The round trip time of the light pulse limits the maximum detectable vibration frequency response range of phasesensitive optical time domain reflectometry (φ-OTDR). Unlike the uniform laser pulse interval in conventional φ-OTDR, we randomly modulate the pulse interval, so that an equivalent sub-Nyquist additive random sampling (sNARS) is realized for every sensing point of the long interrogation fiber. For an φ-OTDR system with 10 km sensing length, the sNARS method is optimized by theoretical analysis and Monte Carlo simulation, and the experimental results verify that a wide-band spars signal can be identified and reconstructed. Such a method can broaden the vibration frequency response range of φ-OTDR, which is of great significance in sparse-widebandfrequency vibration signal detection, such as rail track monitoring and metal defect detection.
XXXXDistributed vibration sensing by phase-sensitive optical time domain reflectometry (φ-OTDR) has been intensively studied for its potential applications in fields like intrusion detection, oil and gas pipeline monitoring, high speed rail or urban rail transit monitoring, and so on [1][2][3][4][5]. It is important for the sensing distance and the frequency response range to meet the requirements of practical applications. Up to now, ultra-long φ-OTDR with 175 km sensing range has been achieved [5], but its vibration frequency response range is limited to less than 285 Hz. In order to broaden the frequency response range, two kinds of approaches have been proposed: the combination of interferometer with φ-OTDR [2], and the frequency multiplexing method [3,4].Taking the φ-OTDR based high-speed rail track monitoring system as an example, the length of sensing fiber should be larger than 50 km or even 100 km, which corresponds to a maximum detectable frequency of 1 kHz or 500 Hz, respectively. However, in terms of the wave propagation along a railway track, the frequency of traininduced vibration is up to 80 kHz [6]. It is difficult for the conventional φ-OTDR system to detect such a broadband signal.In some cases, the vibration waves are sparse in frequency domain, which gives a chance for φ-OTDR system to handle with low pulse repetition rate. As for rail track monitoring, most of the long-range waves decay rapidly because only several vibration modes with certain frequencies are supported [7], owing to the fact that the vibration waves degenerate to sparse signal [8] over 10 kHz to 60 kHz. It should be noted that sparse signal is also used widely in many fields, such as metal damage detection [9].In this letter, we demonstrate a new φ-OTDR system inspired by the concept of non-uniform sampling [10][11][12]. Thanks to its antialiasing characteristic in frequency domain, the proposed method, named sub-Nyquist additive random sampling (sNARS) technique, is capable of sampling the wide-band sparse frequency signals with sub-Nyquist sampling rate [8,12]. The sNARS method is studied theoretically and verified with Monte Carlo simulation, by which the key parameters for φ-O...