Time-expanded phase-sensitive optical time-domain reflectometry

Abstract

Phase-sensitive optical time-domain reflectometry (ΦOTDR) is a well-established technique that provides spatio-temporal measurements of an environmental variable in real time. This unique capability is being leveraged in an ever-increasing number of applications, from energy transportation or civil security to seismology. To date, a wide number of different approaches have been implemented, providing a plethora of options in terms of performance (resolution, acquisition bandwidth, sensitivity or range). However, to achieve high spatial resolutions, detection bandwidths in the GHz range are typically required, substantially increasing the system cost and complexity. Here, we present a novel ΦOTDR approach that allows a customized time expansion of the received optical traces. Hence, the presented technique reaches cm-scale spatial resolutions over 1 km while requiring a remarkably low detection bandwidth in the MHz regime. This approach relies on the use of dual-comb spectrometry to interrogate the fibre and sample the backscattered light. Random phase-spectral coding is applied to the employed combs to maximize the signal-to-noise ratio of the sensing scheme. A comparison of the proposed method with alternative approaches aimed at similar operation features is provided, along with a thorough analysis of the new trade-offs. Our results demonstrate a radically novel high-resolution ΦOTDR scheme, which could promote new applications in metrology, borehole monitoring or aerospace.

High Resolution Distributed Optical Fiber Sensing Using Time-Expanded Phase-Sensitive Reflectometry

“…In Figure 2, we show some of the experimental results obtained from the technique. In particular, in Figure 2A, periodical temperature variations with a resolution of 2 cm and sampling rate of 20 Hz over a fiber of about 200 m have been detected and reported in (Soriano-Amat et al, 2021b). For this experimental demonstration, a random spectral phase modulation was employed in the dual comb.…”

“…First, the phase of each spectral comb line was modulated with a random value between 0 and 2π (Soriano-Amat et al, 2021b). The phases of the probe comb and LO comb were encoded using the same phase profile.…”

“…By using this strategy, the time domain pulses transform into a speckle-like shape covering the full period. Thus, the peak-to-average power ratio (PAPR) of the probe and LO signals can be significantly reduced with respect to the transform-limited case [in almost two orders of magnitude in (Soriano-Amat et al, 2021b)]. The PAPR reduction provides a proportional increase in the SNR of the obtained traces.…”

“…This severe condition can be relaxed if another concept from the dual comb spectroscopy field is implemented, namely, the use of a quasi-integer dual comb (Hébert et al, 2014). This concept has been briefly investigated in (Soriano-Amat et al, 2021b) for extending the range of operation of TE-ΦOTDR. The modification to the original TE-ΦOTDR method consists in using two frequency combs with repetition rates that are quasi-integer multiples.…”

“…This spectral compression is equivalent to an expansion of the detected traces in time domain. For this reason, the technique has been called time-expanded (TE-)ΦOTDR (Soriano-Amat et al, 2021b). The high spatial resolution attained, along with the capabilities of performing real time operation even in relatively long fibers (hundreds of meters) with a simple and potentially costeffective setup, position this sensing method in a unique place, as its performance is not overlapped by any other distributed optical fiber sensing technology.…”