Long-range strain sensing system using a frequency-locked π-shifted grating

We present a passive, all-fiber strain sensing system capable of measuring strain over 75 km using a diode laser locked to a π-shifted grating. The local strain measurement is extracted from the feedback signal sent to the laser for frequency stabilization, thereby reducing the baseline acoustic noise level. Strain variations at frequencies above 10 kHz were obtained from the intensity of the amplified return light. Using both the feedback signal and the light intensity measurement, the strain was measured between DC and 100 kHz, making sound and music recording possible at very long distances. At 10 Hz, the sensitivity was 5.27 mV/µɛ and a minimal detectable strain of <i>dɛ</i>_min = 23 nɛ was obtained. The strain sensing range was approximately 28 dB, from about 30 nɛ to 20 µɛ.