In order to fully understand the research status of cascaded double fiber Bragg grating(CFG) sensor, from the measurement of temperature, strain and refractive index sensor application, the cascaded double Fiber Bragg grating(FBG), cascaded double long period fiber grating(LPG) and cascaded LPG+FBG three CFG sensors are introduced and compared in detail. Summarized the technical advantages and difficulties of CFG sensor at the present stage, finally, the development of CFG sensor technology is prospected.

The phase-sensitive optical time domain reflectometer (Φ-OTDR) has been widely used in anti-intrusion, crack health detection of building structure and pipeline detection, but the accuracy and anti-noise still need to be improved. In this paper, the Φ-OTDR intelligent recognition system is proposed. Through intelligent identification by neural network algorithm and the vibration intensity is identification with probability processing. The experimental results show that the proposed system can accurately identify and locate the intrusion behavior, and the positioning accuracy is within 5 m

Aiming at the high cost and inconvenient operation of the wavelength demodulation fiber Bragg grating(FBG) anemometer system, an intensity interrogated optical fiber thermal anemometer based on the chirp effect of FBG is proposed. After using a pump laser to heat the 15 mm FBG non-uniformly, it produces a noticeable chirp effect. Under the airflow, the chirp effect of the chirped FBG weakens as its temperature gradient decreases, resulting in reflected optical power decline. Based on this principle, the FBG wind speed measurement with intensity demodulation is successfully realized. The experimental results show that the measurement range of the anemometer is 0~11 m/s, and the sensitivity is up to -28.60 μW/(m·s-1) when the wind speed is 0.1 m/s. In addition, the dynamic response time is less than 1 s (about 0.3 s for low to high wind speed, and 0.9 s for high to low wind speed).

With the development of smart grid technology, the demand for current parameter monitoring is increasing. This paper proposes a fiber Bragg grating(FBG) current sensor based on magnetostrictive effect. The FBG is completely fixed on the magnetostrictive material, so that it can sense the magnetic field generated by the energized solenoid to achieve current sensing. By monitoring the wavelength change of FBG, it can be seen that the FBG current sensor can achieve the maximum Bragg wavelength drift of 0.773 nm in the current range of 0~5 A, and the sensing sensitivity is as high as 0.184 nm/A.

Aiming at the problems that the backscattered light of the temperature signal is weak, easy to be submerged under the system noise, and the overall cost of the system is high and the volume is large in the distributed temperature sensing(DTS) based on Raman scattering, an embedded DTS using STM32F407 module is proposed. The cumulative average algorithm, wavelet threshold noise reduction algorithm and polynomial interpolation algorithm are used in the system to reduce the influence of noise, improve the signal-to-noise ratio of the system and eliminate the dispersion effect of light. The experimental results show that the overall temperature demodulation fluctuation range of the system decreases from 3.3~5.9 ℃ to 0.3~0.8 ℃; in a multimode fiber about 5 km long, the temperature is 29 ℃, 45 ℃, 60 ℃ and 80 ℃ respectively, the demodulation temperature fluctuation range of the system is within ±1 ℃, and the data processing time of the system is about 60 ms.*通信作者: 杨军(1982-), 男, 工学硕士, 高级工程师, 主要研究方向为光纤传感技术。

In order to analyze the influence of related factors of demodulation error in Brillouin optical time-domain reflectometry/Brillouin optical time domain analysis(BOTDR/BOTDA), firstly, the noisy Brillouin spectra with different parameter values is simulated generated based on the Lorentzian profile. Then the Brillouin frequency shift error in the different conditions is calculated by use of the least-squares fitting method based on the Lorentzian profile. The results reveal that when the number of sweeps is fixed, the frequency scanning range is not recommended to be too large or too small, and the recommended frequency sweep range is 1.2ΔvB. When the frequency sweep range is constant, the Brillouin frequency shift error decreases exponentially with increasing number of frequency sweep. The Brillouin frequency shift error decreases exponentially with increasing signal-to-noise ratio. When the ratio of frequency sweep range to linewidth and the number of frequency sweep are constant, the Brillouin frequency shift error increases approximately in direct proportion with the increase of Brillouin linewidth.

In order to provide a theoretical basis for the measurement deviation of reflective all fiber current transformer(optical CT), the influence mechanism of the main optical components of reflective optical CT system on the system response is studied, and the influence of the non ideality of the main optical components of optical CT on the accuracy of current measurement is analyzed. Firstly, the main devices and mathematical models of the system are described by Jones matrix, and the expression of the output light of the system and the measured value of the injection current of the system after demodulation and detection are given. Then, based on the system mathematical model, the effects of the linear birefringence and circular birefringence of the sensing fiber and the linear birefringence of the polarization maintaining fiber on the injection current measurement of optical CT are analyzed respectively. The results show that the linear birefringence deviation of sensing fiber has the most prominent influence on the measurement accuracy of optical CT injection current. The circular birefringence of sensing fiber can effectively suppress the influence of linear birefringence of sensing fiber on the measurement accuracy of optical CT injection current.

In order to eliminate the influence of light intensity disturbance and phase modulation depth on the phase generated carrier(PGC) demodulation, this paper proposes a phase demodulation algorithm based on mixing and filtering and phase division differential which is called PGC demodulation improved algorithm. The algorithm makes the demodulated signal without any variables by filtering and differential division and differential and integration operation. PGC demodulation improved algorithm is theoretically analyzed and deduced, and the simulation model is constructed by using Labview platform to verify the advantages of the proposed algorithm over traditional algorithm. A demodulation system based on Michelson interference is built for experimental verification. The experimental results show that the signal to be measured demodulated by the improved PGC demodulation algorithm will not produce harmonic distortion under different phase modulation depth. In the case of different light intensities, the demodulated signal amplitude remains the same.

In order to improve the performance of strain measurement in complex electromagnetic environment, a novel strain measurement method is proposed based on optoelectronic oscillator(OEO) and high-birefringence(Hi-Bi) fiber. The strain will cause the change of phase difference between the two orthogonal polarized lights transmitted on the fast and slow axes of the Hi-Bi fiber, By properly controlling the polarization state of the light wave sent into the Mach-Zehnder modulator, the phase change is mapped to the frequency change of the OEO oscillating microwave signal. Therefore, the demodulation of strain is achieved by monitoring the frequency change. Theoretical analysis and experiment results show that the frequency of the oscillating microwave signal generated by OEO has a linear relationship with strain, and sensitivity of the proposed measurement method is -369.3 Hz/με.

In order to achive three-dimensional seismic wave, a three component fiber Bragg grating(FBG) geophone is designed, and the natural frequency and sensitivity of the seismometer are theoretically deduced. Matlab software and COMSOL software are used to optimize the structure parameters and analyze the modal of the seismometer. The amplitude-frequency response, transverse anti-interference performance and sensitivity response of the geophone are tested. The experimental results show that the flat region of geophone is 0.6~50 Hz. The natural frequencies of the first, second and third order are 56 Hz, 59 Hz and 63 Hz, and the sensitivities in x, y and z directions are 218.22 pm/g, 284.76 pm/g and 249.67 pm/g, respectively. The transverse anti-interference degree in each direction is less than 5%, and the three component detection of low-frequency signal can be realized.

Aiming at the problem that the residual strain caused by steel structure welding is difficult to calculate accurately theo- retically， optical fiber calibration test and I-steel welding test are carried out based on optical frequency domain reflectometer(OFDR) technology， and the application of this technology in steel structure welding residual strain monitoring is studied. The test and analysis results show that OFDR technology can accurately monitor the welding residual strain and the location of strain can be precisely located.

In order to improve agricultural production efficiency, a method of intelligent agricultural internet of things to improve agricultural production is proposed from the perspective of intelligent production. Intelligent agriculture IoT adopted the three key technologies: sensor technology, power line carrier communication technology and computer network communication technology, the first two technologies are used to build a network node, and realize the remote monitoring. Computer network communication technology is used to transmit real-time data through network communication protocols in computer networks. Tests show that intelligent agricultural internet of things can record agricultural growth status in real time, remotely operate production equipment, accurately transmit real-time data, improve work efficiency and reduce production costs.

Metasurface has excellent performance of manipulating electromagnetic waves in the sub-wavelength thickness range. With the development of tuning technology and the need for miniaturization and integration of devices, the design of metasurface has changed from single-function structure to multi-function integration, realizing the switch of specific functions under different excitation factors. Based on several common tuning methods, such as external electric field, thermal control, mechanical stretching, wavelength and polarization manipulation, this paper introduces a variety of tunable metasurfaces with dual and three functions. Finally, the prospect of further research and development of the tunable multi-functional metasurface is suggested.Sb2Se4Te1)的可调谐多功能全介质手征超表面。

In order to meet the requirements of passive heat dissipation of laser source in aerospace applications, project of heat dissipation solution of high power devices in laser source is proposed. The heat conduction channel from device and printed circuit board to metal structure is designed, which can transfer heat from devices to metal structure efficiently, ensure that the device temperature is within the normal operating range. Efficiency of heat dissipation can be promoted by ways of evolving structure and material of laser soucrce, and by ways of adding or evolving dissipation passages. Simulation results show that, when working in high temperature circumstance, In the heat dissipation optimized scheme, the maximum temperature of chip and laser is reduced by about 100 ℃ and 1 ℃ respectively, the optimized project is effective and viable.

In order to reduce the interference and multipath effect of obstacle reflection on visible light communication in the transmission channel. Firstly, asymmetrically clipped optical orthogonal frequency division multiplexing(ACO-OFDM) simulation model and channel model are built in this paper. the simulation results show that the ACO-OFDM modulation based on quadrature amplitude modulation(16QAM) has better anti-noise performance in both direct link and reflection link. the coding method of concatenated BCH and convolutional code is added to ACO-OFDM. The simulation results show that the ACO-OFDM system using cascade BCH and convolutional code has higher anti-noise performance and better anti-jamming performance, and improves the performance of visible light communication.

In order to to improve the reliability of visible light communication(VLC) links and address the limitations of the traditional VLC research paradigm, commercial LUXEON Rebel non-Lambert light beams are introduced into multiple input multiple output(MIMO) visible light emitter configurations in typical indoor applications. The bit error rate(BER) performance of non-Lambert MIMO VLC and Lambert MIMO VLC is compared and analyzed. The simulation results show that the proposed non-Lambert MIMO configuration can reduce the introduced transmission power by 13.1% compared to the traditional Lambert beam MIMO reference configuration when the center of the room receiving position BER≤10-6.

In order to study the influence of atmospheric turbulence light transmission, according to the theory of atmospheric turbulent wavefront aberrations and the basis of the optical imaging principle, Zernike polynomial expansion method is used to simulate the phase and intensity of atmospheric turbulence distortion, the numerical simulation value is consistent with the residual error of the theoretical value. Different input parameters are set to simulate the distorted phase diagram, light intensity diagram and Zernike coefficient under different atmospheric turbulence intensities, and relevant tests are carried out in the self-adaptive optics system based on deep learning, and good correction effects are obtained.

Controlling the fluctuation of bare fiber diameter in the process of optical fiber drawing is an important factor affecting the quality of optical fiber drawing. The data model of the optical fiber drawing process is given, the factors affecting the bare fiber fluctuation in the optical fiber drawing are analyzed, the whole process of optical fiber drawing is introduced, the sampling data of the bare fiber diameter fluctuation by the temperature field and gas flow field in the furnace are tested, and the corresponding experiments are designed according to the furnace structure, gas matching and flow rate. The experimental data show that by changing the structure of heating furnace and drawing tower and the flow rate and configuration of inert gas, the disturbance of drawing manufacturing process can be reduced and the control accuracy of bare fiber diameter fluctuation can be improved. The fluctuation range of fiber diameter is from ±1 μm increased to ±0.5 μm

In order to demonstrate that linearly chirped fiber gratings can be an important development direction of dispersion compensation technology, a coupling mode theoretical model of fiber gratings is introduced in this paper, and a linear chirped fiber gratings with self-constructed apodized functions is proposed for dispersion compensation. The reflection spectrum and delay characteristics of linearly chirped fiber gratings with Gaussian apodized functionsused for dispersion compensation and self-constructed apodized functions are simulated numerically. The simulation result shows that the effect of linearly chirped fiber grating with self-constructed apodized functions is better for dispersion compensation.

Aiming at the problem that the channel gain of the multi-channel radio frequency optical transmission module based on the wavelength division multiplexing technology is small, the channel gain fluctuates greatly due to the spurious signals in the bandwidth generated by the four-wave mixing effect and the wavelength drift at high and low temperature. Based on the principle of frequency mixing and wavelength tuning of the optical transmitter, a solution to the gain fluctuation and in-band spurious of the multi-channel radio frequency optical transmission module is designed. The optimization algorithm of wavelength combina-tion is used to calculate a set of wavelength values that meet the requirements, and the optical transmitter is tuned by temperature. wavelength, and experimentally verified using a tuning system based on tuned wavelengths. The experimental results show that the scheme can debug the spurious signal outside the working bandwidth, and the full temperature gain change of the same channel and the same frequency point is less than 2 dB.