Time-frequency analysis of long fiber Bragg gratings with low reflectivity

Abstract

Abstract:A new technique to investigate the spatial distribution of the reflection spectrum along fabricated long weak fiber Bragg gratings (FBG) is experimentally demonstrated, together with its potential applications for distributed fiber sensing and broadband signal processing. A short pulsed coherent light signal is launched into a FBG and the signal frequency is scanned through the FBG reflection spectrum. When the pulse duration is set much shorter than the transit time through the grating a time-resolved reflected signal can be obtained for each signal frequency. It informs about the distribution of the refractive index periodic perturbation along the entire FBG length, hence the uniformity or frequency chirp information of the fabricated FBG. This technique has been implemented to demonstrate a distributed temperature sensing system with high spatial resolution and to also realize a robust all-fiber tunable delay line for broadband signals.

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

“…The measurements have been performed by implementing a time-frequency domain analysis as demonstrated before in [9,11], combining the OTDR technique with the frequency scanning of the interrogating pulse. However, since the fabrication of a long continuous, ultra weak and homogenous FBG is an expensive and complicated issue, here we have proposed a sensor made of an array of 500 identical ultra weak FBGs written in cascade and covering 5 m of optical fiber, with the purpose of enhancing the measurement range.…”

“…Since fabricating a long, continuous and homogenous FBG with no phase hopping and with very low reflectivity is an expensive and complicated issue, with a maximum reported length of 1 m [10], we propose a sensor made of an array of 500 identical very-weak FBGs written in cascade and covering 5 m of optical fiber. The measurement system is essentially based on a combination of frequency scanning of the interrogating pulse and OTDR technique [11].…”

“…In practical terms, to obtain an optical pulse width of few tens of ps by modulating a continuous wave laser source, a signal generator of some tens of GHz is necessary. On the other hand, complex and bulky configurations, like a nonlinear optical loop mirror [11], were employed to temporally compress the input optical pulse. This demonstrates the existence of a clear trade-off between the minimum value of physical separation between gratings and the interrogating pulse width.…”

Long fiber Bragg grating sensor interrogation using discrete-time microwave photonic filtering techniques

“…The proposed sensor produces delayed replicas of the original signal at the input end of the FBG and at each of the hot-spots to be measured [9,12]. Thus, the response of the FBG sensor is described also by Eq.…”

“…In the context of sensing applications, different methods have been proposed and implemented in order to interrogate the Bragg-frequency distribution along a FBG with the aim of implementing distributed temperature/strain sensors. Some of these salient techniques include optical low-coherence reflectometry (OLCR) [6], optical frequency domain reflectometry (OFDR) [7], synthesis of optical coherence function (SOCF) [8], and timefrequency analysis [9].…”