  The Laser Guide Star Facility (LGSF), as the most important part of the
adaptive optics system of the large ground-based telescope, is aimed to
generate multiple laser guide stars at the sodium layer. Laser Launch Telescope
is employed to implement this requirement by projecting the Gauss beam to the
sodium layer with a small beam size in LGSF system. As the diffraction and
interference effects of laser's long-distance transmission, the conventional
optical design based on the geometrical optics mechanism cannot achieve the
expected laser propagation. In this paper, we propose a method to design
optical system for laser launch telescope based on the physical optics theorem
to generate an acceptable light spot at the sodium layer in the atmosphere.
Besides, a tolerance analysis method based on physical optics propagation is
also demonstrated to be necessitated to optimize the system's instrumentation
performance. The numerical results show that the optical design considering
physical optics propagation is highly rewarding and even necessitated in many
occasions, especially for laser beam propagation systems.
