Frequency Response Enhancement by Periodical Nonuniform Sampling in Distributed Sensing

The sensing fiber length and the use of averaging methods limit the maximum detectable frequency response range of vibration in phase-sensitive optical time domain reflectometry (φ-OTDR), especially for long-haul monitoring. We propose a method to significantly extend the frequency response range of φ-OTDR systems by impressing vibration signals on a set of backscattering signals that are carried by different wavelengths within one measurement interval. The sampled traces are recombined into a trace group using a time multiplexing method, leading to higher effective sampling rate at each sensing point on the sensing fiber, which allows for a larger frequency response range. In experiments, two pulses carried by different wavelengths with a time delay of 0.75 μs are launched into a 3024-m long sensing fiber, and frequency response range of 30 kHz is realized. The system can overcome the tradeoff between the frequency response range and the sensing fiber length.