Radiation-tolerant Raman distributed temperature monitoring system for large nuclear infrastructures

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

“…is the frequency separation between AS and pump signal, is the Planck constant and is the Boltzmann constant [4]. If the differential loss ( ) is characterized as a function of the distance, the exponential factor of (1) can be corrected.…”

“…If the differential loss ( ) is characterized as a function of the distance, the exponential factor of (1) can be corrected. However, in real applications, such a factor is not constant in time since the optical fiber may be exposed to environmental conditions that spectrally change the fiber attenuation during the sensor lifetime [4]. Hence, significant errors in temperature measurements can be induced when no further 1041-1135/$26.00 © 2011 IEEE calibration is performed.…”

“…In such a scheme, the ratio between AS and S signals is obtained in both forward and backward directions, and the geometric average of these two ratios is calculated. 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 most common sensing schemes employ spontaneous Raman backscattering through simultaneous measurements of the backscattered Raman anti-Stokes (AS) and Stokes (S) (or AS and Rayleigh) components in a single-ended fiber configuration [2], [3]. Sensor schemes based on singleended configuration, although representing the common solution in long range applications due to their extended distance, suffer however from the impact of fiber wavelength-dependent losses (WDL) [4]. 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].…”

“…Sensor schemes based on singleended configuration, although representing the common solution in long range applications due to their extended distance, suffer however from the impact of fiber wavelength-dependent losses (WDL) [4]. 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.…”