Abstract

The propagation of ground-based high energy lasers through the atmosphere leads to heating of the air due to molecular and aerosol absorption.¹ This heating changes the local refractive index and the rays diverge from the center of the beam leading to thermal blooming. While the direct energy loss from absorption is small, the phase distortion from thermal blooming degrades the beam quality. To compensate for thermal blooming, one has to correct the beam phase at the transmitter. This is achieved by a closed loop adaptive optics. The conjugate of the incoming beacon phase is imposed on the outgoing laser beam. The phase compensation leads to instability, which places a limit on the critical HEL power (P_c) that can be transmitted through the atmosphere at a given beam diameter (D). The scaling law, P_c ~ D^b is of critical importance for the design of a beam director. The role of the refractive index and velocity turbulence of the atmosphere have been studied. Without velocity turbulence the scaling law is P_c ~ D, and with velocity turbulence the result is P_c ~ D^1.5.

© 1989 Optical Society of America