Novel technique for distributed fibre sensing based on faint long gratings (FLOGs)

Abstract

A novel approach for fibre distributed sensing is proposed, conceived to match as closely as possible to an ideally responding distributed sensor. It is demonstrated that it can be actually realized using fibre Bragg gratings of extremely low index contrast and continuously printed over the entire fibre length. The concept is experimentally validated over a restricted distance range that proves the huge potentialities of the technique in terms of response and precision.

Spot event detection along a large-scale sensor based on ultra-weak fiber Bragg gratings using time–frequency analysis

“…When the input pulse is launched into the photonics sensor composed of low reflectivity FBGs, it is continuously back-reflected while propagating through the entire DUT length. By controlling the central wavelength of the incident pulse, the detection of spot events can be achieved, with a spatial resolution dictated by half the input pulse duration, like any OTDR technique [9]. The here reported measurements show that such a system can offer an attractive, robust and cost-effective approach for quasi-distributed fiber sensing.…”

“…In case of Gaussian-shaped pulse, the pulse spectral width Δf is given as Δf = (2ln2/π)/τ ≈ 0.441/τ [9]. For a 500 ps-long pulse, this leads to a pulse spectral width of 7.056 pm which corresponds to a temperature resolution of less than 0.42 K. Whereas, for τ = 80 ps, the pulse spectral width is 44.1 pm, corresponding to a temperature resolution of less than 2.62 K.…”

“…Subsequently, fiber distributed sensing based on very-weak reflectivity FBG was also demonstrated using optical time-domain reflectometry (OTDR) [9]. By scanning the laser frequency a distribution map of the reflectivity along the fiber has been obtained, demonstrating very good performance in terms of accuracy and signal response.…”

“…Starting from the idea presented in [9], we decided to move toward the possibility to apply a very long FBG as a large-scale distributed sensing device. For a long range sensor, ultra weak gratings are required, which ensures low crosstalk level among units in the array and allows the propagation of the optical incident signal over the entire length of the sensor.…”

Novel Concepts and Recent Progress in Distributed Optical Fiber Sensing

“…The interaction is activated by an intense light pulse that causes a continuous back reflected response, which can be either a simple recapture of spontaneously scattered light in the back direction, or a resonant Bragg-type reflection (stimulated Brillouin scattering and faint long optical gratings [10]). The amplitude of the response is proportional to the pulse energy in a first order approximation (weak signal), which is given by the product of the peak pulse power and the pulse duration.…”