General Method for Rayleigh Signature Interrogation in Distributed Acoustic Sensors based on Optical Time-Domain Reflectometry

We introduce a method to measure the Rayleigh signature of an optical fiber, which is defined as the frequency dependence of the backscattering from each position along its length. The method uses the Short-Frequency Fourier transform to extract frequency-dependent time-resolved information from the spectrum of the detected pulse response in an optical time-domain reflectometry (OTDR) setup. The Rayleigh signature obtained can be used to implement broadband high-sensitivity high-linearity distributed acoustic sensors (DAS) that are immune to signal fading problems commonly affecting other OTDR-based systems. Rayleigh signature interrogation has been widely applied in optical frequency-domain reflectometry sensors and some specialized OTDR configurations. However, to our knowledge, this is the first technique that enables its general use in conventional single-pulse coherent OTDR setups, as well as in other time-domain systems that measure the impulse response of a fiber, such as those that utilize pulse compression. We experimentally demonstrate the method in a conventional heterodyne-detection OTDR DAS and also in a enhanced-performance pulse compression setup employing phase-coded waveforms. Measurements in a 50 km fiber with spatial resolution 2 m and a sensitivity of 113  ${p}\epsilon /\sqrt{Hz}$ demonstrate the capabilities of the technique.