On the measurement accuracy of coherent Rayleigh-based distributed sensors

Abstract

The phase change of back-scattered light due to external perturbations is retrieved in coherent Rayleigh-based distributed sensors by estimating the frequency shift (FS) between the traces of different measurements. The uncertainty associated with the estimator, due to the presence of system noises, can lead to an inaccurate evaluation of the FS. Additionally, in coherent Rayleigh-based sensors, the calculation of the signal-to-noise ratio (SNR) from the jagged back-scattered intensity trace using the statistical estimators can cause an erroneous determination of the absolute value of the SNR. In this work, a method to accurately evaluate the non-uniform SNR caused by the stochastic variation of the back-scattered light intensity along the fibre is presented and validated. Furthermore, an analytical expression to evaluate the uncertainty in the FS estimation using one of the standard estimators, namely cross-correlation, is presented. A direct-detection frequency-scanned phase-sensitive optical time-domain reflectometer (φ-OTDR) is employed for the experimental verification of the expression as a function of two crucial system parameters: the SNR and the spatial resolution. The performance of various distributed sensing system configurations utilising cross-correlation for determining the FS occurring due to the external perturbations can be properly predicted hereafter with the aid of the analytical expression presented in this study.

Enhanced Signal-Associated Noise in a φ-OTDR System

“…resolution, which is nearly 6.5X lower than that of the SMF. This significantly low uncertainty value for the REF is a pure consequence of its ∼6.5X (∼8 dB) higher SNR, and this can be corroborated by our recently published work [2], in which we found out that, for a Rayleigh-based sensing system using a rectangular input pulse, σ f is inversely-related the SNR as given by the following equation:…”

“…The frequency uncertainty (σ f ) of the temperature measurement calculated from the traces for an arbitrary fiber distance of 40 m (shown in the inset of the upper figure of Fig. 9) as well as the theoretical uncertainty calculated using the model presented in [2], [21] are illustrated in the lower figure of Fig. 9.…”

“…Whilst such reflectionenhanced fibers are extensively employed in several experiments based on φ-OTDR [15], the effect of the experimental conditions on the performance of these fibers has not been examined so far and is yet to be discovered further. For instance, in our own previous work making use of such an REF [16], having a ∼10 dB back reflection enhancement, the observed temperature resolution enhancement is just three times even though the inverse relationship exists between the SNR and the uncertainty of the measurement [2]. This observed discrepancy in the inverse relationship between the SNR and the uncertainty of the measurement led us to thorough analysis of the signal from such backreflectionenhanced fiber.…”

“…The signal-to-noise ratio (SNR) of the back-scattered light of any DOFS plays a crucial role when it comes to sensing for long distances yet aiming at achieving highly reliable and fast measurements. The proven inverse relationship of the SNR and the measurement uncertainty in the case of Brillouinbased [1] as well as Rayleigh-based DOFS [2] oriented the highly demanding and purely fundamental research towards improving the SNR of the systems with the ultimate goal of reaching the highest measurement accuracy [3]- [5].…”

Coherently parallel fiber-optic distributed acoustic sensing using dual Kerr soliton microcombs

“…In pursuit of the ultimate performances of DAS, remarkable progress has been made in recent years, such as using functionalized sensing cables ( 11 ), optical amplification schemes ( 12 ), pulse coding strategies ( 13 ), and signal processing algorithms ( 14 ). Nowadays, the commonly used single-frequency light source becomes one of the bottlenecks ( 15 ): (i) for a high-sensitivity DAS with relatively short sensing distance, the main limitation lies in the phase noise of the laser source, which cannot be distinguished from the phase changes along the sensing fiber; (ii) for a DAS with long sensing distance, both the sensitivity and the maximum detectable distance are primarily limited by the signal-to-noise ratio (SNR), which is dependent on the laser energy; however, the laser power cannot be infinitely amplified due to the nonlinear effects in fiber; (iii) in all Φ-OTDR–based DAS schemes, the coherent fading caused by the single-frequency laser is also a critical issue.…”