Research On Multi-Longitudinal Mode Beat Frequency Optical Fiber Sensing System

With the rapid development of Internet of Things technology,optical fiber sensing technology plays an increasingly important role in information network and information sensing,has been paid attention to by researchers in recent years.Optical fiber sensor has been widely studied and applied in scientific research and industry due to its advantages of simple structure,good security and high sensitivity.It plays an important role in the development of medical diagnosis,military engineering,environmental monitoring and urban intelligence.Compared with traditional fiber sensors,fiber grating sensors can achieve large-scale reuse and have certain advantages in practical applications.Because fiber Bragg grating sensors measure physical parameters through wavelength changes,their demodulation requires expensive wavelength demodulation instrument,which is not convenient to carry and difficult to demodulate in optics.Therefore,it is of great practical significance to study a low-cost,easy demodulation and high sensitivity new fiber laser shooting frequency sensing system.The main research contents of this paper include:1.Firstly,the working principle of fiber laser sensor is introduced.Taking distributed Bragg reflection（DBR）sensor as an example,the generation principle of beat frequency signal and how to use the frequency shift of beat frequency signal to detect the change of relevant physical quantity are described in detail.Then,several common peak-finding algorithms are introduced synthetically.Based on the half-peak method,the center point method of rectangular frame is proposed,which can improve the sensor accuracy of the system.2.A new fiber laser beat frequency temperature sensing system is proposed.The fiber Bragg grating（FBG）in DBR resonator is used as the sensing element.When the wavelength of FBG changes due to temperature,the laser wavelength in the resonator will change,reducing the sensing volume and realizing high sensitivity sensing.The platinum resistance thermometer and real-time spectrum analyzer are controlled by Python program,the beat frequency signal modulated with temperature information can be acquired automatically at the second level.Using the center point position method of rectangular frame to process the beat frequency signal automatically collected by Python program can effectively avoid the peak jitter of beat frequency signal and improve the demodulation accuracy.The average sensitivity of the system is 74.039 k Hz/℃.According to the highest accuracy of real-time Spectrum analyzer（RSA）,the measurement accuracy of the sensor is 0.47×10^-3℃through sensitivity calculation,which has good sensitivity and measurement accuracy.At the same time,compared with traditional optical demodulation,the proposed system uses mature electrical demodulation technology to reduce the cost of understanding demodulation.3.Based on frequency division multiplexing technology,a distributed fiber laser beat frequency temperature sensing system is proposed.Two fiber laser resonators can exist in one sensing system by means of coupler,two temperature sensing channels can be realized simultaneously.Because the temperature measurement is achieved by FBG in the resonant cavity,the sensitivity of the proposed system is high.The platinum resistance thermometer and real-time spectrum analyzer are controlled by Python program,the beat frequency signal modulated with temperature information in the multiplexing system can be acquired automatically at the second level.In addition,temperature changes were measured when the exogenous chemical aflatoxin B1（AFB1）was added to the solution of human hepatoellular carcinoma cell line（Hep G2）using the system.In addition to the advantages of simple structure,low design cost and high sensitivity,the ability of the system to two-channel temperature second measurement is of great value in practical applications.4.A fiber optic laser vibration and temperature sensing system is proposed.By monitoring the time-frequency signal of polarimetric mode beat frequency and frequency shift signal of multi-longitudinal mode beat frequency,the vibration and temperature are measured with two parameters.When the vibration signal is demodulated,variational mode decomposition（VMD）algorithm is used to de-noise the time-frequency signal monitored by polarization mode beat frequency,then the corresponding vibration frequency is obtained by fast Fourier transform（FFT）of the de-noised signal.The system realizes low frequency vibration detection of 0.25 Hz.In addition,the sensitivity of temperature sensor using multi-longitudinal mode beat frequency signal is 9.996 k Hz/℃.The measurement results show that the dual-parameter digital sensing system has good low frequency vibration sensing ability,simple structure,high signal-to-noise ratio,good stability of dual-parameter sensing and no mutual interference.It has a certain competitiveness in the field of actual structural health monitoring and medical detection.