High-power fiber oscillators have significant applications in industrial processing and other fields. Fiber Bragg gratings (FBGs) are key components of high-power fiber oscillators. On the one hand, FBGs can act as cavity mirrors of high-power fiber oscillators to select a signal wavelength and couple output signal power. On the other hand, FBGs with special designs such as chirped and tilted fiber Bragg gratings (CTFBGs) can be used to suppress stimulated Raman scattering (SRS) in high-power fiber oscillators. Generally, the traditional approach for fabricating these two types of FBGs is the ultraviolet laser (UV) phase-mask method. However, hydrogen-loaded and thermally annealed treatments are required. When annealing is not thorough, the residual hydrogen and hydroxyl groups in the FBGs will absorb lasers to generate heat, which is the main factor limiting the power FBGs can withstand. To date, the maximum handling powers of mirror FBGs and CTFBGs written using UV lasers are 8.0 kW and 4.3 kW, respectively. The development of femtosecond laser inscription technology provides a promising new method for the inscription of FBGs. FBGs can be directly inscribed into fibers without hydrogen loading. Thus, the heating generated by the hydrogen and hydroxyl groups in FBGs can be avoided. Currently, the handling power of a CTFBG written using femtosecond lasers exceeds 10 kW. However, the maximum output power of the all-fiber oscillator based on femtosecond-laser-written FBGs is 8 kW due to the limitations of transverse mode instability (TMI).

FBGs and CTFBGs used in cavity mirrors are written using the femtosecond-laser phase-mask method. Figure 1(a) shows the reflection spectra of the high-reflectivity FBG (HR FBG) and low-reflectivity FBG (LR FBG). The 3-dB bandwidths of the HR FBG and LR FBG are 4.0 nm and 2.1 nm with reflectivities of more than 99% and approximately 6%, respectively. Figure 1(b) shows the CTFBG spectrum. The central wavelength of the transmission spectrum is 1135 nm with a 3-dB bandwidth of approximately 18 nm and maximum depth of 15 dB. Figure 2 shows the setup of the fiber oscillator. The oscillator employs a counter-pumping scheme with an active 30 μm /600 μm ytterbium-doped fiber (YDF) and pump source of 969 nm+982 nm dual-wavelength diode laser (LD). The dashed box in Fig.2 indicates the CTFBG, which is inscribed on the side of the LR FBG and located in the resonator to ensure the oscillator system is compact and stable.

Figure 3(a) shows the output spectra at maximum output powers. Due to the suppression of SRS by the CTFBG, the Raman light intensity at 1135 nm decreases by approximately 16 dB. In addition, the TMI threshold of the oscillator increases from 8250 W to 8700 W with the CTFBG, as shown in Fig.3(b). Figure 3(c) shows the changes in the output power. The slope efficiency decreases from 85.4% to 83.4% with the CTFBG. Therefore, the insertion loss of the CTFBG is approximately 2%. Despite the decrease in slope efficiency, the output power increases from 8910 W to 9050 W due to the suppression of the SRS and the increase in the TMI threshold.

This study demonstrates an all-fiber oscillator with maximum output power. An all-fiber oscillator is constructed based on femtosecond-laser-written FBGs, and femtosecond-laser-written CTFBGs are used to suppress the SRS, ultimately achieving a 9-kW laser power output.