Research On Optical Fiber Vibration Sensing System Based On Multi-longitudinal Mode Laser Beat Frequency Technology

With the change of the times and the development of science and technology,the theory of vibration sensing technology is constantly improving.New vibration sensing systems based on high sensitivity,high precision,high integration and high stability are continuously being researched and produced.Due to its simple structure,excellent anti-interference ability,extremely high sensitivity and low cost,optical fiber vibration sensing system has entered a period of high-speed development.It has been widely used in seafood,air,industrial,medical and other fields.The key technique of the existing optical fiber vibration sensing system demodulation is the analysis of the wavelength or phase changing caused by the vibration signal,which has high sensitivity and anti-interference,but there is still a lot inadequate.For example,the changing amplitude of the wavelength or the phase caused by the vibration signal at low frequency or with weak energy is small,so that it hard to achieve the high signal-to-noise ratio;the optical signal detection device is expensive and carrying it is not convenient;the optical demodulation is also difficult,etc.Therefore,developing low-cost,high-performance and portable vibration sensing systems has become a hot spot in current research.The main research contents of this paper include:1.The principle and structure of three typical optical fiber sensing systems based on the multi-longitudinal mode beat frequency sensing technology are analyzed.An experimental device is set up to realize the demodulation of single static physical quantity: temperature and stress.The influence of the frequency of the selected beat signal on the experimental demodulation results is also analyzed.2.An optical fiber vibration sensing system based on multi-longitudinal mode beat frequency sensing technology is proposed.The system uses the general software radio peripheral and the principle of frequency modulation demodulation to complete the demodulation of vibration signal frequency.The beat signal generated in the optical fiber laser cavity is modulated by the vibration signal,and the beat signal as the carrier is removed by the general software radio peripheral,only leave the demodulated vibration signal.This is a digital sensing system.Compared with the analog sensing system,the digital sensing system can greatly enhance the intensity of the vibration signal obtained by demodulation by using the frequency conversion superposition technology.Therefore,the digital sensing system can detect the vibration signal with low frequency or weak energy.The operation of the system is simple and has better visualization effect.3.An optic fiber vibration acceleration sensing system for demodulating the acceleration of vibration signals is proposed.On the basis of optical fiber vibration sensing system used to detect the frequency of vibration signal,the optical fiber grating is used to replace the faraday mirror as the sensor probe,and the sensitivity of the sensor probe is improved.A filter is added between the photodetector and the universal software radio peripheral to filter out redundant beat signals in order to reduce interference.After the vibration signal is obtained,the vibration signal is processed in the computer and its acceleration is obtained.The frequency and acceleration of vibration signals at different time points are also demodulated by windowing technique.4.A distributed optical fiber vibration sensing system based on frequency division multiplexing technology is proposed.By using coupler makes a number of sensor probes can exist in a sensor system at the same time,through the general software radio peripheral demodulation of the vibration signals of different parameters applied to different sensor probes at the same time,the parameters of the vibration signal and the position of the sensing probe applied can be accurately obtained.