Investigation on the thermal blooming effect in a high power spectral beam combining fiber laser system

We demonstrate the influence of the thermal blooming effect on the far-field beam quality in a seven-channel spectral beam combining system. Stimulated Raman scattering in the incident narrow-linewidth fiber amplifier is verified to be the dominant factor that induces thermal blooming in the beam combining system. When the power density of Raman light reaches only 180W/cm², the peak intensity of the far-field beam reduces severely and the beam distribution profile spreads. We reveal that H₂O content in the atmosphere has a positive relationship with the thermal blooming effect and study the influence of the humidity on the thermal blooming effect. The influence of the optical path length on the thermal blooming effect is also revealed. The result shows that the focusing property of the far-field beam degrades gradually as the optical path length increases from 100 to 450 mm. The results are conducive to optimize the beam quality of spectral beam combining.