Abstract: With the strehl ratio, power in the bucket, β parameter and beam widths taken as the characteristic parameters of laser beam quality in the far field, the influence of thermal effects in a beam control system and spherical aberration on the laser beam quality in the far field is studied. Detailed calculations are performed using a four-dimensional simulation code, and numerical results are given and analyzed physically. It is shown that the thermal effects in the propagation channel and positive spherical aberration give rise to an expansion of intensity distributions and reduce the focus ability in the far field, and degrade the beam quality. It is interesting that a suitable choice of the negative spherical aberration would result in a higher peak intensity and a better focus abilty at the geometrical focal plane than that of the spherical-aberration-free case, as the emissive power reaches a certain value; whereas a similar result is achieved at the real focal plane due to the focal shift as in the previous work. The physical interpretation is that there exists the nonlinear thermal blooming, which is illustrated with numerical examples.

Keywords: 
• high-power laser /
• thermal effect /
• beam control system /
• spherical aberration