Fiber Strain Sensing at Over 75 km Distance Using the Coherently Amplified Pound-Drever-Hall Method

We present a passive fiber-optic strain sensor system using a π-shifted Fiber Bragg Grating, π-FBG, as a transducer. A frequency-locked laser is used to interrogate the resonance wavelength shifts due to strain applied to the π-FBG. The Pound-Drever-Hall technique combined with a heterodyne amplification method enables strain measurements using a very weak (−58 dBm; < 2 nW) feedback signal, with low detection limit (< 40 nϵ), and high linear dynamic range (30 dB) over an acoustic frequency range of up to 25 kHz. We apply this cost-effective interrogation technique to long-range (over 75 km) strain sensing.