High reflectivity, ultraflat-spectrum chirped fiber Bragg grating written using low energy UV femtosecond pulses

Chirped fiber Bragg gratings (CFBGs) have been extensively used in applications such as ultrafast lasers, fiber sensors, and fiber communications. This work presents a comprehensive investigation of CFBG written using a 257 nm femtosecond laser and a linear chirped phase mask. The CFBG, written on a hydrogen-loaded polarization-maintaining fiber, exhibited a center wavelength of 1031.7 nm and a bandwidth of 11.97 nm. The CFBG achieved an average reflectivity of 98.2 % within its bandwidth, demonstrating an ultrahigh flatness of the reflection spectrum with a low spectral reflectivity variance of 0.00003 (σ2). The high group velocity dispersion (GVD) of 60.7 ps2 signifies its potential in fiber chirped pulse amplification systems and dispersion compensation system. Meanwhile, the fs-laser + PM writing method shows advantages of high efficiency and low energy consumption (as low as 2 nJ), owing to the high photosensitivity of the fiber to the UV band, which holds significant promise in fiber grating inscription.