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In this paper, we describe a novel method for spatially distributed temperature measurement with Chirped Fiber Bragg Grating (CFBG) fiber-optic sensors. The proposed method determines the thermal profile in the CFBG region from demodulation of the CFBG optical spectrum. The method is based on an iterative optimization that aims at minimizing the mismatch between the measured CFBG spectrum and a CFBG model based on coupled-mode theory (CMT), perturbed by a temperature gradient. In the demodulation part, we simulate different temperature distribution patterns with Monte-Carlo approach on simulated CFBG spectra. Afterwards, we obtain cost function that minimizes difference between measured and simulated spectra, and results in final temperature profile. Experiments and simulations have been carried out first with a linear gradient, demonstrating a correct operation (error 2.9 °C); then, a setup has been arranged to measure the temperature pattern on a 5-cm long section exposed to medical laser thermal ablation. Overall, the proposed method can operate as a real-time detection technique for thermal gradients over 1.5–5 cm regions, and turns as a key asset for the estimation of thermal gradients at the micro-scale in biomedical applications.

[EN] In this work, a linearly chirped fiber Bragg grating (CFBG) inscribed in a microstructured polymer optical fiber (mPOF) has been demonstrated for detecting temperature pro- files during thermal treatments. A CFBG of 10 mm length and 0.98 nm bandwidth has been inscribed in a mPOF fiber by means of a KrF laser and uniform phase mask. The CFBG has a high temperature sensitivity of -191.4 pm/°C). The CFBG has been used as a semi-distributed temperature sensor, capable of detecting the temperature profile along the grating length, for scenarios that account minimally invasive biomedical treatments. Two experiments have been designed to validate the CFBG tem- perature reconstruction, using a linear gradient, and a research- grade radiofrequency ablation (RFA) setup to apply Gaussian- shaped temperature spatial profiles. The result is that the higher sensitivity of the CFBG supports the detection of spatially non- uniform temperature fields by means of spectral reconstruction., This work was supported in part by Nazarbayev University, Research Council (ORAU project LIFESTART), in part by Fundacao para a Ciencia e Tecnologia (FCT)/MEC through National funds and when applicable co-funded by FEDER PT2020 partnership agreement under the project UID/EEA/50008/2013, and in part by the Research Excellence Award Programme GVA PROMETEO 2017/103 FUTURE MICROWAVE PHOTONIC TECHNOLOGIES AND APPLICATIONS. The work of C. Marques was supported by FCT through the fellowship SFRH/BPD/109458/2015.

We investigate the theory and feasibility of an inline spatially resolved temperature sensor, suitable for thermal ablation monitoring. The sensor is based o a chirped fiber Bragg grating (CFBG). The CFBG is modelled as a chain of Bragg gratings, each sensitive to local temperature variations. By using a combination of iterative and statistical optimization techniques, it is possible to use demodulate the CFBG, in case of a Gaussian-like spatial temperature profile. A feasibility test based on CFBG simulation shows that the CFBG returns error