Distributed temperature sensor system based on Raman scattering using correlation-codes

Introduction: Fibre-based distributed temperature sensor (DTS) systems have been intensively studied for several years, because of the advantages they can provide compared to conventional sensors, and to optical fibre multiplexed sensor systems (such as grating-based sensors); most common applications include pipeline monitoring (for oil and gas transportation), oil well monitoring and power cables as well as fire detection systems [1]. Spontaneous Raman scattering effect is commonly exploited to implement DTS systems [2]. In Raman-based schemes, the ratio of Raman antiStokes (AS) line to Stokes line intensities is usually used for temperature monitoring, since it allows for measurements which are independent of major fibre loss effects and loss changes owing to fibre ageing and other effects. Temperature sensing along the fibre is then generally achieved through optical time domain reflectometry (OTDR), where light pulses are coupled into the fibre, and backscattered Stokes and AS light are detected. One drawback of such implementation schemes is that, owing to low Raman backscattering power, high peak power and long measurement timemust be used to ensure good spatial and temperature resolution. Coding techniques can be used in OTDR to overcome the resolutionrange trade-off and improve signal-to-noise ratio (SNR) [3]. In this Letter we dealwith the implementation and characterisation of a novel distributed Raman-based DTS (Agilent DTS N4385A= N4386A – proprietary technology) using for first time complementary-correlation (CC) coded OTDR techniques with a single photodiode receiver scheme. Use of correlation coding allowed overcoming of the resolution–range trade-off by enhancing SNR, resulting in a high-performance DTS over 8 km multimode (MM) sensing fibre. The newly proposed single photodetector receiver scheme allows for high measurement repeatability over a wide temperature range. Experimental characterisation has shown temperature sensing over 4 km (8 km) of MM fibres with 1 m (3 m) spatial resolution and 0.3 K temperature resolution.