Bootstrap beacon creation for overcoming the effects of beacon anisoplanatism in laser beam projection system

The task of delivering a sufficient level of airborne laser energy to ground based targets is of high interest. To overcome the degradation in beam quality induced by atmospheric turbulence, it is necessary to measure and compensate for the phase distortions in the wavefront. Since, in general, there will not be a cooperative beacon present, an artificial laser beacon is used for this purpose. In many cases of practical interest, beacons created by scattering light from a surface in the scene are anisoplanatic, and as a result provide poor beam compensation results when conventional adaptive optics systems are used. In this work we present three approaches for beacon creation in a down-looking scenario. In the first approach we probe the whole volume of the atmosphere between transmitter and the target. In this case the beacon is created by scattering an initially focused beam from the surface of the target. The second approach uses an uncompensated Rayleigh beacon at some intermediate distance between the transmitter and the target. This method allows compensation for only part of the atmospheric path, which in some cases may provide sufficient performance. Lastly, we present a novel technique of "bootstrap" beacon generation that allows achieving dynamic wavefront compensation. In this approach a series of compensated beacons is created along the optical path gradually probing the atmospheric turbulence with compensated beacons at increasing distance from the aperture. The performance of these techniques is evaluated by using the average Strehl ratio and the radially averaged intensity of the beam falling on the target plane. Simulation results show that under most turbulence conditions of practical interest the bootstrap technique provides better compensation performance compare to the other two techniques. Power requirements for Rayleigh beacon generation was also investigated. It was found that a laser capable of generating on the order of 3 J of laser pulse energy per pulse is suitable for the laser projection system described in this work.