Numerical Analysis and Design of Active Waveguide Bragg Gratings for High-Contrast Thermal Sensing and Tunable Linear Thermal Gradient Threshold Detection.

Active waveguide Bragg gratings (AWBGs) combine the narrowband reflectivity of Bragg gratings with the optical gain of rare-earth-doped media, representing a versatile platform for monolithic lasers and amplifying reflectors. The lasing regime in these photonic structures is governed by specific design and pumping parameters, exhibiting critical thresholds below which laser emission is suppressed. This work separately explores the potential of AWBGs as high-contrast thermal sensors and threshold-triggering switches. The AWBG thermal response is numerically analyzed by means of the Transfer Matrix Method. Under uniform heating, the Bragg wavelength shift shows sensitivities of 12 pm/°C. The resulting laser peak, with only 1 nm bandwidth and several orders of magnitude above the noise floor, significantly improves the peak-to-background contrast of equivalent passive designs. Furthermore, the impact of linear longitudinal thermal gradients is evaluated. The thermal chirp induced reduces the grating's efficiency, potentially quenching the laser emission if the system is biased near its operating threshold. Our numerical results indicate that AWBGs can function as threshold-triggering thermal gradient detectors where the detection threshold can be adequately chosen at the design stage for a detection range up to approximately 12 °C/mm, and selectively tuned through pump power regulation once fabricated. These findings, obtained under the numerical approximations considered, position AWBGs as a promising solution for advanced thermal monitoring and optical safeguarding applications, pending experimental validation.