FPGA-Based Dynamic Real-time Demodulation System For FBG Wavelength

Being compared to other sensors,fiber Bragg grating(FBG)sensor has many advantages,such as small volume,light weight,high precision,insensitivity to electromagnetic interference and long distance transmission.It is the key to utilizing FBG sensors to accurately detect small shift of FBG reflection wavelength.It means to demodulate FBG wavelength information.The demodulation method based on tunable F-P filter(TFPF)is widely applied in practical projects at present.Due to temperature drifting and nonlinearity from the driving element of TFPF,the FBG wavelength detection accuracy of demodulation system is largely limited.So it is significant to design a kind of FBG wavelength demodulation system with low cost,high precision and real-time.In order to reduce the errors resulted from the driving element of TFPF,which had the characters of temperature drift and nonlinear,the dynamic real-time correction technology for FBG wavelength was studied.Two kinds of real-time correction methods with series and parallel structure were analyzed and compared respectively.Then the real-time correction system with improved series system was proposed.The reference grating module with good thermal stability was designed.The function relation between transmission wavelength and driving voltage of TFPF is adjusted in real time by four FBGs in reference grating module.As a result the demodulation precision of the system was improved.The dynamic real-time demodulation system for FBG wavelength based on TFPF was built.The optical unit is composed of broadband optical source,TFPF,reference grating module and sensing grating array.The circuit unit was designed based on FPGA as the core.It also includes A/D conversion circuit,D/A conversion circuit and human-computer interaction circuit.All digital circuits,such as data acquisition unit,FIR filter unit,peak-searching unit.TFPF driving unit,data calculation unit and human-computer controlling unit were integrated in the FPGA,so the real-time performance of demodulation system is guaranteed.The demodulating experiments were repeated many times using corrected functional relation between transmission wavelength and driving voltage of TFPF in real time.The experimental results show that the maximum error of FPGA-based dynamic real-time demodulation system for FBG wavelength is ±3pm.Being compared with reported real-time correction system with the series structure,the detection accuracy of demodulation system is improved by 70%.For the direct read voltage method,the measurement precision of demodulation system is increased by 83.3%.