Frequency Response Enhancement of Direct-Detection Phase-Sensitive OTDR by Using Frequency Division Multiplexing

The frequency division multiplexing (FDM) technique is first introduced into a direct-detection phase-sensitive OTDR to improve the distributed acoustic sensing performance by using a frequency step sweeping laser source and a dual-pulse heterodyne detection scheme. A raised-cosine-shaped pulse is used to suppress the crosstalk in the FDM technique. By using this technique, a 40-kS/s sampling rate to vibration is realized with a 10-km measurement range, which implies the tradeoff relationship between the frequency response and the measurement range is broken. In the experiment, vibrations with different frequencies are measured to validate the effectiveness of the proposed technique. A 20-kHz frequency response is achieved over a 10-km measurement distance, and the frequency response shows a good flatness with a fluctuation of ~0.5 dB.