Novel auto-correction method in a fiber-optic distributed-temperature sensor using reflected anti-Stokes Raman scattering

Abstract

A novel method for auto-correction of fiber optic distributed temperature sensor using anti-Stokes Raman back-scattering and its reflected signal is presented. This method processes two parts of measured signal. One part is the normal back scattered anti-Stokes signal and the other part is the reflected signal which eliminate not only the effect of local losses due to the micro-bending or damages on fiber but also the differential attenuation. Because the beams of the same wavelength are used to cancel out the local variance in transmission medium there is no differential attenuation inherently. The auto correction concept was verified by the bending experiment on different bending points.

High-Performance Raman-Based Distributed Fiber-Optic Sensing Under a Loop Scheme Using Anti-Stokes Light Only

“…Such a procedure cancels out all loss factors depending on the fiber position, leading to self-calibrated temperature measurements that do not depend on the optical loss variations during the sensor lifetime, as demonstrated in [4]. Some auto-correction methods have been recently proposed and shown to compensate differential WDL using for instance the anti-Stokes signal reflected from a mirror [5] or using two light sources at different wavelength [6]. The first technique actually provides a cost-effective solution for real applications; however, the maximum sensing distance in that case is limited by the mirror reflectivity, which moreover, due to pulse reflection by the mirror, generates a two-fold increase in two-way optical path (and corresponding loss) compared to standard schemes, finally allowing only for short-to-medium sensing ranges [5].…”

“…Some auto-correction methods have been recently proposed and shown to compensate differential WDL using for instance the anti-Stokes signal reflected from a mirror [5] or using two light sources at different wavelength [6]. The first technique actually provides a cost-effective solution for real applications; however, the maximum sensing distance in that case is limited by the mirror reflectivity, which moreover, due to pulse reflection by the mirror, generates a two-fold increase in two-way optical path (and corresponding loss) compared to standard schemes, finally allowing only for short-to-medium sensing ranges [5]. Our proposed loop-RDTS uses AS traces measured in forward and backward directions, with the advantage of standard loop configuration.…”

“…While this issue is inherently solved in standard RDTS by taking the AS S ratio (see (1)), this correction is not straightforward when using AS traces only [5]. An effective normalization in our AS-only loop-scheme can be simply performed: i) by using a temperature-controlled reference fiber [4], ii) by measuring the launched optical pulse power through an optical tap coupler or, iii) by using a laser current monitor proportional to the real-time pulse power fluctuations for each trace.…”

“…The AS intensity in loop configuration can then be obtained from the geometric mean of the normalized singleended AS traces in both forward and backward directions according to: (5) where includes the normalization accounting for the laser power fluctuations. Note that the exponential factor in (5), including effect of WDL, is a constant factor that does not depend on the fiber position , contrarily to the -dependent exponential term in (1).…”

“…In order to make the Raman-based distributed temperature sensor (RDTS) suitable for practical applications, the effects due to WDL and local loss variations must be cancelled out; this is achieved by employing a loop scheme, with double-end fiber interrogation, where AS and S traces are acquired in both forward and backward directions and then properly averaged [4]. For single-ended schemes, the use of AS light only has been proposed in [5] in conjunction with a specific reflective mirror at the far fiber-end, resulting in a four-fold optical path length for light pulses and scattering, and hence increased losses, making this technique more suitable for short-range applications, rather than for long-range sensing; moreover, due to the reflective mirror at the far fiber-end, the system can be more sensitive to multiple optical reflections (ghosts) with unknown connector conditions in real applications. In this letter we propose a loop measurement technique using only the AS backscattered light and a single channel receiver, leading to a simpler, reliable and cost-effective system for long sensing ranges with inherent differential loss correction.…”