Demodulation of overlapping FBG spectra for temperature sensing using a free-running dual-comb fiber laser.

Demodulating changes in the Bragg wavelength of wavelength division multiplexed (WDM) fiber Bragg grating (FBG) arrays presents a practical approach for multi-point temperature sensing in extreme conditions. Traditional methods struggle with overlapping FBG spectra, requiring a greater wavelength spacing between gratings to cover a wider temperature range. Employing a single cavity dual-comb laser rather than a conventional broad-spectrum source makes it feasible to demodulate overlapping FBG spectra for temperature sensing through a free-running bi-directional mode-locked fiber laser. The radio frequency (RF) spectra of the FBGs are obtained for each dual-comb interferogram, even amidst spectral overlap. The WDM array comprises ten gratings with only 0.8-1 nm spacing. Five gratings were maintained at 20 °C, and the others were heated from 20 °C to 300 °C, resulting in substantial spectral overlap. The demodulation results demonstrated a linear temperature-dependent relationship for each heated FBG, with sensitivities ranging from 12.2 to 12.6 pm/°C. Unlike traditional demodulation techniques, the dual-comb measurement technology effectively tackles the issue of wide wavelength spacing, enhances the potential for more wavelength division multiplexers, and shows great promise in high-temperature measurement applications.