Fiber Bragg gratings (FBGs) have important applications in high-power fiber lasers. FBGs can act as cavity mirrors for fiber oscillators, playing a role in frequency selection and coupling output and promoting the development of fiber oscillators toward all-fiber fiber structure. In addition, special FBGs, such as chirped and tilted FBGs (CTFBGs), can act as all-fiber filters to suppress stimulated Raman scattering (SRS) in high-power fiber lasers, improving the output power and spectral purity of fiber lasers. The power handling capability is the key performance index for mirror FBGs and CTFBGs. The traditional fabrication method for mirror FBGs and CTFBGs is the ultraviolet (UV) laser phase mask method; however, hydrogen loading and thermal annealing treatment are required in this method, which leads to a long FBG fabrication period. In addition, if thermal annealing treatment is not complete, the residual hydrogen molecules in the FBG would absorb high-power lasers, limiting the power handling capability of FBGs. With the development of femtosecond (fs)-laser inscribing technology, a new scheme has emerged for fabricating high-power CTFBGs. An fs-laser can directly inscribe a CTFBG in the fiber; hence, the fiber does not need hydrogen loading and annealing treatment, which not only shortens the fabrication period but also avoids the heating caused by incomplete annealing. Moreover, CTFBGs written by fs-lasers have better tolerance to the temperature increase caused by high-power lasers.
A CTFBG is written using fs-laser phase mask technology. Figure 1 shows the spectrum of the CTFBG. The filtering band central wavelength of the CTFBG is 1137 nm, with a 3-dB bandwidth of 8.5 nm and a filtering depth of approximately 15 dB. The homemade high-power fiber amplifier with a maximum output power of 10 kW is used to test the CTFBG, as shown in Fig. 2.
Figure 3(a) shows the output spectra at maximum output powers with and without the CTFBG. The CTFBG has a maximum filtering depth of 10 dB and a filtering width of 12 nm. Figure 3(b) shows the output power variation with and without the CTFBG, as well as the output laser beam profile. After inserting the CTFBG, the output power decreases from 10170 W to 10090 W, and hence the insertion loss of the CTFBG is 0.03 dB. The output beam quality degrades slightly, and the beam quality factor (M2) increases from 3.35 to 3.46. The CTFBG with a cooling package has a small thermal slope of 2.4 °C/kW, as shown in Fig. 3(c).
A CTFBG written by a fs-laser is introduced at the output end of a 10-kW fiber laser to test its power handling capability. The CTFBG has an insertion loss of 0.03 dB and a small thermal slope of 2.4 °C/kW. This study shows that the fs-laser-written CTFBG has excellent power handling capability, which will further promote the development and application of CTFBGs.
