Enhanced and spatially equalized SNR in DAS via digital summation of complex Rayleigh speckles

Fiber-optic Distributed Acoustic Sensing (DAS) make use of Rayleigh backscattering for measuring acoustic signals and vibrations which interact with the fiber along its entire length. However, as DAS methodologies are based on coherent interference of Rayleigh backscattered light, the SNR of each sensor in the array varies randomly along the fiber. Moreover, when long range sensing is implemented, acoustic signals which affect the far-end of the fiber are measured with lower SNR than those affecting the beginning of the fiber due to propagation losses. In this work we experimentally demonstrate a new approach, based on summation of complex Rayleigh spackles, which mitigates the above-mentioned effects. In addition, judicious application of the method along the fiber enables trading sensing range for spatially-uniform SNR without applying any mechanical nor structural changes to the fiber.