Ultra-high-SMSR femtosecond apodized FBG based on variable pulse number inscription technology

This paper reports a novel femtosecond direct-writing apodized fiber Bragg grating (FBG) technology based on variable pulse number control. By combining coupled-mode theory and the transfer matrix method, a theoretical model for apodized FBGs was constructed. Simulations revealed the relationship between the maximum pulse number and the SMSR of the FBG, demonstrating that the variable pulse number technique can precisely control the refractive index modulation depth of the FBG, achieving an ultra-high-SMSR. Experimental results show that the apodized FBG achieves an SMSR of up to 29.18 dB, which represents the highest level reported to date.