Ultra-broadband fiber filter based on chirped highly localized fiber Bragg gratings

Broadband optical filters are highly useful tools in many fields, such as supercontinuum light sources, astronomical observations, and optical coherence tomography. However, conventional broadband filters have limited bandwidth and relatively high insertion loss (IL). In this study, an ultra-broadband fiber filter based on a chirped highly localized fiber Bragg grating (CHLFBG) is fabricated via a femtosecond laser point-by-point inscription technique, which is demonstrated for the first time to the best of our knowledge. This filter provides optical suppression exceeding 90% (-10 dB) over a bandwidth of larger than 500 nm, which extends across the O + E + S + C + L + U bands. Meanwhile, the IL is less than 1 dB, and the fabrication time is within a few minutes. The operating bandwidth can be customized by adjusting the period of CHLFBG. Additionally, increasing the length of the CHLFBG can significantly improve the suppression efficiency, exceeding 99.99% (-40 dB). Moreover, the CHLFBG is insensitive to temperature, axial strain, and bending, making it stable and reliable in practical applications.