Polarimetric Fiber Vibration Sensor Based on Polarization-Diversified Loop Using Short Polarization-Maintaining Photonic Crystal Fiber

In this paper, we demonstrated a polarimetric fiber vibration sensor based on a polarization-diversified loop (PDL) with short polarization-maintaining photonic crystal fiber (PM-PCF) as a sensor head and investigated the dependence of its frequency response on the sensing fiber length. The fabricated PM-PCF vibration sensor is composed of a polarization beam splitter for forming the PDL, PM-PCF as a sensor head, and two wave plates such as a quarter-wave plate and a half-wave plate. A laser diode and a photodetector were also utilized for intensity-based vibration measurement. In particular, two kinds of PM-PCF segments with fiber lengths of similar to 6 and similar to 10 cm were employed to explore the effect of the sensor head length on the sensor frequency response. The frequency response was examined over 1-3000 Hz using a piezoelectric transducer that can apply single-frequency vibration to the sensor head. Every measured frequency response showed a resonance peak at each sensor head length, and cut-off frequencies were measured as similar to 1240 and similar to 794 Hz for PM-PCF lengths of similar to 6 and similar to 10 cm, respectively. That is, a higher cut-off frequency was obtained in a shorter sensor head length. Moreover, it was observed that the sensor head length dependence of the frequency response was maximized in a frequency range from 1000 to 2400 Hz and faded away beyond 2400 Hz. From the measurement results, it is concluded that the short sensor head can provide higher normalized sensitivity, i.e., better signal to noise ratio, at frequencies higher than the resonance frequency, let alone convenience of installation and in sensitiveness to external perturbations in a PDL-based PM-PCF vibration sensor. The phase shift per unit strain and the minimum detectable strain perturbation, regarded as sensitivity and resolution, were measured as similar to 0.377 mrad/mu epsilon and similar to 0.16 n epsilon/Hz(1/2) at 2000 Hz for similar to 6-cm-long PM-PCF, respectively.