Simultaneous distributed fibre temperature and strain sensor using microwave coherent detection of spontaneous Brillouin backscatter

Abstract

Simultaneous optical fibre distributed strain and temperature measurements have been obtained, by measuring the spontaneous Brillouin intensity and frequency shift, using the technique of microwave heterodyne detection. The enhanced stability from using a single coherent source combined with optical preamplification results in a highly accurate sensor. Using this sensor, distributed temperature sensing at 57 km and simultaneous distributed strain and temperature sensing at 30 km were achieved, the longest reported sensing lengths to date for these measurements. As a simultaneous strain and temperature sensor, a strain resolution of 100 µε and temperature resolution of 4 °C were achieved.

Distributed optical fibre sensors based on spontaneous Brillouin scattering employing multimode Fabry-Pérot lasers

“…In this case, a suitable receiver scheme, combining optical and electrical heterodyne detection, is needed. Results report an increase in the threshold value of the input peak power before onset of nonlinear effects, allowing for a better sensing performance compared to single-longitudinal mode lasers.Introduction and theory: Distributed Brillouin-based optical fibre sensors have been widely reported in recent years owing to their possibilities to perform distributed strain and temperature sensing along an optical fibre [1,2]. In particular, sensors based on stimulated Brillouin scattering (SBS) require access to both fibre-ends and the use of two optical fibres to distinguish between strain and temperature effects, owing to the strain-temperature cross-sensitivity of the Brillouin gain [1].…”

“…In particular, sensors based on stimulated Brillouin scattering (SBS) require access to both fibre-ends and the use of two optical fibres to distinguish between strain and temperature effects, owing to the strain-temperature cross-sensitivity of the Brillouin gain [1]. On the other hand, sensors based on spontaneous Brillouin scattering (SpBS) only need access to a single fibre-end and allow for simultaneous strain-temperature measurements along the same optical fibre by measuring simultaneously the SpBS intensity and frequency shift (BFS) [2].In this Letter we propose and characterise the use of multi-longitudinal mode (also called multimode) lasers to improve the sensing performance of SpBSbased sensors. Specifically, in our experiments we have used a Fabry-Perot (FP) laser centred at 1550 nm, which is among the most common laser sources used in optical communications.…”

“…In particular, sensors based on stimulated Brillouin scattering (SBS) require access to both fibre-ends and the use of two optical fibres to distinguish between strain and temperature effects, owing to the strain-temperature cross-sensitivity of the Brillouin gain [1]. On the other hand, sensors based on spontaneous Brillouin scattering (SpBS) only need access to a single fibre-end and allow for simultaneous strain-temperature measurements along the same optical fibre by measuring simultaneously the SpBS intensity and frequency shift (BFS) [2].…”

“…where C vB1 ¼ 0.046 MHz/m 1, C vBT ¼1.07 MHz/8C, C PB 1 ¼ 8 Â 10 24 %/m1, and C PBT ¼ 0.36%/C are the strain and temperature coefficients for BFS and SpBS power [2]. The performance of SpBS-based sensors is essentially limited by the onset of nonlinear optical effects, most notably SBS, which imposes the maximum peak power allowed at fibre input.…”

“…To measure the SpBS components, a coherent-detection scheme is required [2], where the Brillouin signal is mixed with an optical local oscillator (OLO).…”