The System Of Fiber Grating Demodulation Based On FPGA And F-P Filter

As a novel optical sensor,fiber bragg grating has some advantages of corrosion resistance,high precision,anti electromagnetic interference,distributed sensing compared with the traditional electrical sensor, and it has been widely used in security monitoring,construction,aerospace and other fields.The fiber grating demodulation system has been the focus and difficulty in the research of fiber grating sensing technology,it is very important to design a low cost,high precision demodulation system for the development of the fiber optic sensing technology.Five kinds of commonly used demodulation methods is detailed analysis and comparison basing on the theory of fiber grating sensor and according to the requirement of practical engineering application.Based on tunable fiber Fabry-Perot filter a fiber grating demodulation system is designed and developed.FPGA is used to generate the driving power of the tunable F-P cavity based on the scanning characteristics of F-P after testing and analyzing the performance of the optical path and the core components of the system.Then, according to the nonlinear characteristics of FFP piezoelectric ceramics, the linear compensation is carried out by using the direct frequency synthesis technique in FPGA.Finally, A/D data acquisition, serial transmission and reception and other hardware modules in FPGA was designed, sensing grating reflective spectrum data acquisition and FPGA communication with the host computer is successfully implemented.In this paper, the host computer software system is developed based on LabVIEW platform. First of all, he PC software to accept and display real-time FPGA serial upload data function is realized; Secondly, in order to achieve a more accurate measurement of Prague wavelength from the limited data, this paper proposes the use of L-M algorithm to fit the grating reflection spectrum. By fitting the obtained spectral reflectance peak position, in order to achieve the temperature / strain measurement.finally, Finally, in order to solve the problem of FFP’s own drift and other issues on the measurement accuracy of the system, this paper presents a method of measuring the wavelength of a strain sensing grating in series.The measured grating center wavelength is corrected by using the stress calibration grating and the center wavelength difference of the reflected light of the temperature sensing grating to be measured. By doing this,the measurement accuracy of the center wavelength of the grating reflected light wave is improved.After many times of test and measurement, the demodulation system of central wavelength drift less than 15 pm after using wavelength fitting and calibration grating,combined with the use of wavelength of sensing grating temperature sensitivity coefficient 27.3pm/ DEG C, system temperature demodulation precision is less than0.55 degrees.