Abstract

We introduce a method to compensate for the deleterious effects of the phase noise of the laser source on long-range distributed acoustic sensors (DAS) that implement optical pulse compression (OPC). Pulse compression can be used in coherent optical time-domain reflectometry (COTDR) sensors to extend the measurement range without compromising spatial resolution. In fact, OPC-COTDR sensors have demonstrated the longest measurement range to date in passive sensing links that do not require distributed amplification to compensate fiber attenuation. However, it has been found that the limited coherence of the laser source has a degrading effect on the actual performance enhancement that pulse compression can bring because it constrains the maximum duration of the compression waveforms that can be used and makes the use of lasers with extremely low phase noise necessary. We introduce a technique to compensate for the effects of phase noise on OPC-COTDR sensors so that they can demonstrate their full potential for long-range measurements using lasers with less stringent phase noise requirements. The method is based on sampling the phase noise of the laser with an auxiliary interferometer and using this information in a simple signal processing technique to mitigate its deleterious effect on the signal measured. We test our method in an OPC-COTDR sensor that uses 500- ${\mu }\mathrm{s}$ linear frequency modulated pulses to demonstrate 100-km range measurements with 200 $\text{p}\epsilon /\sqrt{\text{Hz}}$ of strain sensitivity at 2-m initial spatial resolution that becomes 10-m after applying the gauge length. To our knowledge, this is the longest compression waveform demonstrated to date in an OPC-COTDR sensor. Its use provides an extra 20-km range compared to previous demonstrations using laser sources of comparable linewidth. Furthermore, comparable performance is also demonstrated when using a laser source with an order of magnitude larger linewidth.

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