Computer simulation of isoplanatic effects on image phase conjugate laser adaptive control systems

Abstract

Because of the distributed nature of the atmospheric turbulence in the imaging and propagation path of a laser beam correction system, it is expected that aberrations introduced by a lack of isoplanatism will limit the effectiveness of the image phase conjugate technique applied to compensate the effects of turbulence. This lack of isoplanatism arises because radiation from different parts of the extended imaged target experience different phase aberrations on their path to the common shared receiver/transmitter aperture. The results of the computer simulation presented show that these limits due to isoplanatism allow a very high degree of correction even with severe atmospheric turbulence. A very considerable increase in far-field irradiance is achieved by this adaptive correction technique, and the limit of achievable results is compared with the diffraction limit using a point source reference in the presence of the atmosphere. This diffraction limit is set by the performance of the wavefront measurement and control system. The simulation procedure used is as follows: The atmosphere is modeled by a discrete number of phase screens which are shown to give the same isoplanatic effects as a fully distributed atmosphere. Through these screens, radiation from several points over an extended region of the imaged object are propagated to the transmitter aperture. The incoherent addition of these and the resultant measurement by a wavefront sensor are simulated. The conjugate phase of the measurement is then applied to an outgoing spherical laser wave, and this wavefront is then propagated through the phase screens to its nominal focus. Both Strehl ratios and encircled energy are calculated as measures of the effectiveness of the technique. The same simulation procedure may be applied to evaluating the technique in the presence of thermal blooming, as well as atmospheric turbulence.