Research On FBG Quasi-Distributed Sensing System Based On Tunable Laser

Due to the wavelength modulation and sensing multiplexing characteristics of fiber Bragg gratings(FBG),it plays an increasingly important role in the sensing field.In the FBG sensing system,as an optical carrier signal varying external parameters,which is essential modulation and demodulation techniques.With the development of optical fiber communication and sensing technology,the system has more and more demand for tunable lasers with wide tuning range,stable performance and narrow line width.In this paper,the development status of fiber laser and fiber grating is introduced firstly,and the theoretical basis of tunable laser and FBG sensing is analyzed.Then a tunable laser based on FFP-TF2 filter is designed.Finally,based on the tunable laser,the FBG quasi-distributed sensing system is built.By calibrating the output wavelength and driving voltage of the tunable laser,the FBG reflected signal is demodulated to realize the FBG addressing.The tunable laser output wavelength is regulated by the drive voltage of the FFP-TF2.The driving voltage of the FFP-TF2 is digitally converted by a function signal generator based on a direct digital frequency synthesis(DDS)design,and then digital-to-analog converted by the DAC module,and finally amplified by an amplifying circuit.Among them,the control program is written in Verilog HDL language,and the hardware design is implemented on the FPGA development board.According to the schematic diagram of the designed tunable laser,the experimental system was built by using related devices,and the output wavelength,output power and wavelength tuning of the system were tested and analyzed.Experiments show that:(1)When FFP-TF2 is at a constant temperature of 25 °C,there is a good linear relationship between the output wavelength of the tunable laser and the driving voltage.It can realize tuning in the 1532~1567nm band,the wavelength change rate is 3.994nm/V,and the 3dB linewidth of the output wavelength is basically maintained at about 0.01nm;(2)The tunable laser can realize two modes of single-wavelength and continuous-scanning wavelength laser output switching,and the operation is simple by the button control on the FPGA development board;(3)The tunable laser output power becomes larger as the pump power increases,and there is agood linear relationship.As the output ratio of the coupler decreases,the system output power threshold gradually increases,and the system output power increases with the increase rate of the pump power;(4)The wavelength output characteristics of FFP-TF2 are affected by temperature.Within a certain range,with the increase of temperature,the center wavelength of the system output decreases at 0V.The FFP-TF2 must be thermostated during the experiment.Finally,the FBG quasi-distributed sensing addressing scheme based on tunable laser is theoretically analyzed,and the experimental system is used to observe the signal on the oscilloscope to verify the feasibility of the scheme.The sensing system collects and processes the signals on the FPGA,and uses Labview to develop the host computer software to realize the functions of peak finding,display and storage of the FBG reflected signals.