Temperature/strain cross sensitivity is a long-standing issue in Brillouin-based distributed sensors, impairing the reliability of such sensors. So far, all the proposed methods perform the discrimination by measuring two quantities showing distinct responses to temperature and strain. Distributed Raman sensing enables temperature measurement without strain sensitivity. However, due to its weak signal intensity resulting from the principle based on spontaneous scattering, the spatial resolution is typically limited to ∼ 1 m. Here, for the first time, we use stimulated Brillouin scattering in gas-filled hollow-core photonic crystal fibers for distributed temperature sensing and we demonstrate ∼ 1 cm spatial resolution and 0.3°C temperature resolution fully free of strain cross sensitivity. Substantially higher performance is obtained thanks to the higher Brillouin gain, narrower gain linewidth and relaxed optical power restrictions when compared to solid silica single-mode fiber. This opens a new avenue in high performance distributed fiber sensing based on gas nonlinear optics.
Fibre optics sensors have been identified as very good candidates for environmental monitoring inside the silicon detectors operated at CERN’s Large Hadron Collider. In this study, we present the results from the first highly sensitive relative humidity distributed sensor with kilometres sensing range. The setup is a 70 cm spatial resolution phase-sensitive Optical Time Domain Reflectometry (OTDR) and is able to monitor fibre lengths up to 10 km. The coating effect is also evaluated, analysing different coating thicknesses, number of coating layers, different manufacturing and different materials. Relative humidity tests were performed at two different temperatures (25°C and 42°C). Polyimide coated fibres show in general a higher humidity sensitivity then a standard acrylate coated fibre, while acrylate fibres offer the fastest response and settling time. The system is able to resolve 0.1% RH and all tested fibres proved to be good candidates to be employed in a distributed relative humidity sensor. If the requirements are a fast time response and short settling time at room temperature, the standard acrylate coated fibres are the best candidates. However, if the requirements are high sensitivity and measurement stability at different temperatures, the polyimide-coated fibres offer advantages on several aspects.
We exploit backward stimulated Brillouin scattering in gases to achieve unprecedented nonlinear optical amplification in fibers. The gain coefficient is 10 times larger than any reported nonlinear gain in gas-filled HC-PCF and 6 times larger than the largest nonlinear gain in standard silica single-mode fiber (SMF). Furthermore, our system can work at any wavelength from vacuum ultraviolet to mid-infrared thanks to the nature of stimulated Brillouin scattering. This massive gain enables us to realize a single-pass gas Brillouin laser with 140 mW threshold power over 50 m of fibre. This platform opens new avenues in gas Brillouin lasing, all-optical signal processing and high performance distributed fiber sensing.