Fiber lasers by laser diode (LD) pumping directly have power scaling due to stimulated Raman scattering (SRS) and transverse mode instability (TMI). Beam combining is an effective way to increase the output power of fiber lasers. Spectral combining is one of the structurally simple and proven effective means. Dichromatic mirror spectral combining technologies employing broad-spectrum laser sources can achieve high efficiency, high beam quality, and high reliability for medium output power. This technology can maintain the beam quality based on flexible structures and low costs. Spectral combining employing dichromatic mirrors requires expanding the central wavelength range. Currently, the output power in short-wavelength fiber lasers cannot meet the requirements of spectral combining. Ytterbium-doped fibers (YDFs) can provide continuous-wave (CW) high-power output mainly focusing on 1030-1100 nm. Short-wavelength fiber amplifiers are susceptible to the amplified spontaneous emission (ASE) effect. ASE and SRS impose serious constraints on the power scaling of short-wavelength fiber lasers. In this paper, a fiber amplifier of 3.5 kW with near-single-mode emitting at 1050 nm is demonstrated. The SRS and ASE suppression methods in short-wavelength fiber lasers are proposed.

The fiber laser amplifier of 1050 nm is constructed by using large-mode-area (LMA) double-cladding YDF in the counter-pump scheme. The core and cladding diameters of the YDF are 20 μm and 400 μm, respectively. The length of gain fiber is optimized to mitigate ASE in short-wavelength fiber amplifiers. The threshold of SRS is raised by optimizing the temporal stability of the seed. The wavelength-stabilized LDs of 976 nm are utilized as the pump source. In order to ensure beam quality, YDF is coiled on a water-cooled plate with a bending diameter of about 8 cm. Employing LDs of 976 nm in the counter-pump and choosing appropriate diameters of YDF can aid TMI mitigation.

Compared with output characteristics of different YDF lengths (Fig. 2), short YDF is beneficial to mitigate ASE. The 4.8 m-long-YDF of seed can suppress ASE and guarantee the absorption of pump power. The SRS threshold of the fiber amplifier adopting the main oscillator is related to temporal stability. The temporal signal stability of the seed is affected by the seed power. The output spectra of the fiber amplifier are recorded at different seed power (Fig. 3). From the spectra at an output power of 2 kW [Fig. 3(a)], the signal-to-Raman noise ratios (SRNRs) corresponding to the seed power of 133, 183, and 232 W are 31.78 dB, 44.66 dB, and 49.36 dB, respectively. As the seed power gets higher, the intensity of SRS gets lower. The spectra at the output power of 3 kW are compared with the seed power of 183 W and 232 W [Fig. 3(b)]. The corresponding SRNRs are 29.76 dB and 34.10 dB, respectively. From the comparison results, the SRS threshold is positively proportional to the seed power. During the increase in seed power, the bandwidth of 3 dB gradually broadens. The broadening of seed spectra indicates that the number of longitudinal modes gradually increases, which leads to an increase in temporal stability. Figure 4(a) shows output power and conversion efficiency corresponding to different pump power. The optical-to-optical conversion efficiency is 86.3% at the maximum output power of 3520 W. The spectrum at the maximum output power is shown in Fig. 4(b). The center wavelength is 1050 nm with an SRNR of 27.6 dB. Figure 4(c) shows the time-domain signal standard deviation (STD) at different output power. With the increase in the output power, the STD always remains at the level of about 0.003 without any features of TMI. Figure 4(d) shows the beam quality of the laser at the highest output power. The beam quality factors in X and Y directions are about 1.33 and 1.25, respectively.

Short-wavelength high-power fiber amplifiers have important applications in fields such as spectral combining. Limited by ASE and SRS, it is difficult to achieve higher power. In this manuscript, a monolithic fiber amplifier of 1050 nm is demonstrated in a counter-pump based on an LMA gain fiber of 20/400 µm. The suppression of ASE in 1070-1080 nm is achieved by shortening the gain fiber length. The temporal stability of the seed is optimized to mitigate the SRS effect. At the maximum output power of 3.5 kW, the optical-to-optical conversion efficiency is 86.3%, and the bandwidth of the output laser spectrum of 3 dB is 4.07 nm. The beam quality factors M² in the X and Y directions are about 1.33 and 1.25, respectively. The intensity of Raman Stokes light is about 27.6 dB lower than the signal light intensity. The fiber amplifier of 3.5 kW and 1050 nm meets the needs of spectral combining and has great potential for application in the field of high-power lasers.