

    Li Gangmin, Li Zhiyuan, Li Zhengran, Wang Jinmin, Xia Li, Yang Zhao, Wei Li,

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    Objective Surface plasmon resonance (SPR) sensors based on Kretschmann prism are bulky and need to be equipped with mechanical movable parts, which is not conducive to the miniaturization and remote sensing of devices. Optical-fiber-based SPR technology has been widely used in food safety, environmental monitoring, and other fields owing to its label-free simple pretreatment, fast analysis, high sensitivity, and other excellent characteristics. However, most SPR sensors do not consider sample consumption in the process of sensor monitoring. The exposed optical fiber is fragile, which easily affects the sensor''s stability. Highly sensitive optical fibers, such as tapered and D-shaped fibers, are vulnerable and unstable. In addition to the abovementioned problems, the sensitivity of existing SPR fiber sensors needs further improvement. As a high-throughput microscale analysis device, the microfluidic chip system has shown great potential in remote monitoring, biological detection, and other fields in recent years. In this paper, a microfluidic chip based on SPR fiber sensor is designed and used to measure the concentration of a solution. This chip has the advantages of small size, compact structure, and low sample consumption; it also reduces the fiber damage and effectively improves the sensing stability by embedding the sensing fiber into Polydimethylsiloxane (PDMS) substrate. Photonic crystal fiber (PCF) is a new type of optical fiber. PCF comprises a single dielectric, in which air holes are closely arranged in the two-dimensional direction but unchanged in the axial structure to form microstructure cladding. Moreover, it can stimulate a more substantial SPR effect for its unique light-guiding characteristics.

    Methods In this paper, PDMS—which has good chemical inertia and good biocompatibility—is used as the main material to fabricate the microfluidic chip. After cooling and forming in 3D mold, PDMS is stably bonded with glass sheet to form the main structure of the chip. The microchip contains a microfluidic channel (diameter: 200nm), whose length is the same as that of the chip. A sensing structure of multimode fiber-photonic crystal fiber-multimode fiber (MMF-PCF-MMF), which is coated with 60-nm gold film on the surface, is embedded in the channel to stimulate SPR effect. Then, mixed solutions of glucose solid sample and deionized water in a certain proportion with refractive index of 1.338--1.425 are used as samples to be tested. The experiment is performed at room temperature (25 °C). The glucose solution in the syringe is injected into the microfluidic chip using a syringe pump at a constant speed; the glucose solution flows through the optical fiber sensing part. When the concentration of the solution injected into the microfluidic chip is changed, the light wave meeting the resonance conditions excites the gold film to produce surface plasmon resonance and the resonant valley is generated on the transmitted spectrum. The position of the resonant valley shifts in the samples with different refractive indexes. By recording the resonance spectra of a series of samples, the sensitivity of the sensor with respect to the change in refractive index can be calibrated.

    Results and Discussions When the refractive index of the liquid injected into the microfluidic chip increases from 1.338 to 1.425, the resonant wavelength of SPR spectrum shifts to longer wavelength because of the SPR resonance effect. The relationship between the refractive index and resonant wavelength of the liquid to be measured is extracted, and the sensitivity curve can be obtained by taking the resonant wavelength at n=1.338 as the starting point. The increasing speed of the sensitivity curve is related to the chip sensitivity. The experimental results show that the refractive index sensitivity of PCF SPR sensors can reach up to 8240.6nm/RIU, in which RIU is refractive index unit, thereby showing that these have good high-sensitivity characteristics and meet the application requirements of high sensitivity.

    Conclusions In this paper, a novel microfluidic chip embedded with SPR sensor is designed and manufactured by combining the optical fiber structure, SPR effect, and microfluidic system. The volume of chip is approximately 3.5cm×1cm×5cm, which is much smaller than that of the traditional measurement instrument and is conducive to the integration of sensor. After testing the MMF-PCF-MMF structure based on SPR effect, the sensor sensitivity obtained is high (up to 8240.6nm/RIU) in a wide refractive index measurement range of 1.338--1.425. The proposed structure has good refractive sensitivity, small size, acid resistance, and corrosion resistance; these characteristics enable a broader application prospect in the field of biochemistry.

    Chinese Journal of Lasers, 2021, 48(1): 0106002      

    Chen Yufang, Shen Xiao, Zhou Quan, Zhang Shuai, Mao Jingyi, Wan Hongdan,

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    Objective A tunable fiber filter (TFF) takes an optical fiber as the medium to realize wavelength-selective reflection or transmission of optical signals. TFFs play an important role in optical fiber sensing and communication owing to their inherent merits of anti-electromagnetic disturbance, compact size, and low fabrication cost. Compared with traditional interferometer TFFs, such as Mach-Zehnder interferometer, fiber Bragg gratings, Fabry-Perot interferometers, and microstructure interferometers, an optical fiber microcavity has a high quality factor, high energy density, and whispering-gallery mode (WGM) resonance spectrum with an ultra-narrow band. Moreover, research on WGM microcavities based on new materials is of great significance for realizing an all-optical controllable TFF with high flexibility and tunable filtering. An all-optical TFF with a simple structure can eliminate the need for applying additional mechanical devices or heating devices to realize dynamic tuning. In this paper, an all-optical TFF based on phosphate glass microspheres (PGMS) is proposed to facilitate a systematic study of optic-thermal tunability. With the advantages of all-optical control, compact structure, high stability, and ultra-narrow bandwidth, all-optical TFFs based on PGMS could be widely used in fiber sensing elements or mode selection of fiber lasers, providing good application prospects in the field of optical fiber communication.

    Methods The components of a PGMS microcavity coupling system include the phosphate glass optical fiber (PGOF), PGMS and single mode tapered fiber. The PGOF was prepared by preform-drawing method. After high temperature melting, stirring, pouring, annealing and cooling to room temperature slowly, the optical fiber preforms of core and cladding were prepared, respectively. Then, the PGOF was fabricated by wire drawing with the preforms and then fused and stretched by CO2 laser heating with a certain power. At the same time, the diameter changes of the PGOF were observed by high resolution microscope, and the microsphere was formed based on surface tension effect. The single mode tapered fiber with low loss and good size was tapered by controlling the cone drawing speed and hydrogen flow rate strictly. The sizes of PGMS and the single mode tapered fiber were characterized by optical microscope, including the diameter and waist width. According to the above preparations of three optical components, the WGM resonance spectrum with high Q value and low insertion loss was obtained by efficiently coupling PGMS with the single mode tapered fiber.

    Results and Discussions The optic-thermal tunability of PGMS was studied by scanning WGM resonance spectrum under transient process. In the measurement of low power optic-thermal tuning, with the increase of optical pump power, the WGM resonance wavelength drifts to a shorter wavelength (blue shift). The maximum drift is about 18.25pm and the optic-thermal tunable sensitivity is about 49.46pm/mW with a linearity more than 0.99. In the test of high power optic-thermal tuning performance, the WGM resonance wavelength of PGMS drifts to the shorter wavelength (blue shift) with the increase of optical pump power. The maximum drift is about 179.58pm and the optic-thermal tunable sensitivity is around 72.727pm/mW with a linearity more than 0.99. According to the experimental results, the WGM resonance spectrum drifts and optic-thermal tuning sensitivity of PGMS increases with the increase of optical pump power. Furthermore, in order to verify the effectiveness of the experimental results, the all-optical tuning characteristics of the TFF based on silica microsphere were studied with the same method. The experimental results show that the WGM resonance wavelength of silica microsphere shifts to the longer wavelength (red shift) with the increase of optical pump power. The results of silica microsphere and PGMS are different, which depend on the characteristics of the material itself. The maximum drift is about 0.28pm and the optic-thermal tunable sensitivity is only about 0.086pm/mW with a linearity less than 0.7. By comparison, it can be seen that the all-optical TFF of PGMS based on strong optic-thermal effect combines with high Q value, high energy density and narrow linewidth characteristics, and achieves an optic-thermal tunable sensitivity up to 72.727pm/mW.

    Conclusions An all-optical TFF based on PGMS was proposed and demonstrated. Fabricate the microsphere with high Q value was frabricated using the high power CO2 laser by melting and heating the fiber. The microsphere was coupled efficiently by a single mode tapered fiber and the WGM resonance was excited by a tunable laser source. With a varied optical pump power, the PGMS has higher optical sensitivities than the silica microsphere. As for the PGMS, an increased pump power results in WGM resonance wavelength shifting to a shorter wavelength (blue shift), and the optic-thermal tunable sensitivity is about 72.727pm/mW with a high linearity of >0.99. However, in the same condition, the WGM resonance wavelength of the silica microsphere shifts to a longer wavelength (red shift), and the optic-thermal tunable sensitivity is only about 0.086pm/mW with a much lower linearity. The proposed all-optical TTF based on PGMS has the advantages of all-optical control, compact structure, high stability, ultra-narrow bandwidth and highly tunable efficiency, which will be used widely in the field of power system, optical fiber sensing and optical fiber laser.

    Chinese Journal of Lasers, 2021, 48(1): 0106003      

    Fang Shasha, Wu Xuqiang, Zhang Gang, Shi Jinhui, Zuo Cheng, Zhang Wujun, Gui Lei, Yu Benli,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Optical fiber sensors have been extensively used to measure the external physical properties, such as temperature, refractive index, pressure, and strain, because of their characteristics such as simple structure, compact size, low weight, electromagnetic immunity, and high sensitivity. Generally, the fiber optic temperature and strain sensors are manufactured using fiber Bragg gratings (FBG) and optical fiber interferometers. However, the existing optical fiber sensors cannot meet the requirements in some scenarios that require high measurement sensitivity. Therefore, in this study, we propose using the optical vernier effect to enhance the performances of the optical fiber sensors. Normally, the optical vernier effect is generated by cascading two interferometers, among which one is set as the sensing interferometer and the other is set as the reference interferometer. The current interferometers based on the optical vernier effect generally include two cascading interferometers exhibiting almost identical responses to external disturbance, decreasing the sensitivity of the system. A complex isolation structure is usually applied to the reference interferometer to increase the response difference of the two interferometers with respect to the external disturbance, increasing the complexity of the sensors. Therefore, it is considerably important to design a reference interferometer not sensitive to the changes in external environment and achieve high-sensitivity and high-accuracy sensing of temperature and strain.

    Methods In this study, a reference interferometer based on the fiber Sagnac interferometer (FSI) is constructed by introducing an elliptical core polarization-maintaining fiber (ECPMF), which exhibits insensitivity to temperature, strain, bending, and torsion. The cascaded sensor is constructed by cascading a reference interferometer and a sensing interferometer based on polarization-mode interferometers (PMI). The optical vernier effect is obtained by controlling the free spectral range (FSR) of the two interferometers to be close but not equal. Thus, high-sensitivity and high-accuracy temperature and strain sensing is realized. The pigtail of the 3-dB coupler of the reference FSI is cut as short as possible to minimize its response to environmental disturbance. The length, major-axis radius, minor-axis radius, and cladding diameter of ECPMF (IVG PME-1300-125) are 10m, 3μm, 1μm, and 125μm, respectively. The PMI is considered to be the sensing interferometer, which is obtained by splicing a polarizer and a polarization-maintaining fiber (PMF) with the fast/slow axis at of an angle of 45°; the other end of the PMF is gold-coated. The length, core, and cladding diameters of the PMF (YOFC PM1017-A) are 0.94m, 6.5μm, and 125μm, respectively. Furthermore, theoretical analysis, numerical simulation, and experimental verification have been successively conducted based on the above design.

    Results and Discussions The output spectra of single FSI, single PMI, and cascaded sensors are analyzed theoretically and simulated to verify that the vernier effect can improve the system sensitivity. Results show that the FSR of PMI and FSI are 2.87 and 3.15nm, respectively. When the birefringence parameter of the PMF is increased from 3.5 × 10 -4to 3.51 × 10 -4, the interference spectrum of single PMI exhibits a 1.12-nm red shift, whereas that of the cascaded sensor exhibits a 12.7-nm red shift. The magnification factor is 11.34. When no strain is applied to the fiber at room temperature, the FSR of PMI, FSI, and cascaded sensors are 2.83, 3.1, and 35nm, respectively. Based on theoretical analysis, the birefringence parameter and beat length of ECPMF are 6 × 10 -5and 25.8mm at 1550 nm, respectively, which are consistent with the product specifications. The magnification factor with respect to the vernier effect obtained via theoretical calculation is 11.48, which is consistent with the simulation results. FSI and FMI are placed on a heating plate with a precision of 0.01 ℃ to investigate the temperature sensing characteristics of FMI and the relative temperature insensitivity of FSI. In the experiment, the temperature measurement ranges from 30 ℃ to 37 ℃ with a step of 1 ℃; each temperature state is maintained for 20min. The temperature sensitivities of dip 1 and 2 are 1.40 and 1.38 nm/℃ for a single PMI, respectively, whereas those of dip 1 and 2 are 168.38 and 157.44 pm/℃ for a single FSI, resulting in a temperature sensitivity that is approximately 1/8 of the single PMI temperature sensitivity. Thus, FSI is insensitive to temperature. After being amplified by the vernier effect, the temperature sensitivity of the cascaded sensors becomes 15.56 nm/℃, which is 11.12 times greater than that of a single PMI and is consistent with the theoretical value of 11.34. FSI and PMI are fixed using an optical adhesive (NORLAND 81) on a microdisplacement platform (Thorlabs, KMTS25E/M) with an accuracy of 1 μm to investigate the strain sensing characteristics of PMI and the relative strain insensitivity of FSI, and the distance between the two platforms is 40cm. Controlled by software, one of the microdisplacement platforms move outward; further, a displacement step of 16μm and a corresponding PMI strain step of 40με are realized, and the strain measurement range is from 0 to 280με. The strain sensitivities of dip 1 and 2 are 13.04 and 12.90pm/με for a single PMI, whereas those of dip 1 and 2 are 2.52 and 2.50pm/με for a single FSI, resulting in a strain sensitivity that is approximately 1/5 of the single PMI strain sensitivity. Thus, FSI is insensitive to strain. After being amplified by the vernier effect, the strain sensitivity of the cascaded sensors becomes 15.56nm/℃, which is 11.81 times greater than that of a single PMI and is consistent with the theoretical value of 11.34. The deviation in values can be attributed to the manual reading of the spectrum.

    Conclusions In this study, a highly sensitive optical fiber temperature and strain sensor is proposed by introducing ECPMF into the Sagnac ring as a reference interferometer. The sensor includes a reference interferometer FSI and a sensing interferometer PMI, and the lengths of PMF and ECPMF are reasonably designed to make the FSR of FSI and PMI close but not equal, resulting in a vernier effect. The experimental results show that the temperature and strain sensitivity of the cascaded sensor become 15.56 nm/℃ and 154.04 pm/με, respectively, which are 11.12 and 11.81 times greater than that associated with a single PMI, respectively, and consistent with the simulation results. Thus, the effectiveness and reliability of the proposed scheme are verified based on the experimental results. The obtained sensor exhibits high sensitivity, a simple structure, good stability, and a good application prospect in various fields such as aerospace and industrial production.

    Chinese Journal of Lasers, 2021, 48(1): 0106004      

    Cao Yang, Dang Yuchao, Peng Xiaofeng, Li Yue,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Compared with wireless local area network(WLAN)/ultra wide band(UWB) and other electromagnetic wave wireless communication technologies, indoor visible light communication technology has the advantages of low cost and an advantageous edge. Research on the indoor localization method based on received signal strength(RSS), time of arrival(TOA), time difference of arrival(TDOA), angle of arrival(AOA), and other classical localization algorithms is the key to realizing localization technology. However, compared with traditional electromagnetic wave wireless communication technology, in the visible light communication environment, the localization methods based on most localization algorithms are not mature, and the improvement in the localization performance is often limited by the singular localization information. In this paper, TOA and RSS data are combined to reduce the effect of nonlinear errors on the localization accuracy in indoor visible light communication environments. Combined with the inertial sensing data at the receiving end, the robustness and low localization delay of the proposed localization method are guaranteed. Additionally, the localization accuracy of the system is further improved.

    Methods Four sources are evenly distributed on the ceiling to simulate the indoor environment with a length and a width of 5m and a height of 3m. A channel model of indoor visible light communication is established, which has a direct line of the sight link and a multiple order reflection indirect line of the sight link. Then, the distribution of indoor received optical power is obtained, and the empirical formula of signal strength and distance is established using the mapping relationship between the received optical power and linear distance, between the source and receiver. Moreover, the time stamp record is used to measure the signal transmission time at the receiver. The particle filter based on an unscented Kalman filter is used to combine TOA and RSS data to improve the accuracy of distance estimation. Furthermore, the least-square method is used to estimate the localization coordinates. Finally, based on the inertial sensing data of the receiver, the movement trend is analyzed, and high-precision localization results are obtained.

    Results and Discussions Based on the channel model, a localization simulation is conducted. A total of 625 test points are selected in the room, and the localization results are obtained by coordinate estimation. The indoor localization error fluctuates from 1.6 to 3.2cm, and the overall localization error is low lying with a small center and rising edge, exhibiting only a small fluctuation range. First, the simulation parameters are fixed, and the localization performance of the proposed localization method, RSS method based on trilateral localization, and traditional fingerprint localization method are compared. For the proposed localization method, the probability of the localization error at less than 3cm is 98.1% and the average and maximum localization errors are 2.02 and 3.39cm, respectively. For the traditional fingerprint localization method, the probability of the localization error at less than 3cm is 40.8% and the average and maximum localization errors are 3.11 and 6.12cm, respectively. For the RSS method based on trilateral localization, the probability of the localization error at less than 3cm is 1.6% and the average and maximum localization errors are 5.61 and 9.67cm, respectively. Second, the localization performance of 12-, 6-, and 3-W LEDs is compared. Under 12-W transmitting power, the probability of the localization error at less than 3cm is 98.1% and the average and maximum localization errors are 2.02 and 3.39cm, respectively. Under 6-W transmitting power, the probability of the localization error at less than 3cm is 91.2% and the average and maximum localization errors are 2.52 and 3.77cm, respectively. Under 3-W transmitting power, the probability of the localization error at less than 3cm is 42.4% and the average and maximum localization errors are 3.18 and 5.08cm, respectively. Finally, the localization time of the proposed localization method, RSS method based on trilateral localization, and traditional fingerprint localization method are compared. For this, 30 positioning processes of the three localization methods are selected. The RSS method based on trilateral localization exhibits the shortest localization time, while the localization time of the proposed localization method and fingerprint localization method fluctuates by approximately 1s.

    Conclusions The simulation results are listed below:

    1) Under fixed parameters, the maximum localization error of the proposed method is 44.61% less than that of the traditional fingerprint localization method, and the average localization error is reduced by 35.04%. Compared with the RSS method based on trilateral localization, the maximum and average localization errors of the proposed method are reduced by 64.94% and 63.99%, respectively. Therefore, the localization accuracy of the proposed localization algorithm is better than that of the other two localization methods.

    2) The localization performance clearly decreases with a decrease in the LED transmitting power. However, no significant difference is observed between the performance of the proposed localization method at 3?W transmission power and that of fingerprint localization method at 12?W transmission power. Moreover, the proposed localization method shows better performance than the RSS method under 12?W transmission power, thus demonstrating the robustness of the proposed localization method.

    3) The localization time of the proposed localization method is stable. Although it is nearly the same as that of the traditional fingerprint localization method, the overall trend is stable and it is only slightly longer than that of the RSS method based on trilateral localization. Thus, the proposed localization method can achieve an improved localization effect only by sacrificing a small amount of time resources.

    In conclusion, the overall localization effect of the proposed method is good, and the localization error do not fluctuate significantly, thereby ensuring the robustness of the proposed localization method as well as low localization delay and good localization performance.

    Chinese Journal of Lasers, 2021, 48(1): 0106005      

    Li Xiaoming, Wang Longming, Li Xiang, Zhang Lizhong, Zhang Jiaqi,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective In the one-to-many laser communication indoor principle verification system, the antenna adopts multi-faceted tilt-mirror with independent optical axis alignment function to realize one point and multi-point laser communication in a broad range. To reduce the size and working envelope of the tilt-mirror in the laser communication ground test system to meet the limitation for size, a compact and integrated tilt-mirror with SiC/Al is proposed in this paper. The tilt-mirror can be connected to the turntable’s pitch axis seat to reduce the reflecting surface’s eccentric distance and the whole machine’s outer envelope size. The tilt-mirror and supporting structure’s mounting surface can only be separately grinded and then installed since the structure restrict the system. The test shows that when the grinding accuracy is 5μm, the tilt-mirror profile accuracy cannot meet the design requirements. The mounting surface must also be repaired by grinding several times according to the surface inspection results after the tilt-mirror is installed. However, the system has three independent tilt-mirrors, which makes the repair grinding cycle long and complicated. Thus, the tilt-mirror must be optimized to consider the impact of grinding errors on the surface quality and reduce the requirements for the surface quality of the mounting surface. This study focuses on the tracking tilt-mirror structure in the networked laser communication ground demonstration prototype, which causes the reflector and mounting base unable to use the traditional bonding process and can only be installed with screws, resulting in serious degradation of the reflector surface during the screw connection process. Theoretical and simulation analyses are conducted to verify the reasons and laws of the problem based on the original structure’s test results. It then optimized the design to improve the mirror surface’s stability, ensuring that the design requirements are met.

    Methods First, the initial design mirror’s surface shape changes under different mounting points are tested. The theoretical calculations are conducted based on the experimental and test results of the original reflector structure. The analysis proves that the unevenness of the structure’s installation points affects the mirror surface’s accuracy. The law is consistent with the test results. Next, through finite element analysis, the influence of each installation point of the original structure is simulated, and compared with the test result, the reason and law of the problem are verified. It is obtained that the conventional grinding and other mechanical methods cannot meet the requirements. Thus, the tilt-mirror needs to be optimized and improved. The flexible structure is then used to isolate the stress transmission, and the tilt-mirror is optimized based on the original design. The flexible structure with isolation trenches is used to solve the problem of surface degradation. Therefore, isolation grooves are designed on the mounting plate to reduce the mounting surface’s flexibility and effectively isolate the installation force’s transmission. The analysis showed that the surface deformation of the high-quality surface tilt-mirror reduced significantly. Also, the surface stability of the tilt-mirror has significantly been improved. The ability to suppress the external forces influence is greatly enhanced, ensuring that the tilt-mirror meets the design index requirements after installation. Finally, it verified the mirror surface’s accuracy through static surface testing and verified its practical application effect through dynamic spot detection.

    Results and Discussion The surface quality test shows that the tilt-mirror surface shape’s peak-valley value is better than λ/6, and the root-mean-square value is better than the λ/52 (Fig. 7 and Table 2). It shows that the integrated tilt-mirror with high surface stability guarantees its surface stability under the influence of installation flatness and temperature load and fully meets the design requirements. In multi-laser communication antennas using the high-surface stability integrated tilt-mirror mirrors and indoor demonstration experiments, the camera’s image processing software is used to identify the light spot in the fine tracking camera when capturing static (pointing but no tracking) and dynamic (dynamic tracking), and extract the light spot’s pixel information in the x- and y-axes directions (Fig. 9). Comparing the changes in spot size in different states and at different times, it can be seen that the spot roundness is better, and the spot roundness is slightly reduced under dynamic conditions, but the effect is small (Table 3).

    Conclusion Aiming at the problems that the structure limits the compact tilt-mirror in the networked laser communication system, the grinding accuracy of the mounting surface affects the surface accuracy of the reflecting surface, and the surface accuracy after installation is remarkably reduced and cannot meet the design requirements, the stability of the tilt-mirror surface is optimized through theoretical analysis and finite element simulation. The mechanical characteristics of the integrated tilt-mirror installation are analyzed and investigated. The influence of the installation point parameters on the surface of the tilt-mirror is analyzed. The installation point position of the integrated tilt-mirror is optimized, and the parameters of the integrated tilt-mirror are designed. Because of the particularity of the tilt-mirror processing and installation methods, the flatness’ influence law and strength are analyzed. The isolation structure is designed to suppress the flatness’ influence, ensuring the integrated tilt-mirror’s surface stability after installation. The surface accuracy test results show that the tilt-mirror fully meets the design requirements after installation.

    Chinese Journal of Lasers, 2021, 48(1): 0106006      


        Laser devices and laser physics

    Sun Shaowei, Qi Naijie, Kong Yan, Liu Cheng, Gao Shumei,

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    Objective Fused silica has been used in a variety of applications, including high-power laser devices, owing to its excellent chemical stability and optical properties. However, the fused silica optical elements in high-power laser devices are easily damaged owing to various complex physical and chemical mechanisms, thereby the system stability is affected. Researchers have successively conducted a lot of basic and applied researches on laser induced damage. However, most of the existing researches focus only on the dynamic process of material surface damages caused by the interaction between optical materials and lasers, such as temperature distribution, material evaporation, and damaged pit morphological change. Many researches have been conducted on the thermal stress distribution on fused silica material surfaces. However, there are few studies on the stress field distributions inside materials along the direction of an incident laser. This study presents a detailed research and an analysis on temperature and damage morphological distributions of materials and clarifies the interaction mechanism between laser and matter from the three-dimensional stress viewpoint.

    Methods To study the three-dimensional stress distribution of laser damaged optical components, this study establishes a finite element thermodynamic model describing the interaction between pulsed CO2 laser and fused silica. This model can simulate temperature evolution inside fused silica during laser irradiation, and can be used to analyze the initial damage morphology of the specimen and three-dimensional stress distribution inside the material after cooling. To ensure the accuracy of the constructed thermodynamic model, this study considers classical heat conduction, heat radiation, and heat loss caused by heat convection on the specimen surface. Solving the heat conduction equation, one can get the internal temperature distribution when laser interacts with fused silica. Simultaneously, using the obtained surface temperature of the specimen, one can get the depth of damage pit. However, a single thermoelastic equation is not enough to completely describe the change in the material cooling process, and the viscoelasticity of materials is also needed to be included to investigate the variation of strain and stress with time. Therefore, a generalized Maxwell model with a single element is introduced to represent the viscoelastic materials, and the three-dimensional stress distribution of laser damaged fused silica can be calculated after the material is cooled. Further, a more in-depth analysis of laser damage can be conducted.

    Results and Discussion Based on the model we established above, we obtained the three-dimensional radial stress and hoop stress in fused silica along the direction of the incident laser. As for the difference between the two stresses, the corresponding parameters in the numerical simulation are selected according to the experimental ones, and the comparison between the experimental and simulation results shows that the two stresses have a completely consistent trend, which proves the accuracy of the numerical model describing the interaction between the pulsed CO2 laser and fused silica. Moreover, according to the interpretation of the obtained three-dimensional stress distribution, the radial stress within the depth of the damage pit appears as a compressive stress. The radial stress first increases to the maximum. After exceeding the damage depth, the radial stress gradually decreases until approaches zero. In addition, the internal radial stress of fused silica first reaches the maximum compressive stress value near the bottom of damage pit and then gradually transforms from the radial compressive stress to the tensile stress before gradually decreasing to zero along the axial direction. The hoop stress near damage pit appears as the compressive stress, similar to the radial stress. With the radius value decreasing, the hoop compressive stress is transformed into the tensile stress. The hoop stresses first increase along the z-axial direction until they reach the maximum value and then gradually decrease with the increase of depth until they become zero. In addition, the increase of laser pulse energy leads to the significant increase of the hoop and radial stresses and their influence ranges. These numerical calculation results, especially the three-dimensional hoop and radial stress distributions, are difficult to obtain with the traditional optical measurement technology.

    Conclusion The traditional laser damage stress measurement experiment is complicated. It has a huge margin of error, and it is difficult to directly measure the radial and hoop stress distributions through this experiment; only the difference between these two stresses can be measured through this experiment. In this study, a finite element analysis method is used to establish a thermodynamic model describing the interaction between a pulsed CO2 laser and fused silica. Based on the obtained temperature evolution inside fused silica and the initial damage morphology of specimen during the laser heating process, the three-dimensional stress distribution inside the material is calculated. The thermodynamic model considers the classical heat conduction, heat radiation, and heat loss caused by heat convection on the specimen surface. The three-dimensional distribution of the difference between the radial and hoop stresses calculated using this numerical model has the same changing trend as that from the experiment, which proves the accuracy of the numerical model. Based on the calculation of the three-dimensional stress distribution, the relationship between the radial and hoop stress distributions, the depth of damage pit, the distance from damage pit, and laser pulse energy are also analyzed in detail. These results are helpful to establish a three-dimensional stress field inside fused silica and provide a theoretical basis for the improvement of CO2 laser repair process.

    Chinese Journal of Lasers, 2021, 48(1): 0101001      

    Wei Lei, Wu Decheng, Liu Dong, Zhao Shuyun, Chen Guo, Li Bao, Fang Cong, Han Long, Wang Yingjian,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective The 8--12μm long-wave infrared laser is just within the transmission bands of atmosphere and widely used for gas composition detection and electro-optic countermeasure. As traditional long-wave lasers, CO2 lasers can output lasers with a specific wavelength within the range of 9--10μm. Beyond them, a long-wave infrared optical parametric oscillator (OPO) shows an enormous advantage because of its wavelength tuning. However, OPO-based lasers with wavelengths longer than 8μm and high optical-to-optical conversion efficiency are still scarce. Herein, we construct a ZnGeP2 OPO and experimentally test its long-wave infrared output. The experimental result shows that a long-wave laser with high conversion efficiency is obtained, which provides a reference to engineer the laser based on ZnGeP2 OPO.

    Methods The scheme of the OPO-based long-wave infrared laser pumped by a 2μm laser is discussed, in which the selection of a 2μm laser crystal and a long-wave infrared nonlinear crystal is included. In this scheme, the selected nonlinear crystal is ZnGeP2, and the selected pumping laser source is 2.05μm Ho∶YLF laser with a maximum output power of 27W (10kHz). The two end faces of the ZnGeP2 crystal are polished and coated with an antireflection film at 2.05, 2.7, and 8.2μm bands, which are the key processes for reducing the optical loss in the crystal and for reducing the risk of damage. The resonator of the ZnGeP2 OPO is a flat cavity and the resonant mode is double resonance OPO. The Ho∶YLF laser is linearly polarized, which is helpful for ZnGeP2 OPOs to achieve a high optical-to-optical conversion efficiency. The Ho∶YLF laser, pulsed using an acousto-optic Q-switch, is pumped by a Tm∶YAP laser (CW) with a wavelength of 1.94μm and a maximum output power of 62W. Without damaging the elements of the Ho∶YLF laser, the laser’s repetition rate is minimized, the OPO’s threshold is reduced, and the conversion efficiency is improved. The ZnGeP2 crystal, Ho∶YLF crystal, and Tm∶YAP crystal are all wrapped in thin indium foils and placed in copper heat sinks to collect the heat absorbed by them. During the operation of the experimental apparatus, there is the water flow with 20 ℃ in the Q-switch and all heat sinks, and a microchannel structure for the water flow is indicated. Finally, the typical parameters of the long-wave infrared laser, including average power, wavelength, laser beam quality, repetition rate, and pulse duration, are measured.

    Results and Discussions The laser experimental apparatus (corresponding to the scheme mentioned above) achieves good experimental results with high power, efficiency, and repetition rate. The long-wave laser is generated when the 2.05μm pulsed laser with an average power of 10.5W is injected. The maximum output power of the long-wave laser is 3.2W when the 2.05μm pulsed laser with an average power of 26.68W is injected. Meanwhile, the corresponding optical-to-optical conversion efficiency is up to 12% and the slope efficiency is up to 19.3%. A spectrum analyzer is used to measure the spectrum of the long-wave laser with an output power of 3.2W and the peak wavelength of 8.135μm is disclosed. A CCD laser beam analyzer is used to measure the laser beam quality factor of the long-wave laser with an output power of 3.2W. The focusing lens method is used for these measurements. After the measurements, the quality factor is 4.5 in the X direction and 4.2 in the Y direction. The laser parameters including repetition rate of 10kHz and pulse duration of 27.11ns are measured using a photoelectric detector. The simple calculation shows that the single pulse laser energy is 0.32mJ and the peak power is 11.8kW.

    Conclusions We verify that a ZnGeP2 OPO is feasible to realize high efficiency and tunable long-wave laser output. First, the phase-matching mode and the phase-matching angle of the ZnGeP2 crystal are analyzed and designed according to the principle that the output laser wavelength of a ZnGeP2 OPO corresponds to its phase-matching angle. Second, to realize the 8μm laser, the ZnGeP2 crystal is processed according to the above phase-matching angle. Third, the experimental apparatus is set up and the effect of the long-wave ZnGeP2 OPO laser is verified, and the ZnGeP2 OPO laser pumped by the 2.05μm Ho∶YLF pulsed laser can generate a long-wave laser output with a specific wavelength, high efficiency, and high power. In the future, long-wave infrared lasers with wavelengths longer than 8μm can be achieved just by reducing the phase-matching angle of the ZnGeP2 crystal (changing its cutting angle as an example) or reducing the incident angle of the pump laser.

    Chinese Journal of Lasers, 2021, 48(1): 0101002      

    Ouyang Xinchuan, Yang Bowen, Wan Jinyin, Xiao Ling, Cheng Huadong,

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    Objective Laser are used in many research fields such as quantum communication, atom cooling, atom clock, and materials processing. The power stability of laser is very important, especially in the field of quantum precision measurements where it directly affects the experimental measurement accuracy. For the atom clock, the power stability of the laser affects its stability and accuracy. Therefore, it is necessary to make the active laser power stabilization system. As a general control method, the fuzzy proportional-integral-differential (PID) control has been widely used in the closed-loop control systems, such as temperature control, path planning, flight attitude adjustment, etc. A recent study investigates the laser power stabilization with the analog circuit PID, but in which the values of PID parameters need to be readjusted if it is used in different environments and the stabilized value of laser power cannot be changed during the experiments. In order to solve these problems, the fuzzy PID control scheme is proposed. We hope that our solution can reduce the stable time of the feedback loop, improve the relative intensity noise, and achieve long-term stabilization of laser power.

    Methods There are two types of feedback loop for the laser power stabilization, one is feedback to the laser current (internal loop), and the other is feedback to an acoustic optical modulator (AOM) (external loop). Generally, the external lock loop is used because the internal loop will disturb the laser current and thus the frequency. In this paper, an embedded system of laser power stabilization based on fuzzy control is investigated. The lock loop is realized by feedback to an AOM. After passing through AOM, the laser generates diffractive light. By adjusting the diaphragm only +1 order diffraction light is allowed to pass through. After passing through the beam splitter, it is divided into two beams. One beam is detected by the photodetector, the other beam is used for experiments. The digital control circuit consists of an analog-to-digital (AD) converter, a digital-to-analog (DA) converter, and a digital signal processing chip. First of all, the laser power is detected by photodetector. Then the voltage signal is converted to a digital signal by an AD conversion. The error signal is obtained by comparing with the standard set voltage. After that the error signal and its rate of change as well as the three parameters of PID are fuzzy, and the fuzzy algorithm controller performs the calculations. The results of the parameters of PID are clarified. Finally, the amplitude modulation voltage of AOM is output through DA after the PID operation. The key to the performance of the laser power stabilization is setting the fuzzy rules. Table 1 shows the fuzzy rules adjusted according to the actual situation.

    Results and Discussions The set voltage of laser power is 3.5V. It is defined that the loop stable time is the one required for the photodetector voltage that increases from 0 V to 3.5V. The stable time of laser power after fuzzy control can be obtained by monitoring the voltage of the photodetector in the feedback loop. Compared to traditional PID, the stable time is reduced from 4.7ms to 1.8ms due to the absence of overshoot (Fig. 5). The relative intensity noise of the laser power can be measured by placing the photodetector outside the loop (the beam for physical experiments). The results show that the power spectral density of relative intensity noise of the laser is depressed from -88dBc/Hz to -110dBc/Hz at 1 Hz and from -93dBc/Hz to -110dBc/Hz at 10Hz, and is lower than -110dBc/Hz over a wide frequency range, meanwhile the relative intensity noise of DA output voltage is lower than that of the laser (Fig. 6), meeting the experimental requirements. In addition, the relative fluctuation of the laser power is measured over three hours and improved from 0.29% to 0.035% after power stabilization (Fig. 7). Here, the relative fluctuation of the laser power is the ratio of the laser power fluctuation to the average.

    Conclusions In this paper, a fuzzy control is applied to laser power stabilization using an embedded technique. The amplitude modulation voltage of the AOM is used to change the diffraction efficiency of the laser and thus achieve the laser power stabilization. Compared with traditional PID, after adding the fuzzy control, the feedback loop will not oscillate due to overshoot, and the stable time of the feedback loop is reduced from 4.7ms to 1.8ms. After power stabilization, the power spectral density of laser relative intensity noise is greatly improved in the low-frequency part, which is suppressed by 22 dB at 1Hz, and is lower than -110dBc/Hz over a wide frequency range. The time domain test results show that the relative fluctuation of the laser power improves from 0.29% to 0.035% within 3h. In the field of quantum precision measurement, the power stabilization technique is important for improving the measurement accuracy, such as improving the stability of atom clock and the accuracy of interferometer measurements, and because the power stabilization technique can change the stabilized laser power in real time, it is suitable for some experimental procedures that need to change the laser power in specific situations.

    Chinese Journal of Lasers, 2021, 48(1): 0101003      


        Rapid communications

    Xi Xiaoming, Wang Peng, Yang Baolai, Wang Xiaolin, Zhang Hanwei, Ning Yu, Han Kai, Wang Zefeng, Zhou Pu, Xu Xiaojun, Chen Jinbao,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Chinese Journal of Lasers, 2021, 48(1): 0116001      


        Letters

    Wang Shijie, Cai Yiming, Zhang Zhilun, Lin Xianfeng, Xing Yingbin, Liao Lei, Li Jinyan,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective All-fiber, high-power lasers play important roles in practical applications and scientific research. In the research field of high-power pulsed fiber lasers, the master oscillator power amplifier (MOPA) configuration seeded by a pulsed fiber laser oscillator is a typical design, which is widely used to achieve high-power pulsed laser emission. However, there still exist too many factors influencing the practical realization of such devices, such as 1) nonlinear effects, especially stimulated Raman scattering (SRS), 2) overly high intrinsic temperature of the active fiber caused by quantum loss, 3) parasitic oscillation induced by gain saturation, and 4) fiber damage due to instantaneous and violent pulsed power peaks. Appropriate fiber design can mitigate these effects to some degree. The realization of more than ten thousand watts in continuous-wave lasers based on homemade fibers has been reported. However, the fiber damage requires more attention in pulsed fiber lasers because of their high peak powers. Currently, most active fibers utilized in a high-power pulsed-laser system rely on imports. In this paper, we report the realization of a 1000W nanosecond pulsed laser using homemade double-cladded Yb-doped fibers (DCYDFs). To our knowledge, this is the first fiber laser based on homemade fibers to break through the kilowatt level in average power.

    Methods We construct an acousto-optic-modulator-based seed source with a tunable repetition rate of 20-60kHz. Two amplification stages are followed to boost the average power to 1000W. The waveforms and spectra are captured by an oscilloscope (Lecroy WaveSurfer 44MXs-B) and a spectrograph (YOKOGAWA AQ6370D), respectively. We utilize the homemade 10μm /130μm, 30μm /250μm, and 100μm /400μm DCYDFs to provide the laser gain media in the seed source, preamplifier, and main amplifier, respectively. At the output end, a coreless fiber with a cleaved angle of about 8° is used to mitigate the laser intensity.

    Results and Discussion A pulsed power of 2.87W, at a repetition rate of 60kHz, and a pulse width of 150ns, is generated by the Q-switched seed source. After two stages of amplification, the average power is boosted to 1000W, with the pulse width broadened to about 260ns [Fig. 2(a)]. The pulse energy is 16.7mJ and the peak power is about 64kW. Neither SRS nor high-frequency parasitic oscillation appears at the highest power [Fig. 2(b)]. The slope efficiency is about 72.5%. A linearly fitted curve indicates the possibility of further power scaling, but this is limited by pump promotion [Fig. 2(c)].

    Conclusion Using homemade DCYDFs, we have demonstrated the realization of a 1000W nanosecond pulsed fiber laser based on an acousto-optic-modulator-based and Q-switched seed source and two stages of amplification. Neither SRS nor parasitic oscillation is generated in the spectra and power scaling is limited by pump promotion. To our knowledge, this is the first kilowatt pulsed fiber laser based on homemade fibers.

    Chinese Journal of Lasers, 2021, 48(1): 0115001      

    An Yi, Yang Huan, Xiao Hu, Chen Xiao, Huang Liangjin, Pan Zhiyong, Wang Xiaolin, Xi Xiaoming, Ma Pengfei, Wang Zefeng, Zhou Pu, Xu Xiaojun, Jiang Zongfu, Chen Jinbao,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective In recent years, there has been a rapid progress in the development of high-power fiber lasers, which are widely used in the fields of laser marking and material processing as well as numerous industrial applications. The main factors that limit the output power of fiber lasers are the mode instability and nonlinear effects, including stimulated Raman scattering and stimulated Brillouin scattering. To suppress these nonlinearities, the core size of large mode area active fibers should be increased. However, this may lead to the degradation of beam quality. The core diameter of tapered fibers gradually increases as the length increases, therefore suppressing the nonlinear effects. Moreover, the tapered area, which satisfies the adiabatic taper principle, facilitates in achieving excellent beam quality. Tapered active fibers have been used in various applications such as continuous-wave fiber laser oscillators or amplifiers, ultrafast laser systems, and single-frequency fiber amplifiers. In early 2020, researchers from National University of Defense Technology proposed the Yb-doped double-tapered double-cladding fiber (DT-DCF), which consists of a thin-core section at both ends and a large-core section in the middle. In the present study, all-fiber high-power laser amplification is performed based on the self-fabricated Yb-doped DT-DCF. The laser system achieves a single-mode laser output with a maximum power of 4 kW and a mass factor M2 of 1.33.

    Methods We construct a master oscillator power amplifier system based on the homemade DT-DCF. This system is 21-m long and has small-core and large-core sections with core/cladding diameters of 22/413 and 32/600mm, respectively. Fusion splices connect all the components. The seed has a center wavelength of 1080 nm and a output power of 103 W. After passing through the cladding light striper (CLS), the seed light is injected into the amplifier. Subsequently, bidirectional pumping is applied to laser amplification. Seven laser diode (LD) modules with a center wavelength of 976 nm are divided into two groups comprising two and five modules to pump the DT-DCF through the forward and backward couplers, respectively. After the amplification stage, the CLS is utilized to strip out the residual pump power and the laser is finally output to free space through the endcap for the measurement of power, spectrum, and beam quality.

    Results and Discussion The output power increases linearly with the pumping power. When the pumping power is 4.75kW, the output power reaches 4kW. The corresponding optical efficiency and slope efficiency are 82% and 83%, respectively. The M2 at the highest power is 1.33 [Fig. 3(a)], exhibiting the single-mode output characteristic of the system. Mode instability limits further power scaling of the single-mode output. If the pump power increases to over 4.75kW, time domain fluctuation of kHz can be observed, indicating the initiation of mode instability. The output laser has a center wavelength of 1080nm, and its spectrum broadens as the output power increases. For the spectrum under the highest output power, the Raman suppression ratio reaches up to 44dB [Fig. 3(b)], demonstrating the ability of the DT-DCF to inhibit the nonlinear effects.

    Conclusions In summary, we have established a 1080-nm all-fiber amplifier based on DT-DCF. This amplifier can achieve a 4-kW single-mode output laser with a slope efficiency of 83% and M2 of 1.33. Our results indicate that the DT-DCF can simultaneously suppress the nonlinear effects and transverse mode instability, thus providing a beneficial reference for further power scaling of single-mode fiber lasers.

    Chinese Journal of Lasers, 2021, 48(1): 0115002      


        Holography and information processing

    Xu Fuyang, Yang Xin, Yao Jianyun, Liu Zimo, Song Qiang, Li Yong,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Holographic 3D display has gained considerable attention owing to its ability to completely reproduce the wavefront information of a 3D scene. To reconstruct a real and virtual 3D scene, computer-generated hologram (CGH) is found to be more flexible than optical holography. The CGH has two main branches, namely, the dynamic holography reconstructed by liquid crystals on silicon (LCoS) and the static high-resolution holography realized by photolithography. However, due to the large pixel size and low-resolution, dynamic holographic 3D display based on LCoS can only achieve a small field of view and small display size, thus limiting its application. High-resolution CGH can be calculated and printed using photolithography and copied for mass production. However, the computational time is a big challenge. To overcome this challenge, holography is regarded as an information encoding method, which is employed to encode light-field images, thereby significantly reducing the calculation time. Moreover, high-resolution holography can only reproduce a static 3D image, indicating that the viewer can see the perspective information of the 3D image from different angles, but with a static 3D image. Thus, more vivid holographic 3D display needs to be realized. In this study, multiview dynamic holographic 3D display is proposed by coding the dynamic light-field images fused from multiple rendered light-field images via 3D animation. In this new display, the viewer can see a 3D dynamic image when the viewpoint changes.

    Methods In the hologram calculation, the sequence of the light-field image of each frame of the 3D model in the 3D animation was first rendered according to the pinhole array projection model. Then, the light-field images that correspond to the view angle information were extracted from the rendered multiple sets of light-field image sequences and fused to achieve the fused dynamic light-field image sequence. In the hologram coding, the frame of images in the dynamic light-field image sequence was taken as the object light amplitude, and the phase of the diverging spherical wave coming from the pinhole was taken as the object light phase. In addition, the plane reference light was used to obtain a unit hologram. Since the calculation of the dynamic light-field image and each unit hologram are independent of each other, we used parallel acceleration in the calculation process. Light-field image fusion and holographic coding of a high-resolution hologram with a size of 32mm×32mm and a resolution of 100000pixel×100000pixel only took 27min.

    Results and Discussion A 3D model with a size of 18.8mm×30mm×17mm containing 4.06×10 5 object points was employed for high-resolution hologram calculation. The distance between the 3D model and the hologram plane was 17mm, and the 3D animation was rotated horizontally, including 50 frame. The 355×139×50 light-field images of the 3D animation were first calculated, and the 355×139 dynamic light-field images fused from the rendered light-field images were then utilized for hologram calculation. The high-resolution hologram with a size of 32mm×32mm and a resolution of 100000pixel×100000pixel is calculated and printed using our holographic output system. The dynamic light-field image fusion and hologram calculation only took 27min. In the hologram reconstruction, a white LED light from a mobile phone was used to illuminate the hologram at the back of the hologram at a proper distance and illumination angle. A USB camera on a one-dimensional rail was used to take pictures of the hologram. When the camera was focused on the holographic plane, the holographic plane became clear, whereas the reproduced 3D image became blurred. When the camera was focused only on the 3D-reproduced image, the reproduced image became clear, whereas the holographic plane became blurred. This phenomenon indicates that the proposed display can realize the 3D light-field reconstruction. By moving the camera, the reproduced images of the other two perspectives were taken. Based on the results, we can infer that the 3D image actions are different in different perspectives. In the actual view, human eyes can see the continuously changing 3D animation by changing the viewpoints.

    Conclusion High-resolution multiview dynamic holographic 3D display is realized based on the direct coding of dynamic light-field images. To produce the dynamic light-field images from the 3D animation, the mapping relationship between the viewpoints and the time sequence of 3D animation is established. The proposed 3D display is vivid with a dynamic 3D display from different viewpoints. The calculation method is also found to be effective. Although the display is a monochromatic 3D display, the color 3D display can be achieved using the combination of the color rainbow holographic method and the proposed method here. This will be the focus of our future work. Moreover, a large hologram can be produced for a more complex 3D animation to improve the practicality of the proposed method.

    Chinese Journal of Lasers, 2021, 48(1): 0109001      


        Biomedical photonics and laser medicine

    Jiang Jianta, Wei Meng''en, Xiong Zhengdong, Wu Xianyou, Cheng Tingqing, Jiang Haihe,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Er∶YAG laser crystals can produce laser at a wavelength of 2.94 μm, which is close to the infrared absorption peaks of water and hydroxyapatite. Laser at a wavelength of 2.94 μm possess numerous advantages in the ablation of biological tissues and the cutting of hard tissues such as bones and teeth. In clinical applications, erbium lasers are mainly used in two modes: free-running and Q-switched. However, free-running erbium lasers have a pulse width of a few hundred microseconds. Long pulses used to affect tissues cause heat diffusion into the surrounding healthy tissues resulting in damage or necrosis. Due to the low gain of 2.94 μm erbium laser crystals, it is difficult to obtain Q-switched erbium lasers with high pulse energy at high repetition frequency. The low ablation efficiency caused by low pulse repetition frequency limits their efficacy in dental treatment. To solve the problems mentioned above, we have developed a sub-pulse sequence mode laser at a high repetition frequency. In this mode, a standard long pulse is divided into several short sub-pulses with the same sub-pulse interval. This enables the sub-pulse sequence mode to deliver short, high finesse pulses with a photoelectric conversion efficiency of long duration pulses without sacrificing the ablation precision of short duration pulses.

    Methods We set four groups of sub-pulse widths as 20, 30, 40, and 50 μs, and the sub-pulse interval time as 85 μs. The repetition frequency of the laser was 20 Hz. An Er∶YAG crystal rod with 4 mm diameter and 104 mm length was used as a laser-active medium. Doping concentration of the Er∶YAG crystal was 50%(atomic fraction) for Er 3+. Two facets of the Er∶YAG rod were antireflection-coated at 2.94 μm. A resonator was formed using two plane mirrors separated by 194 mm. The reflectivity of the high reflective (HR) mirror exceeded 99% and reflectivity of the output coupling mirror was 70%. An insulated gate bipolar transistor module that can control pulse width and laser frequency through the external pulse signal was used in the laser power supply. In addition, the effects of sub-pulse width on erbium laser ablation in sub-pulse sequence mode were investigated. We used the Er∶YAG laser in sub-pulse sequence mode as the light source. The laser beam was reflected toward the dental sample using a 45° reflector. After being focused by a lens focal length(focal length f= 50 mm), the beam vertically irradiated the surface of the dentin. Cooling water mist was not required during the experiment.

    Results and Discussions A sub-pulse sequence mode erbium laser with high energy laser outputs of 80 sub-pulses per second was developed . When the sub-pulse widths were 50, 40, 30, and 20 μs, the maximum energy values were 671.1, 741.1, 814.1, and 798.8 mJ, respectively. The corresponding maximum slope efficiency was about 1.8% (Fig. 2). Through the experiment of dentin samples ablation, we found that the ablation mass would increase with decreasing sub-pulse widths. When the sub-pulse width was 20 μs, the mass of ablation was 90 mg after 60 s laser irradiation. When the sub-pulse width was 50 μs, the ablation mass was 62 mg. The ablation mass of the former was 45% higher than that of the latter (Fig. 4). Under conditions of 20 Hz repetition frequency, the samples were treated with the sub-pulse sequence mode laser at energy of 45 mJ. With decreasing sub-pulse width, the temperature rise in the pulp chamber decreased. When the pulse width was 50 μs, the temperature reached 42 ℃ within 25 s, but when the pulse width was set as 20 μs, the temperature reached 42 ℃ within 33 s (Fig. 5). In addition, the dentin samples were sprayed with gold and dehydrated to study the cavity structure after ablation before observation with a scanning electron microscope (SEM). When the sub-pulse widths were 20 μs and 30 μs, no carbonization, melting, or debris were observed on the surface of the pot hole, and the lower dental tubules were completely open. However, when the sub-pulse width reached 40 μs or 50 μs, no obvious melting and debris were observed by the scanning electron microscope, but the dentinal tubules were partially sealed (Fig. 6).

    Conclusions A sub-pulse sequence mode erbium laser at a high repetition frequency was developed, which obtained high energy laser outputs of 80 or 100 sub-pulses per second. The effects of sub-pulse width on erbium laser ablation in sub-pulse sequence mode were investigated. The pulse widths of the sub-pulse during ablation were set to 20, 30, 40 and 50 μs, respectively, and the pulse energy of the laser was maintained at 45 mJ. The influence of sub-pulse width on the ablation mass, the temperature rise in the pulp chamber, and the cavity microstructure were analyzed without cooling water mist.Results indicate that shorter sub-pulse widths can increase the ablation mass, reduce the temperature rise in the pulp chamber, prolong the operation time, and improve efficiency. In addition, improved cavity microstructure and more open dentinal tubules were obtained, which are both beneficial to adhesive repair and treatment.

    Chinese Journal of Lasers, 2021, 48(1): 0107001      


        Micro-nano optics

    Yang Feng, Wen Ping, Zhang Zhiqiang, Li Danyang, Chen Li, Li Shunbo, Xu Yi,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective Surface-enhanced Raman spectroscopy (SERS) can provide fingerprint information on target molecules with high detection sensitivity without being affected by water, which makes it an attractive non-destructive analysis technology. The SERS active substrate is key part of inspection applications. Therefore, a significant research effort has been devoted to the development of a stable, uniform, and repeatable SERS substrate. Unlike rigid SERS substrate, flexible SERS substrate can be flexibly deformed, which is convenient for in situ detection on irregular curved surfaces and can even be used for wipe sampling detection directly. However, the characteristics of the flexible substrate itself significantly impact the SERS performance, and the commonly used sol drop casting method to prepare SERS active substrates is significantly affected by the “coffee ring” effect. Because the effect caused uneven distribution of nanoparticles, uniformity of the SERS signal is affected. In this paper, simulation analysis and experimental research on the reflectivity of the substrate are carried out, and optimization of the substrate is performed to suppress the “coffee ring” effect. A high-throughput and array-type flexible SERS chip with excellent performance is successfully fabricated, which has application potential in biomedicine, food safety, environmental pollution, and other detection fields.

    Methods To study the influence of substrate reflectivity on SERS performance, the Raman signal intensities of different substrates are compared and analyzed by simulation (COMSOL Multiphysics) and experimental tests. To eliminate the “coffee ring” effect generated from the evaporation of nanoparticle suspensions, we control the solvent composition by adding a certain proportion of ethylene glycol in Ag sol, and use inward Marangoni flow induced by the surface tension gradient, which ensure uniform deposition of nanoparticles. The array detection unit of the SERS chip is fabricated on aluminum foil through laser printing. Then, Ag sol mixed liquid is dropped into the detection area and confined by the hydrophobic toner film. The SERS chip is formed after the droplets dried in a vacuum drying oven. Rhodamine 6G (R6G) is used as a probe molecule for the SERS test, and the performance of the SERS chip is evaluated by calculating the Raman enhancement factor and performing a signal uniformity test.

    Results and Discussions Simulation analysis results show that the greater the reflectivity of the substrate, the higher the intensity of the Raman signal. The Raman detection results for R6G molecules also show that the substrate with high reflectivity is helpful for Raman signal collection, which is consistent with the simulation results. Aluminum foil is chosen as the substrate because it has the strongest reflectivity under 532nm excitation light among the several materials used, and the SERS chip is manufactured using the Ag sol drop casting method. After adding ethylene glycol to the Ag sol, a tension gradient is formed on the surface of the droplet, resulting in an inward Marangoni flow, which prevents the nanoparticles from gathering on the edge of the droplet, thereby suppressing the “coffee ring” effect. The experimental results show that when 200μL of ethylene glycol is added to 1mL of Ag sol, the “coffee ring” effect is eliminated, and the Ag nanoparticles are uniformly distributed on the substrate. The Raman test results indicate that the SERS chip exhibited a high Raman enhancement factor of up to 1.32×10 8 and a detection limit of down to 10 -11 mol for R6G molecules. Furthermore, the chip showes good signal uniformity.

    Conclusions Flexible SERS chips have many advantages in Raman detection applications. In this work, simulation analysis and experimental tests show that the high reflectivity of the substrate has a significant impact on improving the SERS performance of the chip. By adding a certain proportion of ethylene glycol solution to the Ag sol, the surface tension state of the solution can be changed, ensuring uniform distribution of silver nanoparticles on the chip, thereby eliminating the “coffee ring” effect generated during deposition of the nanoparticles. The flexible SERS chip shows good Raman detection performance. The array structure of the SERS chip enables it to achieve high-throughput, multiparameter detection. It has strong application potential in fields such as biomedicine, food safety, and environmental pollution.

    Chinese Journal of Lasers, 2021, 48(1): 0113001      


        Remote sensing and sensors

    Yang Hao, Xie Chenbo, Fang Zhiyuan, Wang Bangxin, Xing Kunming, Cao Ye,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Objective As aerosols easily spread biological organisms (such as viruses and germs) and exert an extinction effect, they pose serious threats to public health and travel. In addition, with the rapid development of China''s economy, intensive industrial carbon emissions and automobile exhaust caused the increase of the amounts of man-made aerosols in many areas of China. Therefore, monitoring these aerosols by remote sensing is necessitated. Joint observations by space-borne and ground-based lidars can capture the temporal and spatial changes in aerosol emissions. The comparative studies under various weather conditions were rarely reported. The differences among the different weather conditions are still unclear. In this paper, we filtered the observation Aerosol-lidar data by matching the time at which the spaceborne CALIOP transits through Hefei under four typical weather conditions: dusty, cloudy, moderately polluted without clouds, and moderately polluted with clouds. We conducted joint aerosol observations in Hefei, and analyzed the types of aerosols, their changes, and the causes and sources of aerosol pollution.

    Methods We compared the vertical distributions and profiles of the aerosols in the observation data of Aerosol-lidar and CALIOP. Using the PM2.5and PM10 concentration data at the ground stations, we also determined the changes, vertical distributions, and causes of aerosols. The horizontal aerosol distributions were determined from the remote-sensing true-color images of MODIS. The causes of aerosol changes were deduced from the wind speeds and directions near the ground. Finally, the backward trajectories in the four study cases were analyzed in HYPSLIT mode. The lidar equation by the traditional Fernald algorithm was used in this article.

    Results and Discussions According to the joint observations, the polluted dust aerosol in dusty (lightly polluted) weather was concentrated in the 0.8- to 1.6-km height range, and the dust type were dust and polluted dust. The depolarization ratio was concentrated in the 0.18--0.20 range. In cloudy weather, the main dust type was polluted continental type concentrated in the 0.4- to 1.2-km height range, with depolarization ratios between 0.015 and 0.020. The aerosol content was very small and the particles were fine and spherical. In moderately polluted cloudless weather, polluted dust coexisted with polluted continental-type aerosols. The particles were concentrated in the 0.3- to 1.3-km height range, and the depolarization ratio was below 0.080, indicating an obvious spherical attribute. In moderately polluted cloudy weather, polluted dust coexisted with polluted continental-type aerosols again, but the main height range was 0.8--1.4km, and the depolarization ratio ranged from 0.075 to 0.100, indicating small-sized spherical particles. Small aerosols showed the properties of spherical particles with low depolarization ratios. The joint observations of space-borne and ground-based lidar more accurately captured and characterized the aerosol distributions at different time and from different locations than single observations. Although the ground-based lidar results were more accurate, the space-borne lidar provided better resolution for observing the spatial changes of aerosols. NOAA provided the HYSPLIT backward trajectory model for analyzing the sources and transport paths of the aerosols in the four weather cases. The HYSPLIT results confirmed different sources of the aerosols, to determine different conditions of aerosol formation by combining with the wind speeds and directions.

    Conclusions Cloudy weather conditions affected the detection of the bottom aerosols by the space-borne lidar. To better obtain the aerosol content and the characteristics of its change, the range-corrected signal was continuously observed at fixed point by the ground-based lidar. The aerosol changes largely differed under different weather conditions, and the types, causes and sources of aerosols were also highly variable. When the stratification of the atmosphere is stable, aerosols tend to accumulate locally and cannot diffuse; in contrast, when the atmospheric fluidity is strong, a small amount of pollutant dust mixes with the local aerosols and the weather becomes hazy, resulting from the change and transmission of aerosols. The combination of various data, such as lidar observations and ground particle-concentration data, wind speed, and wind direction, can explain the changes and causes of aerosols. In future work, we should combine these data into comprehensive observations of weather changes, and thereby build a model for monitoring urban aerosol pollution.

    Chinese Journal of Lasers, 2021, 48(1): 0110001      


        Reviews

    Hu Xinxin, Wang Yahui, Zhao Le, Zhang Qian, Zhang Mingjiang, Zhang Jianzhong, Qiao Lijun, Wang Tao, Gao Shaohua,

          Abstract + Free to read the article if you have an account for login, or you need to pay.

    Significance The distributed optical fiber sensor based on Brillouin scattering can continuously measure temperature or strain along the optical fiber and has become a research hotspot at home and abroad. Currently, it is widely used in many fields such as modern industries, civil structural health monitoring, and national defense security. There are four main types of Brillouin distributed optical fiber sensing technologies: Brillouin optical time domain reflectometry (BOTDR),Brillouin optical time domain analysis (BOTDA), Brillouin optical correlation domain reflectometry (BOCDR),and Brillouin optical correlation domain analysis (BOCDA). Among them, the BOCDA has the unique advantages of high spatial resolution, high-speed measurement, and random accessibility of measuring position, so it has extremely high potential application value. According to their different operating principles, this paper reviews the research progress of sine-frequency-modulation BOCDA (sine-FM BOCDA), phase-coded BOCDA, and broadband-source-based BOCDA in recent years. Broadband-source-based BOCDA includes amplified-spontaneous-emission-based BOCDA (ASE-based BOCDA) and chaos-based BOCDA,and the latter is proposed by our group. Additionally, in view of their limiting factors of sensing distance, spatial resolution, and measurement speed, the performance improvement of these BOCDA technologies is analyzed, and their future developments are also prospected.

    Progress The BOCDA is a novel distributed sensing method based on stimulated Brillouin scattering (SBS). The interference of the pump wave and the counter-propagating probe wave results in stimulated Brillouin acoustic field through electrostriction effect. The two waves are modulated in phase or frequency by the same waveform, and their frequencies are detuned around the Brillouin frequency offset of the fiber. The magnitude of the SBS acoustic field is stimulated at a specific position referred as correlation peak (CP), and the spatial resolution is only determined by the full-width at half-maximum (FWHM) of the CP ( Fig. 1). Consequently, compared to time-domain technology with inherent predicament of 1-m-spatial resolution, the correlation-based method does not suffer from the spatial resolution limitation. For the sine-FM BOCDA, the highest spatial resolution (1.6 mm) is achieved by applying the beat lock-in detection scheme, but the sensing distance is limited to less than 5 m ( Fig. 4). In order to increase the sensing distance, a differential measurement scheme with dual modulation and temporal gating is proposed to achieve a measurement range of 10.5 km, but the measurement time is too long [ Fig. 7 (b)]. Later, time-domain data processing is proposed in the differential measurement BOCDA system, which effectively improves the measurement speed of the system ( Fig. 8). For the phase-coded BOCDA, by using the short optical pulse source modulated by the pseudorandom sequence (PRBS), a highest spatial resolution (0.64 mm) of the current BOCDA system is realized ( Fig. 13). By using Golomb codes to replace PRBS and applying temporal gating, the coding noise is effectively suppressed ( Fig. 14), and then the optimal phase-coded BOCDA system based on temporal gating is proposed, by which the longest sensing distance (17.5 km) and the maximum number of resolution points (beyond 10 6) are achieved. The improvement of the measurement speed of the phase-coded BOCDA system is mainly studied from three aspects: the decrease of the number of averaging (incoherent sequence compression, Fig. 15), the reduction of the position addressing (double-pulse-pair analysis, Fig. 16), and the elimination of frequency scanning (transient SBS gain analysis without spectral scanning, Fig. 17). Therefore, the measurement speed is significantly promoted, and the dynamic monitoring could be further explored. For the broadband-source-based BOCDA, the millimeter-level spatial resolution could be easily achieved because the FWHM of the CP is determined by the source bandwidth. Consequently, spatial resolution of 4 mm is realized with ASE source of 25 GHz despite that the sensing distance is only 5 cm ( Fig. 19). In order to get better performance, the chaos-based BOCDA is proposed by our group, higher resolution of 3.5 mm is achieved with chaotic laser of 10 GHz, and the sensing distance reaches 165 m (Figs. 23--24). Moreover, by suppressing the time delay signature and using the time-gated scheme, the noise background is largely inhibited, and the sensing distance is greatly improved to 10.2 km with a spatial resolution of 9 cm ( Fig. 25).

    Conclusion and Prospect The sine-FM BOCDA system is easy to achieve high spatial resolution by adjusting higher modulation amplitude and has preferable signal-to-noise ratio (SNR). However, the direct modulation of semiconductor laser induces frequency modulation superimposed on amplitude modulation through carrier density modulation and temperature change effects, which leads to a predicament that is hardly to set both modulation frequency and modulation amplitude at a higher bandwidth simultaneously. In order to obtain mm-order spatial resolution, a special laser diode (LD), three-electrode LD, is used as light source, and intensity modulation (IM) is used synchronously to compensate for intensity chirp, which leads to complexity and high cost of the system. Phase-coded BOCDA system combines long sensing distance with high spatial resolution, and the measurement speed is also greatly improved. The off-peak Brillouin interactions will introduce coding noise and degrade the sensing performance of the system, although the application of Golomb codes and temporal gating has effectively improved the SNR. In addition, to obtain a higher spatial resolution, a higher modulation rate is required, but high-performance modulation devices are seldom available, which will further increase the cost and the complexity of the system. The spatial resolution of the broadband-source-based BOCDA system can easily reach millimeter level. However, the ASE-based BOCDA system has poor SNR and the sensing distance is largely limited. In the chaos-based BOCDA, the spatial resolution is theoretically determined by the chaotic bandwidth. The chaos of 10 GHz is easily obtained, and the sensing distance is successfully extended to 10.2 km. However, the main weakness of the current chaos-based BOCDA is the location of a single CP scanned by the variable delay line, which results in a time-consuming system and poor practicability. In general, these BOCDA technologies involved in this paper have made a significant progress and they have been used for static temperature or strain measurement. In modern industry, the demand for distributed measurement of dynamic parameter continues to increase. Domestic and foreign researchers have made some progresses in these aspects, including fast frequency sweeping and slope-assisted method, but the dynamic strain range and vibration frequency still have great potential for development. In summary, the BOCDA will develop in the direction of long sensing distance, high spatial resolution, and high-speed real-time measurement, and has broad application prospects in modern industry and civil structural health monitoring.

    Chinese Journal of Lasers, 2021, 48(1): 0100001      

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