Performance analysis of peak tracking techniques for fiber Bragg grating interrogation systems

Abstract

Abstract-In this paper, we propose a spectral correlation-based technique for tracking the wavelength shift of a fiber Bragg grating. We compared this approach, by means of a Monte Carlo numerical simulation, to the typical peak tracking techniques applied in classic interrogation systems. As result, we obtained a considerable gain in terms of noise tolerance (about 20 dB), which can be further incremented by selecting large-bandwidth gratings. This permits to increase the power budget of a fiber Bragg grating interrogator without changing the optical layout, overcoming classical limitations of commercial and custom systems. Penalties due to the non-idealities have been evaluated through the same Monte Carlo approach. Finally, we discuss a practical application of the peak tracking techniques to a fiber Bragg grating-based weight sensor, in which we applied the spectral correlation to track both the Bragg wavelength position, spectral deformations due to high strain, and spectral non-linearity. Within the last few years, many commercial interrogators have converged to similar optical and processing architectures, most of them based on the straightforward and reliable principle of operation of spectral reconstruction; interrogators measure the wavelength shift of a FBG array simply by scanning a spectral bandwidth with a tunable laser, a tunable filter, or a spectrometer. In this case, a technique for evaluating the peak wavelength from the measured spectrum has to be implemented; typical interrogators define the peak wavelength either as the wavelength correspondent to the spectral 253 maximum or as the middle point of the X-dB bandwidth (usually 3 dB for weak gratings and 10 dB for strong gratings). Index Terms-A spectral correlation technique, however, permits improving performances of the Bragg wavelength tracking, for all spectrometric and spectral scan-based FBG interrogators, which represent the vast majority of commercial FBG sensing instruments [12]. The correlation method computes the mutual correlation between the reference spectrum and the measured one progressively shifted, and evaluates the FBG wavelength shift as the maximum of the correlation. We performed a Monte Carlo statistical simulation in order to compare the behavior of all peak tracking techniques, using the theoretical expressions of the FBG spectra and reproducing a typical FBG interrogation system, with the purpose of finding the minimum signal-to-noise ratio (SNR) that assesses a correct detection. As result from our simulation, we demonstrate that a FBG interrogator based on spectral reconstruction is capable of gaining a power budget of about 20 dB only by replacing the peak tracking algorithm, and this margin can be even increased by proper choice of the FBGs. Also, the Monte Carlo technique return quantitative results for FBG design, whereas the optimization of the FBG profile can lead to superior performance in peak tracking.Finally, peak tracking techniques have been compared in an applicative context of FBG sensor, validating t...

Review and Analysis of Peak Tracking Techniques for Fiber Bragg Grating Sensors

“…Finally, the Bragg wavelength is estimated as the center of the inner bandwidth. The value X is often set to 3 dB [ 48 ]: in this case the X-BW coincides with the evaluation of the center of the FWHM; however it is possible to adjust X in order to select a narrower or wider portion of the FBG. In the case of a perfectly symmetrical FBG spectrum, the estimated FBG wavelength coincides with the Bragg wavelength in Equations (1)–(3).…”

“…The method, as described in [ 48 , 60 ], compares the spectrum of the FBG in reference condition R ref ( λ ) and the measured spectrum R ( λ ), which is a replica of R ref ( λ ) shifted by the quantity Δ λ . The method consists of identifying the estimated wavelength shift Δ λ e as the value of wavelength shift (discretized over the pixels of the spectrometer) for which the mutual correlation is maximum.…”

“…In the first part of this work, FBG peak tracking methods are reviewed, including direct methods, correlation-based methods, fitting methods, digital oversampling methods, transform-based methods, and optimization-based methods. A thorough review, that extends the peak-tracking analysis released so far [ 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ], is outlined.…”

“…In the second part of the work, the several methods are compared in a simulation-based scenario, in order to analyze the performance. FBGs are simulated with Erdogan’s coupled-mode theory (CMT) [ 38 ], and a statistical analysis is used to evaluate the performance of all reviewed methods as a function of the wavelength resolution and signal-to-noise (SNR) ratio of the interrogation setup and on the grating strength coefficient [ 48 ]. This analysis allows comparing the different techniques in different working conditions, with the aim of optimizing each method and providing a guideline for their implementation.…”

Solving the inverse scattering problem with differential evolution: an experimental validation

“…Spectral shifts of the FBG resonance are the basis for the device operation as sensor. Most techniques of interrogation are based on algorithms used to detect the peak position of the FBG resonance [2], [3]. This strategy allows, e.g., punctual or quasi-distributed sensing of strain and temperature [1], [4]- [6] if the spectral wavelength shifts result from a uniform FBG deformation or a temperature change that affects the whole grating.…”