Simulation of the Diffraction Propagation of the Output Beam of Laser Expanding System

In order to study the effect of the laser expanding system on the control of the beam transmission in laser propulsion technology, the expanding system and the annular incidence Gaussian beam due to the telescope's center obscure were founded by the optical design software named ZEMAX. By the method of Physical Optics Propagation, the output beam's characteristics including the waist, the waist position, the far field divergence, and the energy distributing characteristics were obtained; rules of the image aberration's effect on the output beam's characteristics and the expanding ratio were analyzed. Then, taking encircled power and uniformity of the output beam for standards, the relation curve of the optimal expanding ratio and the launch distance was achieved. The results reveal that the width of the output beam at a distance of 100 m is less than 250 mm, a maximal collimation distance can be about 2 km, and the energy focus degree and the uniformity of the far field laser are also quite well. These indexes can satisfy the demands of transmission and control of laser beam in laser propulsion technology. Firstly, this paper found the models of the expanding system and the incidence beam. Then, by the method of Physical Optics Propagation, the process of transmission and gradual change of the output laser beam is studied, the characteristic parameters of space and energy distribution were analyzed, the optimal distance between the primary and secondary mirror at definite launching distance were acquired. Finally, taking encircled power(PIB) and uniformity of the laser beam for standards, the optimal expanding ratio at some distance was achieved. The results can be used in the transmission and control of the laser beam in laser propulsion technology.