Atmospheric turbulence compensation using coherent optical adaptive techniques

Abstract

Measurements using an experimental, visible wavelength, eighteen-element, multidither, self-adaptive, planar, optical phased array have been made on a well-characterized outdoor 100-m propagation range. The measurements have proved that this type of COAT system can remove most of the beam distortions produced by atmospheric turbulence and by fixed optical system errors. The system has demonstrated the ability to form a beam with a nearly diffraction-limited peak intensity for turbulence levels characterized by structure constants (C(N)(2)) ranging from 1 x 10(-16) cm(-2/3) to 6 x 10(-14) cm(-2/3). Convergence times for the COAT system range from 1.5 msec to 3.0 msec for a servo system with a 500-Hz unity gain bandwidth. Spectral analysis of the COAT correction signals indicates, however, that only a 50-Hz bandwidth is required for correction to within tenth-wave residual wavefront errors for static targets, even in strong turbulence. The experimental phase error spectra agree well with theoretical calculations that use a Von Karman spectrum for the refractive index fluctuations. Multiple glint discrimination and tracking of the strongest glint in a multiple glint target are demonstrated in high turbulence. Good target tracking is observed at rates up to 14 mrad/sec. The convergence stability of the COAT system is good, limited only by the inability of planar, stepwise phase control to remove atmospheric beam wander and scintillation effects. Receiver aperture size has had no appreciable effect on system performance except in multiple glint cases where the glints are within 2-3 dB in net reflectance.