Simultaneous measurement of vibration and temperature based on FBG and DBR fiber laser beat frequency digital sensing system

A fiber Bragg grating (FBG) and distributed Bragg reflection (DBR) fiber laser sensor combined digital sensing system for both vibration and temperature sensing is proposed. To the best of the authors' knowledge, this is the first FBG and DBR fiber laser combined dual parameters digital sensing system ever reported. By utilizing both FBG and DBR fiber laser sensor, not only a low frequency vibration detection as low as 0.25 Hz can be achieved, but also temperature and vibration dual parameters sensing can be achieved. External vibration is applied on the FBG which is part of the DBR fiber sensor. The wavelength change of DBR fiber laser sensor's polarization laser modes is controlled by the FBG. Frequency change of polarization laser modes' beat frequency (PMBF) signal is closely related to the reflected wavelength change of the sensing FBG. Variational mode decomposition (VMD) algorithm is utilized to denoise the monitored time-frequency signal of PMBF and the denoised signal is fast Fourier transformed (FFT) to get the corresponding vibration frequency. By utilizing this method, extremely low frequency vibration signal can be accurately demodulated. Beat frequency signals (BFS) of multi-longitudinal mode lasers which are generated in the DBR fiber laser sensor are utilized for temperature sensing. Measurement results demonstrate that the dual parameters digital sensing system has an excellent low frequency vibration-sensing capability, simple structure, high SNR, stability and accurate dual parameters sensing without mutual interference.