提出一种利用极大倾角光纤光栅(Ex-TFG)谐振峰的3 dB点进行悬臂梁低频振动检测的优化方法。对Ex-TFG的轴向应变和弯曲应变特性进行理论分析和实验验证;对基于光强调制的Ex-TFG振动传感的光谱响应特性进行理论分析,采用Ex-TFG谐振峰的3 dB点对振动进行实验研究。实验结果表明:1)Ex-TFG在轴向应变和弯曲应变条件下,其谐振波长均发生蓝移,横电(TE)模和横磁(TM)模的轴向应变灵敏度分别为-2.8 pm/με和-1.8 pm/με,透射强度基本不变,在0~0.4 m ^-1的曲率范围内基于光强变化的弯曲应变灵敏度分别为2.6 dB/m ^-1和1.2 dB/m ^-1,基于波长漂移的弯曲应变灵敏度分别为-3.34 nm/m ^-1和-2.53 nm/m ^-1;2)在低频振动的检测中,Ex-TFG的TE模和TM模的谐振峰3 dB点的振动加速度灵敏度分别为113.54 mV/g和100.93 mV/g,比谐振峰的峰值波长点(100%点)的振动加速度灵敏度高2倍多,且信噪比(SNR)高约10 dB。此外,SNR随悬臂梁厚度的增大而增大;3)在相同条件下,TE模具有比TM模更高的振动响应灵敏度,但TM模比TE模具有更好的输出稳定性。

This study proposes an optimization method for detecting the low-frequency vibration of a cantilever beam using the 3 dB point of the resonant peak of an excessively tilted fiber grating (Ex-TFG). First, the axial and bending strain characteristics of Ex-TFG are theoretically analyzed and experimentally verified. The spectral response characteristics of the Ex-TFG vibration sensor modulated by light intensity are also theoretically analyzed. The 3 dB point of the Ex-TFG resonance peak is proposed to detect the vibration signal, and subsequently, a corresponding experimental research is conducted. The results are as follows. First, the resonance wavelength of Ex-TFG is blue shifted owing to both axial and bending strains. The axial strain sensitivities are -2.8 and -1.8 pm/με for the TE and TM modes, respectively, whereas the transmission intensity remains unchanged. Bending strain sensitivities based on the light intensity modulation in the curvature range of 0-0.4 m -1 are 2.6 dB/m -1 and 1.2 dB/m -1, respectively. Bending strain sensitivities based on wavelength shift are -3.34 nm/m -1 and -2.53 nm/m -1, respectively. Second, during vibration detection, the vibration acceleration sensitivities of the resonance peaks of Ex-TFG at the 3 dB points are 113.54 and 100.93 mV/g for the TE and TM modes, respectively, which are over two times higher than those (100% point) of the resonance peaks. The signal-to-noise ratios (SNRs) of the TE and TM modes at the 3 dB point are also higher (approximately 10 dB) than those (100% point) of the resonance peaks. In addition, the SNR increases with the increasing cantilever beam thickness. Third, under the same condition, the TE mode exhibits higher vibration response sensitivity than the TM mode; however, the output of the TM mode has better stability than the TE mode.