Linearly chirped fiber-optic Bragg grating as distributed temperature sensor for laser ablation

Review of Chirped Fiber Bragg Grating (CFBG) Fiber-Optic Sensors and Their Applications

“…The method shown in [ 35 , 45 , 46 , 47 , 48 , 49 ] and outlined in Figure 1 b provides a model of the FBG that can be defined as incoherent, as it assumes that the FBG can be modeled as a chain of M uniform FBGs each having different Bragg wavelength and with no intrinsic standing waves existing between each grating element. Calling L the grating length, and M the number of gratings, the length of each short grating element is L g = L / M .…”

“…The Bragg wavelength of each i -th layer, instead, is spatially varying. In a linearly chirped FBG [ 35 , 36 , 37 , 38 , 39 , 46 , 47 , 48 , 49 ] the reference value for the Bragg wavelength has a linear dependence upon the grating length: which can be converted into the discretized grating model as: …”

“…In Figure 3 the concept of temperature profiling through CFBG sensors is exposed. Three different profiles are simulated in Figure 3 a, including a linear profile throughout the whole CFBG length, that can mimic the temperature obtained in a gradient setup [ 35 ], and two Gaussian profiles with peaks in a different part of the grating length that mimic a thermo-therapies temperature pattern [ 38 , 45 , 46 , 47 , 48 , 49 ]. The grating parameters are the same of Figure 2 , including 50 mm length and 1 nm/mm chirp rate, and the temperature coefficient has been set to 10 pm/°C.…”

“…If the temperature or strain pattern along the direction z exhibits a pattern that is known a priori, or that can be assumed to be a function depending on some parameters (for example a polynomial of specific order, or a Gaussian function that depends on amplitude, variance, and central value), it is possible to run an optimization technique that compares the measured spectrum with the CFBG spectrum subjected to the temperature or strain pattern, until the best match is found. This approach, here labeled spectral reconstruction (because it aims at reconstructing the spectrum of the measured CFBG from its reference value and temperature or strain sensitivity), has been successfully demonstrated in [ 35 , 45 , 46 , 47 , 48 , 49 , 57 ].…”

Linearly chirped fiber Bragg grating response to thermal gradient: from bench tests to the real-time assessment during in vivo laser ablations of biological tissue

“…5,[12][13][14] Different from uniform FBGs, the characterization of LCFBGs for thermal gradient measurement is not straightforward, and a preliminary analysis is mandatory to find a correlation between the spectrum features and the thermal gradient. 12,15 The aim of this article is to fill the gap in the literature by performing a careful assessment of LCFBG response to thermal gradients, under various conditions. First, a constant temperature was applied along the active length, to perform the static calibration of the sensor.…”