Abstract-A distributed fiber-optic temperature sensor technique inherently allowing for system calibration, compensating time-dependent variations of the fiber losses as well as local external perturbations, is proposed using a loop-scheme together with Raman anti-Stokes-only measurement. A temperature resolution enhancement with respect to a standard loop configuration is shown by experiments, providing a robust and reliable high-performance sensing technique for long sensing ranges.

No abstract

We demonstrate a hybrid distributed acoustic and temperature sensor (DATS) using a commercial off-the-shelf (COTS) distributed feedback (DFB) laser, a single-mode optical fiber, and a common receiver block. We show that the spectral and frequency noise characteristics of the laser, combined with a suitable modulation scheme, ensure the inter-pulse incoherence and intra-pulse coherence conditions required for exploiting the fast denoising benefits of cyclic Simplex pulse coding in the hybrid measurement. The proposed technique enables simultaneous, distributed measurement of vibrations and temperature, with key industrial applications in structural health monitoring and industrial process control systems. The sensor is able to clearly identify a 500 Hz vibration at 5 km distance along a standard single-mode fiber and simultaneously measure the temperature profile along the same fiber with a temperature resolution of less than 0.5°C with 5 m spatial resolution.

A distributed sensor system employing spontaneous Raman scattering with use of correlation-coding techniques and a single-detector scheme is discussed and experimentally characterised. A sensing distance of up to 8 km is achieved with high spatial and temperature resolutions; use of correlation-coding significantly reduces measurement time and allows use of low-power laser sources.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 time must 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 deal with 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.

No abstract