Coherent Pound–Drever–Hall Technique for High Resolution Fiber-Optic Sensors at Low Probe Power

The Pound-Drever-Hall (PDH) technique has been widely adopted in high-resolution fiber-optic sensors, but its performance degenerates as the probe power drops. In this work, we develop a coherent PDH technique for detection of very weak probe light, in which the probe beam is coherent detected with a strong local oscillator. Assisted with an analog frequency doubler and a band-pass filter, the configuration of proposed coherent PDH technique is highly compatible with classical PDH technique. The influence of fiber dispersion is also assessed. In the demonstrational experiments, the signal-to-noise ratio of the extracted PDH signal is dramatically improved compared with classical PDH technique, especially under weak probe power. Using a π-phase shifted fiber Bragg grating as the sensing element, a nε-order strain resolution is achieved at a low probe power down to -43 dBm, which is about 15-dB lower compared with classical PDH technique. The proposed technique has great potentials in high-resolution large-scale fiber sensor networks.