We experimentally investigate the benefits of a new optical pulse coding technique for long-range, meter and submeter scale Raman-based distributed temperature sensing on standard single-mode optical fibers. The proposed scheme combines a low-repetition-rate quasi-periodic pulse coding technique with the use of standard high-power fiber lasers operating at 1550 nm, allowing for what we believe is the first long-range distributed temperature measurement over single-mode fibers (SMFs). We have achieved 1 m spatial resolution over 26 km of SMF, attaining 3°C temperature resolution within 30 s measurement time. © 2011 Optical Society of America OCIS codes: 060.2370, 280.1350 Distributed fiber-optic sensors based on Raman scattering are becoming a widely adopted technology [1], with a range of industrial applications spanning from oil and gas pipelines (for fire and leakage detection), to firealarm systems, reservoir and power cables monitoring. Raman-based distributed temperature sensor (RDTS) systems exploit the strong temperature sensitivity of the anti-Stokes Raman backscattered light, which is, however, characterized by extremely low power values, resulting in challenging measurements and requiring the use of high-sensitivity photodiodes, as well as the acquisition of many traces to decrease the noise impact through averaging. To partially overcome the trade-off among temperature resolution, sensing range and acquisition times, RDTS systems commonly employ multimode fibers (MMFs) [2,3], which are characterized by higher backscattering coefficients and also allow for higher input peak power levels before the onset of nonlinearities [3]. Unfortunately, modal dispersion ultimately limits the RDTS spatial resolution when using MMFs. Although this issue can be partially overcome by using graded-index MMFs, the best achievable spatial resolution is still limited to several meters when operating over long sensing ranges (tens of kilometers). In spite of these limitations, for many applications a better spatial resolution would be highly desired over long distances, together with fast acquisition times and high temperature resolution. In order to enhance the sensing performance of RDTS systems, optical pulse coding techniques have been proposed based on either directly or externally modulated semiconductor lasers in MMFs [2] and SMFs [4]. In both cases the strong potential in terms of signal-to-noise ratio (SNR) enhancement provided by optical coding has been somehow limited by the available power in semiconductor lasers. In particular, for RDTS systems operating on standard SMFs with pulse coding, the maximum peak power level from a semiconductor laser that can be reasonably coupled into the sensing fiber is of the order of few hundred milliwatts; however, in principle, up to ∼3-4 W could be used before exciting detrimental nonlinear effects, such as stimulated Raman scattering.Hence, the use of new coding schemes, which could be used with high-power pulsed lasers [such as Q-switched and rare-earth-doped fiber lasers ...

We experimentally demonstrate the use of cyclic pulse coding for distributed strain and temperature measurements in hybrid Raman/Brillouin optical time-domain analysis (BOTDA) optical fiber sensors. The highly integrated proposed solution effectively addresses the strain/temperature cross-sensitivity issue affecting standard BOTDA sensors, allowing for simultaneous meter-scale strain and temperature measurements over 10 km of standard single mode fiber using a single narrowband laser source only.

A cyclic pulse coding technique is proposed and experimentally demonstrated for fast implementation of long-range Brillouin optical time-domain analysis (BOTDA). The proposed technique allows for accurate temperature and strain measurements with meter-scale spatial resolution over kilometers of standard single-mode fiber, with subsecond measurement times.

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.

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.