Objective Spectral beam combining (SBC) is an effective method to achieve a high-power, high beam quality fiber laser. In the SBC system, multi-channel incident lasers are arranged spatially and are combined into a single laser beam via an optical element. The transmitted laser power density in the SBC system is very high due to the high power and small beam diameter. In this case, the thermal blooming effect becomes a nonnegligible factor that influences the far-field beam quality. In addition, a narrow linewidth is required in SBC to eliminate the dispersion effect. As a result, nonlinear effects are easy to stimulate, e.g., stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), which generates a new wavelength laser. The new wavelength laser may cause enhanced atmospheric absorption and degrade the far-field beam quality. Atmospheric thermal blooming of high-power laser propagation has been studied extensively in the outer path (optical path in atmosphere) and inner path (optical path in the launching system). However, relevant studies of the inside optical path of a high-power laser have not been sufficiently thorough. Therefore, in this paper, the influence of thermal blooming on far-field beam quality in a high-power spectral beam combining system is studied. SRS in the incident narrow linewidth fiber amplifier is verified to be the dominant factor that induces thermal blooming in the beam combining system. In addition, N₂ injection into the combining system essentially eliminates the influence of thermal blooming on far-field beam quality, which can be considered an efficient suppression method.

Methods A three-channel spectral beam combining system is constructed with central wavelengths of 1064, 1072, and 1084 nm, respectively. Each incident laser can deliver 2 kW power. Taking the SRS effect into consideration, we employ 1400 W and 1800 W laser power in this experiment. First, the far-field beam patterns of each incident laser at 1400 W and 1800 W are tested. Then, the far-field beam patterns of the three-channel combined beam at different power are measured. By analyzing the measured results, the causing factor of thermal blooming in SBS system is confirmed. Then, the relationship between the optical path length and thermal blooming effect is investigated by adjusting the beam splitter position of a 1084 nm laser. Finally, suppression of the thermal blooming effect by N₂ injection is verified.

Results and Discussions When the power of each incident laser increases from 1400 W to 1800 W, apparent degradation is observed in the far-field beam pattern [Fig. 5(a)--(c)]. The peak intensity degrades severely, and the beam distribution disperses badly, which reduces the laser’s focusing property. For the combined beam, when the power reaches 2000 W to 3800 W, no obvious beam quality degradation is observed [Fig.5(d)]. The results demonstrate that the 1 μm signal laser does not cause obvious thermal blooming effect in the SBC system. The thermal blooming effect of the incident laser at 1800 W can be attributed to the SRS light. When the incident laser power is 1800 W, the SRS is measured as 100 W. In addition, the SRS wavelength covers from 1.1 μm to 1.3 μm for the 1 μm signal laser, which is in the strong absorption band of the H₂O molecule in atmosphere. Thus, air in the beam propagation path is heated to cause the thermal blooming effect. We found that optical path length has direct influence on the thermal blooming effect. When the optical path length increases from 100 mm to 450 mm, the thermal blooming effect becomes increasingly significant (Fig. 6). The far-field beam patterns of the 1084 nm laser at 1400 W and 1800 W after N₂ injection are measured (Fig. 7). The peak intensity increases significantly from 1400 W to 1800 W, and no beam quality degradation is observed, which means that the thermal blooming effect did not occur. This result demonstrates that N₂ injection can be an effective method to eliminate the thermal blooming effect in the SBC system.

Conclusions In this paper, we investigate the thermal blooming effect in an SBC system. By comparing the far-field beam patterns of a sub-beam and combined beam with identical power density, stimulated Raman scattering in the incident narrow linewidth fiber amplifier is verified as the dominant factor that induces thermal blooming in the SBC system. When the power density of Raman light reaches 180 W/cm ², the peak intensity of the far-field beam is reduced significantly, and the energy spreads. The influence of optical path length on thermal blooming is investigated. The focusing property of the far-field beam degrades gradually and finally spreads as the length increases from 100 mm to 450 mm. By injecting N₂ into the combining system to reduce H₂O content, the thermal blooming effect is effectively suppressed. The results presented in this paper are expected to facilitate optimization of the beam quality of high-power spectral beam combining.