Broadband and High-Filtering-Efficiency Band-Rejection Filters Fabricated Using Femtosecond Laser Line-by-Line Technology

We have proposed and experimentally demonstrated a broadband-adjustable band-rejection filter based on a chirped and tilted fiber Bragg grating (CTFBG). The CTFBG is fabricated using femtosecond laser line-by-line technology. By precisely controlling the tilt angle, chirp rate, and pitch of the CTFBG, the filter's parameters such as wavelength, bandwidth, and efficiency can be flexibly customized. Through increasing the length of CTFBG, the large bandwidth and ultra-high filtering efficiency at the specific wavelength can be realized simultaneously. The proposed filter exhibits adjustable bandwidth ranging from 51 nm to 112 nm, low insertion loss (<1 dB), high filtering efficiency (up to >99.99%), and negligible back-reflection (<-30 dB). Moreover, the filters are insensitive to temperature, axial strain, and bending variations, which demonstrates their excellent spectral stability and robustness against disturbances.