The Noise Lower-Bound of Rayleigh-Scattering-Pattern-Based Distributed Acoustic Sensing With Coherent Detection

As a Rayleigh-scattering-pattern-based distributed sensing scheme, recently coherent optical time-domain reflectometry (COTDR) exhibits high performance in distributed acoustic sensing, particularly for those demonstrations with coherent detection. As the noise lower-bound (NLB) determines the minimal detectable strain, the dedicated study is of prime importance. Straightforwardly, key parameters like pulse width, sweeping bandwidth and signal-to-noise ratio would affect NLB, however, so far there has been no explicit expression between key parameters and NLB in the COTDR with coherent detection. This paper derives the Cramér-Rao lower bound (CRLB) of the COTDR with coherent detection, which illustrates the quantitative relationship between the minimal detectable strain and the related parameters. Besides, the method to reach the CRLB is discussed, and the experimental results exhibit good consistent to the theoretical analysis. This work gives a guidance for the sensing system design based on coherent detection COTDR.