Due to the multipath effect and time-varying effect of underwater channel, the underwater radio communication rate and signal-to-noise ratio are very low. In the actual underwater communication application scenario such as underwater sensor network, it is necessary to obtain the code stream signals of multiple signal sources at the same time. An underwater visible multi input multi output(MIMO) communication system based on CMOS camera is proposed. The system samples Manchester code to recover the clock signal of different distance signal sources. At the same time, the method of multiple signal sources identification and light intensity fitting is proposed. The influence of baud rate on the signal waveform distortion in underwater environment is explored. The experimental results show that the system can achieve 21 kb/s underwater communication after the LED array is optimally arranged in a 4 m×3 m×3 m test cell. It can be used in short distance and small information data exchange occasions such as underwater sensor network.

In view of the existing calculation of the attenuation of blue-green laser underwater, the same kind of sea water is estimated by fixed attenuation coefficient, but the actual attenuation coefficient of different sea areas is different, which leads to the problem that some theoretically feasible areas cannot communicate normally. A new method is proposed to calculate the transmission of blue-green laser underwater based on the transparency remote sensing observation data. According to the moderate-resolution imaging spectroradiometer(MODIS) remote sensing data is used to retrieve the attenuation coefficient of blue-green laser in some areas of the Yellow Sea, this paper analyzes the temporal and spatial distribution characteristics of the attenuation coefficient of the Yellow Sea, estimates the transmissivity of 50 m underwater, and evaluates the feasibility of blue-green laser communication. It is concluded that the higher the transparency is, the smaller the attenuation coefficient is, and the higher the transmittance is when the transmission distance is fixed; When the transparency is greater than 5.295 m, the attenuation of the laser at 50 m underwater is less than 40 dB, and the blue-green laser communication can be carried out.

As a kind of channel coding, fountain code is an important technique to solve the interference of atmosphere on free space optical communication. Degree distribution is a key factor affecting the performance of fountain code. In the light of the degree distribution of fountain code, robust soliton distribution and fixed degree distribution is firstly analyzed from the principle and the deficiency on the performance, after that, the fixed degree distribution function is improved, then a new joint degree distribution design is proposed to combine the moved robust soliton distribution with the improved fixed degree distribution by a proportion ratio. The simulation results show that the decoding performance of the joint degree distribution proposed in this paper is much higher than the robust soliton distribution and fixedness distribution.

Dynamic range and spatial resolution are the key indexes of optical time domain reflectometry(OTDR) system. On the basis of expounding the relationship between OTDR dynamic range and spatial resolution, this paper summarizes the new methods of improving OTDR dynamic range and spatial resolution at present, including pulse code modulation, OTDR signal processing algorithm, linear optical sampling method and chaotic optical time domain reflection method, introduces the research progress of these methods at home and abroad, and the future of improving OTDR dynamic range and spatial resolution is also prospected.

The inherent cross sensitivity of optical fiber sensor is always an urgent problem to be solved in the application of optical fiber sensing technology. A multi-objective particle swarm optimization(MPSO) demodulation algorithm is proposed. The demodulation of two fiber Bragg grating(FBG) central wavelengths in the same and different sensing systems is discussed respectively, and the influence of different reflectivity on the demodulation results is also discussed. This method not only has high detection accuracy, the temperature measurement accuracy is 0.1 ℃, the strain accuracy is 0.5 με, but also the system structure is simple and easy to realize, which has no strict restrictions on the material, structure, central wavelength and reflectivity of FBG.

In order to explore a new real-time demodulation method of impact energy for fiber Bragg grating(FBG) sensor, the impact energy of sheet metal is measured by FBG edge filtering method. For the impact process of low quality sphere acting on aluminium plate, the right edge moving filter demodulation method of FBG spectrum is used in the experiment. For the impact process of high quality sphere acting on acrylic(PMMA) and aluminium plate, a new demodulation method based on damping attenuation principle is proposed. The experiment results show that the peak value can be directly used as the demodulation basis for a weak impact energy signal, and its goodness of fit is 0.988, sensitivity is 0.0007476 v/g. For a stronger impact energy, the linear fitness of PMMA plate and aluminium plate is 0.94 and 0.89 respectively when using the time length corresponding to the peak attenuation of 10% as the demodulation.

In order to reduce the error caused by temperature in the magnetic field measurement system based on magnetic fluid (MF) and long period fiber grating(LPFG), a magnetic field measurement correction scheme based on MF and banded twin-core optical fiber is proposed. The LPFG with different cladding thermo-optic coefficients is integrated in the ribbon twin-core optical fiber, and the difference between the temperature sensitivity is used to correct the magnetic field measuring results. Through theoretical analysis and system simulation, the temperature characteristics of MF and LPFG are demonstrated, and the relationship between MF refractive index and LPFG resonant wavelength in a certain temperature range is revealed. Compared with the system without temperature compensation, the proposed scheme's intensity to accuracy is increased by at least 79.1%.

The main problem oil and gas leakage in pipeline detection is that the pipeline distance is too long, and the effect of traditional detection mode is not good. A detection method of oil and gas leakage in pipeline using conjugate gradient echo state network(ESN) based on optical fiber technology is proposed. Firstly, the real-time data of long-distance oil and gas pipeline is monitored and collected by optical fiber detection technology, and the dimension compression method of unmarked data is used to reduce the dimension and simplify the data by combining the data series method and rough set algorithm. Secondly, ESN is selected to analyze and process the dimension reduced oil and gas pipeline leakage data. The weight of ESN is optimized preliminarily, and then the preliminary weight results are further optimized by using the semi positive qualitative of Hessian matrix to realize the recognition ability of ESN to the leakage data of oil and gas pipelines which is based on regularization. Finally, through the simulation of the measured data, the proposed leakage monitoring model of oil and gas pipeline has higher recognition accuracy and calculation efficiency.

The phase-sensitive optical time domain reflectometer(Φ-OTDR) system uses the amplitude differential accumulation method to count the position of the vibration signal. There exists the problem of low recognition rate and need to be modified the N for different frequencies. An algorithm combining empirical mode decomposition(EMD) and neural network is proposed. The EMD is used to denoise and analyze the time domain signal at the vibration point, the signal characteristics are extracted by EMD and input to an improved neural network as features for training. The trained neural network is then employed to identify different vibration signals at different frequencies. The experiment shows that the recognition rate of vibration signal is 96.49%.

In order to obtain the strain information quickly and accurately, several methods for calculating the central wavelength variation of fiber Bragg grating(FBG) spectrum signals are studied and analyzed based on the fiber sensing application system of Fabry Perot(F-P) filter. According to the symmetrical characteristics of the power distribution waveform of the FBG reflection spectrum, an algorithm for optimizing the central wavelength value is proposed. Based on the two side data of the peak value of the two adjacent sampling spectral waveforms, the alternative value of the central wavelength is obtained by the optimization calculation according to the principle of the least square method. The experimental results show that the variation of the central wavelength of FBG spectrum calculated by this method is not more than one sampling point.

The spectrum allocation in elastic optical network(EON) needs to meet the spectrum consistency and continuity constraints of subcarriers at the same time. The spectrum fragments of different sizes will be generated after the allocation or release of services with different bandwidth requirements, which will affect the allocation of subsequent services and the utilization of spectrum resources. An improved EON defragmentation algorithm based on resource saving strategy(RSS) is proposed. In the process of defragmentation, the number of continuously available frequency gaps and the number of frequency gaps needed for arriving services can be as equal as possible to avoid the generation of new spectrum fragments. Theoretical analysis and simulation results show that in 100~ 600 Erl, compared with the traditional defragmentation algorithm, the improved algorithm can reduce the bandwidth blocking rate by 8.54%.

In order to improve the precision of demodulation phase of coupler in optical fiber pickup system, a 3×3 coupler phase demodulation algorithm is proposed in this paper. Based on the demodulation of 3×3 coupler, this algorithm firstly compensates the direct current component of two interference signals by iterative operation, and then demodulates the audio signal. The experimental results of audio measurement show that this algorithm can effectively restore audio signal and it is practical.

In the actual digital baseband communication system, in order to transmit information smoothly in the baseband, non-return to zero(NRZ) signal must be coded into code elements suitable for baseband transmission. Because the sender does not send clock information when sending, but in order to ensure the synchronization of the signal between the sender and the sender, it is necessary to extract clock information from the information stream at the receiver to recover the data. A novel encoding and decoding circuit for third-order high density bipolar code(HDB3) is proposed. The principle of all digital phase-locked loop(PLL) is used at the receiver. The decimal frequency divider is used to design a digital controlled oscillator(DCO) with loop filtering characteristics. Experiments show that the new HDB3 codec circuit can successfully recover timing information and data in the input signal frequency range of 2.048 MHz(±70 ppm).

In view of the problem that the testing optical path of the Brillouin optical time domain analyzer(BOTDA) is prone to failure in long-term use, which leads to the interruption of BOTDA testing, an optical path automatic switching protection system based on the principle of optical time domain reflectometer(OTDR) is designed. The system takes K60 controller as the control core, uses the FLEXBUS interface to mount SRAM, FPGA and W5300, integrates the OTDR module of K60+FPGA architecture, realizes the optical path switching through the optical switch, monitors the failure of each optical path, and can switch the optical path and send the short message alarm when the optical path fails in the BOTDA test; introduces the system principle, main hardware modules, ethernet guidance and upgrading and optical path self dynamic switch protection software process. The experimental results show that the OTDR dynamic range is more than 35 dB, which meets the monitoring requirements of BOTDA test optical path.