Fibre optic distributed temperature sensor with an integrated background correction function

Abstract

Raman scattering in optical fibres provides a means of measuring continuous temperature distributions over extended distances. However, in many applications it has been found that the background transmission properties of the optical fibre either change after deployment of the fibre or vary over time due to ageing effects. Changes in the fibre transmission can be distinguished from thermal effects by measuring the Rayleigh backscatter, which is relatively insensitive to temperature. The combined use of Raman and Rayleigh data is discussed here in the context of a particularly simple and cost-effective sensor design, where a single, fixed optical filter and a single light source are used for both measurement modes. The Rayleigh backscatter measurement allows the accurate correction of background transmission changes in a probe arrangement that includes several splices between different fibres with different attenuations. In this way, the temperature in a 290 °C test region is accurately recovered from the anti-Stokes Raman signal. However, preliminary measurements of the fibre attenuation as a function of temperature and exposure to an accelerated ageing environment show that these two effects will be difficult to separate in practice. Therefore it remains challenging to perform accurate, unambiguous background corrections in situations where high-temperature ageing occurs.

A Review of Distributed Fibre Optic Sensors for Geo-Hydrological Applications

“…It is worth mentioning that improved and alternative designs have been proposed over the years, including commercially-e.g., photon-counting Raman OTDRs [53][54][55][56][57], pulse compression/coding schemes [58][59][60][61], or incoherent Raman OFDRs, in respect of which we direct the reader to the corresponding references. Remarkably, in [62], a resolution of 0.09°C in a 1100 m-long fibre with 0.39 m spatial sampling was achieved with direct detection approach.…”

Impact of Loss Variations on Double-Ended Distributed Temperature Sensors Based on Raman Anti-Stokes Signal Only

“…Hence, the ratio AS/S (or AS/Rayleigh) is expected to change during the sensor lifetime due to the ageing process or external harsh environmental conditions, leading then to significant errors in the temperature estimation [6], [13]. Such errors actually consist in the gradual deviation of the temperature trace with respect to the real value, effect that increases with the distance and turns to be a critical factor at long sensing distances.…”

“…Equation (12) then allows us to evaluate the sensing performance improvement provided by the AS-only scheme in comparison to the standard AS/S loop scheme. Actually, while in the case of AS-only scheme, and represent the signal-to-noise ratio of the forward and backward AS traces, in the case of standard loop configuration, and represent the signal-to-noise ratio of the AS/S ratios in both directions, which can be evaluated as (13) where and are the signal-to-noise ratio of the respective anti-Stokes and Stokes traces.…”

Empirical mode decomposition based dynamic error correction in SS covered 62.5/125µm optical fiber based distributed temperature sensor

“…These schemes include the use of Rayleigh in place of St signal [10], loop back [6] and double-ended (DE) configuration [11] where both ends of sensing fiber are accessed by the OFDTS. Lee [7] and Suh and Lee [12] suggested the schemes based on the use of two laser sources.…”