Sub-Nanometer Thermal and Spectral Stability of Fibre Bragg Gratings in Dual-Mode Optomechanical Sensors

Precise wavelength stability and narrow spectral linewidths are essential for enhancing the performance of optical sensors in biomedical diagnostics and environmental gas detection. This work presents a detailed investigation of fibre Bragg gratings (FBGs) integrated within a dual-mode laser sensor system based on intra-cavity absorption spectroscopy. The Bragg wavelength stability was characterised over extended periods, demonstrating fluctuations of up to ±0.3 nm without thermal insulation, which were significantly reduced to ±0.1 nm with the implementation of styrofoam enclosures. Furthermore, the laser’s full width at half maximum (FWHM) was analysed under varying injection currents, attaining a minimum linewidth of 0.06 nm, thereby improving spectral selectivity. To the best of our knowledge, this is the first systematic investigation of long-term Bragg wavelength stability and FWHM behaviour in a dual-mode intra-cavity absorption spectroscopy configuration, providing quantitative stability limits directly relevant to the design of highly selective gas sensors.