Quasi-single-mode signal output of sapphire fibre Bragg grating based on single mode fibre offset coupling measurement

Abstract In this study, we demonstrate a method for achieving quasi-single-mode signal output in sapphire fibre Bragg grating (SFBG) based on single-mode fibre offset coupling measurement. The few-mode spectral output of SFBG was obtained by directly coupling a single-mode silica fibre to the SFBG. The effects of offset coupling distance on the side-mode suppression ratio (SMSR), peak reflectivity and wavelength of the SFBG reflection spectrum were studied. A significant improvement in SMSR was observed during the offset coupling process. The optimized spectrum achieved an SMSR of 3.5 dB, a full width at half maximum of only 0.09 nm, and a signal-to-noise ratio of 10 dB. This method offers a straightforward and practical solution for enhancing the spectral quality of SFBG without requiring complex structural modifications to the fibre.