Femtosecond Laser Edge-Constrained Composite Apodization Method for Simultaneously Enhancing the Bandwidth and Side Mode Suppression Ratio of FBG

Conventional fabrication methods for fiber Bragg grating (FBG) struggle to simultaneously enhance the bandwidth and the side mode suppression ratio (SMSR), which limits their applications in photonic systems and sensing systems. We propose a femtosecond laser edge-constrained composite apodization method for simultaneously enhancing the bandwidth and SMSR of FBGs. Building on the multi-layer composite apodization technique, this method simultaneously controls the position and length of the writing lines to constrain the coupling strength at both edges of the FBG, thereby expanding bandwidth while improving SMSR. The writing results show that, compared with the unconstrained case, the SMSR of the edge-constrained composite apodized FBG (ECAFBG) improves by approximately 4.7 dB while maintaining an essentially unchanged bandwidth of 3.7 nm and undegraded reflectivity, with a length of only 0.7 mm. Dynamic strain and intensity demodulation is applied to the ECAFBG. The results demonstrate that the ECAFBG is capable of dynamic strain measurement over a relatively large range. The proposed method provides an effective approach to simultaneously achieve short-length FBGs with wide bandwidth and high SMSR and has potential applications in compact laser and sensing devices.