We investigate second-harmonic generation (SHG) from aperiodic optical superlattices in the regime of pump depletion, where the influence of typical fabrication errors which can be introduced by the random -fluctuation of the thickness for each domain in the simulation is considered according to the actual case. It is found that both the SHG conversion efficiencies calculated in undepleted pump approximation (UPA) and an exact -solution decrease when the fluctuation gets larger. However, the decreasing degree is related to the wavelength of the fundamental wave (FW), and the longer the FW wavelength, the lesser the corresponding conversion efficiency reduction. A relative tolerance with respect to SHG conversion efficiency calculated in UPA and exact solution is defined in a previous work, in which a typical model based on the relative tolerance curves is proposed to estimate the SHG conversion efficiency. The simulation results exhibit that the relative tolerance curves are basically coincident with the standard curve when the random fluctuation is very small (typically below 1%). However, as the fluctuation increases, the relative tolerance curves exhibit a large deviation from the standard one, and the deviation is also determined by the wavelength of the FW.

We study the decoding performance of low-density parity-check (LDPC) code based on Watermark scheme under different percentages of water-mark bits over 100 Gb/s differential quadrature phase shift keying (DQPSK) high-speed optical communication system. We also find the optimum percentage of water-mark bits according to the performance comparison. Simulation result shows an improvement of 0.5 dB net code gain by using the optimum Watermark scheme at a post-forward error correction bit error rate (BER) of 10-9, comparing with the traditional log-likelihood ratios belief propagation decoding algorithm. Also, for the same BER, there is a decrease in the number of iterations used in LDPC decoding.

In order to improve the bandwidth and ability to resist electromagnetic interference of phased array antenna, we use the real-time delay technology of optical fiber. By using phase control principle of optically controlled phased array, we deduce the relation formula of time delay and phase control. Based on multiple optical carriers and optical switch delay technology, we analyze design method of optical fiber time delay system and give the results of the experimental test. From the results, we find that the system can improve ability of phased array antenna about phase control and resisting electromagnetic interference in broadband condition.

We investigate optimal twin-hole poling optical fiber configurations, including distances between core and each electrode, and poling voltage based on the two-dimensional charge dynamics model. We propose a poled fiber with optimized .(2) as well as single-polarization property. Small distance between core and anode -guarantees the poled fiber with large .(2) in fiber core and large polarization-dependent loss. A maximum .(2) in the core region either outside or inside nonlinear layer can be realized by appropriately selecting edge-to-edge distance between core and cathode. The maximum .(2) in the core region can be even larger by increasing the poling voltage.

We experimentally demonstrate the multiband orthogonal frequency-division multiplexing ultra-wideband (MB-OFDM UWB) over fiber system with direct detection. Different sub-carrier modulation formats (quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (QAM)) are investigated in the MB-OFDM UWB over fiber system. The experimental results show that a 3.84 Gb/s 16 QAM-encoded MB-OFDM UWB signal can be successfully transmitted over 70 km standard single-mode fiber without chromatic dispersion compensation.

We use fundamental matrix (F-matrix) method derived from coupled wave theory to simplify the diffraction simulation of chirped volume Bragg grating (CVBG) and it can be applied to arbitrary grating phase profiles. With the F-matrix method, we study the diffraction in CVBG. The spectral response of CVBG is a gate-like function, and the passband width of spectral response is related to the product of grating thickness and spatial chirp rate. The peak diffraction efficiency of CVBG increases monotonously as the amplitude of refractive index modulation increases. Incident beams with different wavelengths will be mainly diffracted at different depths of CVBG to match the Bragg condition.

We present a simple method for the subsurface imaging of a nucleated cell, which is realized by measuring the difference in wrapped phase between a nucleated cell and its enucleated cell model. The latter one called as the reference phase can be simulated according to the axial thickness and the cytoplasmic refractive index. We illustrate the proposed method with theoretical analysis and numerical simulation of a binucleated cell, and prove its validity on real biological cells by imaging the HeLa cell based on its experimental phase. It shows that this method is suitable for imaging of relatively simple nucleated cells.

We present a method to extract foreground object regions efficiently from image sequences. Scale-invariant feature transform algorithm is adopted to estimate the descriptor firstly by matching between two consecutive frames. Given local descriptor matching results, dense motion vector of each pixel is calculated by large displacement optical flow with variational optimization, which integrates detailed descriptors into the variational model. Then the foreground object boundaries and regions are detected by computing the optical flow gradient and magnitude. Experiments demonstrate that the method can achieve better segmentation results than alternative methods and adapts well to moving objects in relatively stationary background image sequences.

Phase diversity (PD) is a kind of wavefront sensing technology based on image collecting and post-processing. We apply the PD technology to align an off-axis three-mirror reflecting anastigmatic system precisely. It can be concluded that the wavefront error obtained by PD agrees well with the interferometric result. The focused images are also restored according to the testing results of PD, and the qualities of restored images are improved.

We present an improved compressive sensing algorithm with negative transformation and piecewise-nonlinear transformation. The reconstruction characteristics of the improved algorithm are studied by conducting numerical analysis research. Watch gear and handwritten character are used in the experiments. The results validate the application value of the improved algorithm in improving 2D reconstructed image quality in terahertz (THz) Gabor inline digital holography.

In order to solve the difficulty of testing large mirror, the sub-aperture stitching interferometry (SSI) is proposed and expatiated. The basic theory and principle of this method are introduced and analyzed. A reasonable stitching algorithm and mathematical model are established based on least-squares fitting, triangulation algorithm, homogeneous coordinate transformation, etc., and the relative program and flow chart are established. Some marked points are used to accomplish the alignments between sub-apertures and calibrate the relationship between the coordinate of the mirror and the pixel. With engineering examples, a large rectangular mirror with an irregular aperture of 720×165 (mm) is tested by SSI. The peak-to-valley and root mean square of the stitched surface error are 0.451 λ and 0.042 λ (λ is 632.8 nm), respectively.

Alignment and testing of optical axis parallelism are the key problems in alignment of a multi-optical axis system. We propose a method which can adjust optical axis parallelism and single-channel wavefront aberration simultaneously in multi-axis imager. Firstly, the imager's installation base surface is adjusted to be perpendicular to the optical axis of the interferometer and remain motionless precisely by using theodolite. Then, according to the measurement principle of wave aberration with interferometer, each channel axis is adjusted to be parallel to the axis of the interferometer. While imaging quality in each channel is measured, alignment on each channel axis parallelism is accomplished. By this method, three-axis parallelism in annular three-channel imager is aligned. The accuracy of axis parallelism is up to 15″, and the imager requirement (30″) is satisfied. Feasibility and precision of this method are verified.

We propose a Y-type polarization beam splitter based on internal polarization-selective defects within crystal waveguides in a two-dimensional square-lattice photonic crystal with solid rods. When the nonpolarized light launches from the input port, different polarizations will be separated and can only transmit through their own channel. It is demonstrated by finite element method that the proposed structure can achieve good performance for both the transverse electric and transverse magnetic polarizations in a wide range of wavelength, with the polarization extinction ratio more than 25 dB, the degree of polarization nearly 1, and the insert loss less than 0.5 dB, respectively.

We theoretically study the optimum design of anisotropic acousto-optic modulator (AOM) based on lithium niobate crystal. Four different kinds of operating modes in the XOZ and YOZ acousto-optic (AO) planes are systematically analyzed by the tangent condition theory and the optimized operating mode is determined. Furthermore, the dependence of the AO merit on the operating frequency and the off-axis angle of the AOM are also obtained by numerical simulations.

We build up a common-path optical coherence tomography (OCT) system using reflected light of sample surface as reference light. As the zero path length reference point has nothing to do with the distance between probe and organ, it can be utilized in endoscopic system. Besides, an optical delay stair is used in this common-path OCT to reconstruct the exact morphology of tissue surface, diminishing the distortion caused by sample surface reference.

We present a theory to investigate the existence and the propagation properties of incoherently coupled single-hump and dipole soliton pairs in self-defocusing media with parity-time symmetric lattice. These soliton pairs can exist provided that they are composed of two optical beams with the same polarization and wavelength. It is found that single-hump soliton pairs are always stable when the components copropagate in the lattice, whereas high-power dipole soliton pairs are unstable. If one of the components is absent, the propagation behavior of the other one is also studied.

Based on the general mechanism of the coherent population oscillations, we propose the fundamental -harmonic fractional delay (FHFD) to evaluate the superluminal and slow light propagation in semiconductor optical amplifier (SOA). The sinusoidal and square-wave signals in SOA are investigated with the propagation equations. It is shown that the superluminal and slow light always accompany the signal distortion, and FHFD depends on the signal distortion as well as the incident power, the modulation frequency, and the optical gain.

Aspheric elements are widely applied in optical systems and the vertex radius of curvature (VROC) is one of the important fundamental parameters of an asphere. We present a method for measuring the VROC of asphere. We use a portable laser tracker to measure the optical interval of the null testing path and then determine the VROC of the asphere through ray tracing. Based on this method, we carry out an accurate measurement. The accuracy can reach up to 0.056 mm on an asphere with VROC of approximately 2 m and the relative error is 0.003%.

In order to obtain precise optical free-form convex mirror, we present a perfect process specification for fabricating and testing optical free-form convex mirror. Some technical requirements of 84×84 (mm) optical free-form convex silicon carbide (SiC) mirror are introduced. Firstly, the SiC blank is milled to the best-fitting sphere by means of DMG Ultrasonic 100-5 computer-controlled machine. Secondly, the best-fitting sphere is grinded and polished to optical free-form surface with certain figure accuracy by computer-controlled small tool fabrication. Finally, in order to meet the requirement of design, the optical free-form convex mirror is fabricated by advanced ion beam figuring. The contour testing technique is used for measuring the optical free-form convex mirror in milling and grinding processes, and the computer-generated hologram null testing technique for measuring the optical free-form convex mirror in polishing process is studied. The final testing result indicates that the figure accuracy of the optical free-form convex mirror is 0.02. (root mean square).

Silicon carbide (SiC) is a wide bandgap semiconductor which exhibits outstanding mechanical, chemical properties, and potential for a wide range of applications. Laser technology is being established as an -indispensable powerful tool to induce structural or morphological modifications on hard brittle materials. SiC (6H-SiC wafer) is irradiated by nanosecond pulsed Nd:YAG laser to evaluate microstructure and mechanical properties of irradiation areas. Raman spectroscopy analysis reveals that irradiations produce homonuclear Si-Si bonds and disordered phase of crystalline SiC. Crystal structure changes are observed as a consequence of laser-induced melting and resolidification. Hardness in the irradiation area exhibits a significant decrease. The formation of silicon film facilitates material removal rate, surface electrical conductivity, and ceramics conjunction.

We study a convex off-axis aspheric mirror which works as secondary mirror in space optical system. The parameters of the mirror are described. In order to test the surface error, the mirror is made up of fused silicon and is tested by the backside transmission type. The shape accuracy while grinding is controlled using coordinate measuring machining testing. The distortion of the measurement is corrected by affine transformation. The ion beam figuring is used for surface finishing and to achieve root mean square of 0.015λ (λ = 632.8 nm).

We theoretically study the nonlinear surface wave propagation at the interface between superconductor media and nonlinear metamaterials. The dispersion equation is analytically derived and solved numerically. Moreover, we present the power for the propagating waves at the interface. The results display different behaviors of the propagating waves as the nonlinear term or temperature is tuned. These results indicate that this structure can have potential applications in superconductor waveguide devices and integrated optics.

We describe a compact beam splitter based on grating-assisted coupler which consists of Bragg grating sandwiched between two parallel waveguides on the silicon-on-insulator platform. The coupled-mode theory is an important method to analyze waveguide structure. The coupling effect is affected by the grating refractive index perturbation due to the phase mismatch between two waveguides with different widths and refractive indices. The power difference between the transmitting and the reflecting directions in waveguide A is nearly 0 when the Bragg wavelength is 1.3464 μm, the index perturbation is 0.245, the period of grating is 0.2 μm, and the distance of two waveguides is 1 μm. At this time, cross couple neighbor waveguides are significantly suppressed. Beam splitter based on grating-assisted coupler is very useful in integrated optical circuits and photonic network-on-chip.