Ultrahigh return loss LPFGs fabricated via femtosecond laser direct writing of ultrashort TFBGs.

Abstract

In this study, we propose, for the first time to our knowledge, the use of femtosecond laser direct writing to fabricate ultrashort tilted fiber Bragg gratings (TFBGs) for the development of long-period fiber gratings (LPFGs) with ultrahigh return loss (RL). A series of spectral transmission characteristics of the TFBG-LPFGs, constructed from ultrashort TFBGs, were systematically investigated, focusing on the effects of tilt angles and grating periods. As the tilt angle of the ultrashort gratings increases (from 0° to 10°), the RL of the LPFGs improves to 56.31 dB, representing a reduction of approximately 39.42 dB compared to the LPFG with a 0° tilt angle. Notably, the TFBG-LPFGs, composed of multiple 100 μm ultrashort TFBGs, were fabricated using the femtosecond laser direct writing technology, reducing the fabrication time for a single TFBG-LPFG to approximately 30 s, thereby significantly shortening the overall manufacturing time for LPFGs. High-quality LPFGs with LPFGs with ultrahigh RL (RL > 56.31 dB) demonstrate promising applications across areas such as fiber lasers, optical communications, biosensing, chemical detection, and structural health monitoring.