Active constellation expansion (ACE) and iterative clipping and filtering (ICF) are simple and effective techniques for reducing the peak-to-average ratio (PAPR) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems, but effective PAPR suppression requires a lot of iterations. To overcome this shortcoming, a joint algorithm based on improved active constellation expansion (IACE) and ICF (IACE-ICF) is proposed. The simulation results show that at the complementary cumulative distribution function (CCDF) of 10^-4, the PAPR of IACE-ICF (G=4, iter=4) algorithm is optimized by 1.507 dB, 1.13 dB and 0.204 dB compared with that of the IACE, ICF (iter=4) and ICF-IACE (G=4, iter=4) algorithms, respectively. Meanwhile, when the bit error rate (BER) is 10^-3, the optical signal to noise ratio (OSNR) of the proposed scheme is optimized by 2.04 dB, 1.75 dB and 1.4 dB compared with that of clipping, ICF (iter=4) and ICF-IACE (G=4, iter=4) algorithms, respectively. On the other hand, the proposed scheme can reduce the number of complex multiplications by 14.29% and complex additions by 28.57% compared with the ICF (iter=14) scheme.

A Mach-Zehnder interferometer (MZI) based on two spherical structures is proposed and temperature and humidity are measured simultaneously. The device is fabricated by inserting two spherical structures into a single mode optical fiber (SMF). The results of the experiment indicate that the temperature sensitivities are 0.079 nm/oC and 0.090 nm/oC from 10 oC to 60 oC, respectively. When the humidity changes from 30% to 70%, the humidity sensitivities are 0.148 nm/%RH and 0.06 nm/%RH, respectively. Therefore, temperature and humidity are measured simultaneously by the sensitive matrix. The new structure is demonstrated to be a particularly useful approach to detect temperature and humidity.

An in-line Mach-Zehnder interferometer (MZI) for simultaneous measurement of temperature and refractive index (RI) is proposed and demonstrated. The sensor is composed of cleaved taper, single-mode fiber (SMF) and spherical structure. Using precision device to measure the position of waist, the cleaved taper structure is obtained by cutting the taper structure. The sensitivities of the temperature are 0.052 nm/℃ and 0.037 nm/℃ in the temperature range of 25—70 ℃, respectively. The RI sensitivities are -56.59 nm/RIU and -43.53 nm/RIU in the RI range of 1.335—1.38, respectively. This sensor has many advantages such as compact structure and good stability.

A single-mode-few-mode-thin-core-single-mode (SFTS) structure based optical fiber sensor is fabricated and experi-mentally studied. The sensing principle relies on the inter-modal interference. Since the core diameter of few-mode fi-ber (FMF) is larger than that of single-mode fiber (SMF), the FMF helps to allow more light to enter the cladding of thin-core fiber (TCF), which helps TCF to excite cladding modes. The interference between core and cladding modes in TCF occurs at the joint of lead-out SMF and TCF. Experimental results demonstrate a refractive index (RI) sensitiv-ity of .103.34 nm/RIU and a temperature sensitivity of 0.05 nm/℃. The proposed sensor not only can measure tem-perature, but also can measure RI. In addition, the proposed sensor is simple for without complicated fabrication proc-ess.

A widely tunable microwave photonic notch filter with adjustable bandwidth based on multi-wavelength fiber laser is proposed and demonstrated. The multi-wavelength fiber laser generates the multi-taps of the microwave photonic filter (MPF). In order to obtain notch frequency response, a Fourier-domain optical processor (FD-OP) is introduced to control the amplitude and phase of the optical carrier and phase modulation sidebands. By adjusting the polarization controller (PC), different numbers of taps are got, such as 6, 8, 10 and 12. And the wavelength spacing of the multi-wavelength laser is 0.4 nm. The bandwidth of the notch filter is changed by adjusting the number of taps and the corresponding bandwidths are 4.41 GHz, 3.30 GHz, 2.64 GHz and 2.19 GHz, respectively. With the additional phase shift introduced by FD-OP, the notch position is continuously tuned in the whole free spectral range (FSR) of 27.94 GHz. The center frequency of the notch filter can be continuously tuned from 13.97 GHz to 41.91 GHz.

An all fiber magnetic field sensor with peanut-shape structure based on multimode fiber (MMF) is proposed and experimentally demonstrated. The sensing structure and magnetic fluid (MF) are both encapsulated in capillary, and the effective refractive index of MF is affected by surrounding magnetic field strength. The measurement of magnetic field is realized by observing the wavelength drift of interference peak. The transmission spectrum generated by Mach-Zehnder interferometer (MZI) includes core-core mode interference and core-cladding mode interference. Experimental results demonstrate that the core-cladding mode interference is sensitive to magnetic field, and the magnetic field sensitivity is 0.047 8 nm/mT. In addition, two kinds of interference dips are sensitive to temperature, and the sensitivities are 0.060 0 nm/°C and 0.052 6 nm/°C, respectively. So the simultaneous measurement of magnetic field strength and temperature can be achieved based on sensitivity matrix.

A magnetic field sensor with a magnetic fluid (MF)-coated intermodal interferometer is proposed and experimentally demonstrated. The interferometer is formed by sandwiching a segment of single mode fiber (SMF) between a segment of multi-mode fiber (MMF) and a spherical structure. It can be considered as a cascade of the traditional SMF-MMF-SMF structure and MMF-SMF-sphere structure. The transmission spectral characteristics change with the variation of applied magnetic field. The experimental results exhibit that the magnetic field sensitivities for wavelength and transmission loss are 0.047 nm/mT and 0.215 dB/mT for the interference dip around 1 535.36 nm. For the interference dip around 1548.41nm, the sensitivities are 0.077 nm/mT and 0.243 dB/mT. Simultaneous measurement can be realized according to the different spectral responses.

A continuously tunable microwave photonic notch filter with complex coefficient based on phase modulation is proposed and demonstrated. The complex coefficient is generated using a Fourier-domain optical processor (FD-OP) to control the amplitude and phase of the optical carrier and radio-frequency (RF) phase modulation sidebands. By controlling the FD-OP, the frequency response of the filter can be tuned in the full free spectral range (FSR) without changing the shape and the FSR of the frequency response. The results show that the center frequency of the notch filter can be continuously tuned from 17.582 GHz to 29.311 GHz with FSR of 11.729 GHz. The shape of the frequency response keeps unchanged when the phase is tuned.

A tunable erbium-doped fiber (EDF) laser with a cascaded structure consisting of in-line Mach-Zehnder interferometer (MZI) and traditional MZI is proposed. The transmission spectrum of the in-line MZI is modulated by the traditional MZI, which determines the period of the cascaded structure, while the in-line MZI’s transmission spectrum is the outer envelope curve of the cascaded structure’s transmission spectrum. A stable single-wavelength lasing operation is obtained at 1 527.14 nm. The linewidth is less than 0.07 nm with a side-mode suppression ratio (SMSR) over 48 dB. Fixing the in-line MZI structure on a furnace, when the temperature changes from 30 °C to 230 °C, the central wavelength can be tuned within the range of 12.4 nm.

A microwave photonic notch filter with a complex coefficient is proposed and demonstrated based on four wave mixing (FWM). FWM effect of two single-frequency laser beams occurs in a highly nonlinear fiber (HNLF), and multi-wavelength optical signals are generated and used to generate the multi-tap of microwave photonic filter (MPF). The complex coefficient is generated by using a Fourier-domain optical processor (FD-OP) to control the amplitude and phase of the optical carrier and phase modulation sidebands. The results show that this filter can be changed from bandpass filter to notch filter by controlling the FD-OP. The center frequency of the notch filter can be continuously tuned from 5.853 GHz to 29.311 GHz with free spectral range (FSR) of 11.729 GHz. The shape of the frequency response keeps unchanged when the phase is tuned.