Femtosecond Cascade Chirped and Tilted Fiber Bragg Gratings for Raman Filtering

Objective The chirped and tilted fiber Bragg grating (CTFBG) is an important component for filtering Raman light in high- power fiber laser systems. The filtering bandwidth and depth of CTFBG determine the filtering effect, so it is necessary to increase its filtering bandwidth and depth. The tandem inscription method can effectively increase the bandwidth by cascading two CTFBGs with different tilted angles. However, the tandem inscription method based on the traditional ultraviolet laser phase mask technology has the following shortcomings. 1) The fiber needs to be processed by hydrogen loading and heat annealing before and after the CTFBG inscription, respectively, which increases the fabrication time and cost. 2) When cascade CTFBGs with different tilted angles are inscribed, it is necessary to change the tilted angle of the phase mask and realign the inscription system, which increases the inscription complexity. 3) The Bragg reflection bandwidth of cascade CTFBG will also increase, which may provide feedback to Raman light and affect the Raman filtering effect of CTFBG. The proposed femtosecond laser inscription system for cascade CTFBG in this paper can effectively overcome the above shortcomings. Methods The femtosecond laser arrives at the cylindrical lens and the chirped phase mask in turn and finally forms interference fringes on the fiber core. The tilted grating plane is formed by oblique scanning of the fiber via a piezoelectric stage. At the same time, the femtosecond laser scans the chirped phase mask along the fiber axis, thereby increasing the length of the grating and introducing a larger chirp. When the femtosecond laser scans along the fiber axis, the grating planes with different tilted angles can be formed by changing the amplitude of the piezoelectric stage, thereby realizing the inscription of cascade CTFBGs. The schematic of the grating structure of the cascade CTFBG is shown in Fig. 1, which consists of sub-CTFBG. and sub-CTFBG. with different gratings. Results and Discussions Figs. 2(a) and 2(b) show the spectra of single- stage CTFBG and cascade CTFBG, respectively. The tilted angle of the former is 6. 4 degrees with a grating length of 20 mm. The latter consists of two sections of CTFBG with different tilted angles, and its grating length is 20 mm. The bandwidth of cascade CTFBG is wider than single-stage CTFBG, and the filtration depth can be maintained greater than 20 dB. In order to test the performance of cascade CTFBG for filtering Raman light, a test system is built (Fig. 3). The test source is a continuous- wave high- power fiber oscillator of 1080 nm with a maximum output power of about 1. 5 kW and output fiber length of about 18 m. The output spectra measured without and with cascade CTFBG at different output powers are shown in Figs. 4(a) and 4(b), respectively. Raman light is almost completely filtered out by cascade CTFBG at the maximum output power. Conclusions Here, a CTFBG is fabricated by the femtosecond laser tandem inscription method, and the filtering bandwidth and depth of which are about 15. 2 nm and greater than 20 dB, respectively. By introducing the CTFBG at the output end of a high- power fiber laser long-distance transmission system of 1080 nm, the output spectrum without Raman light is realized, which greatly improves the purity of the output laser. This work provides a method for the fabrication of the wideband CTFBG and demonstrates its Raman filtering effect, which is of significance for the development and application of CTFBG.