A comprehensive study of the interactive effects of multidither adaptive optics (COAT) systems with the spurious signals induced by speckle modulations is presented. An analysis based on a statistical model of a COAT system is developed in order to predict convergence levels in the presence of such modulations. A computer simulation study of these effects is also presented. Good agreement is found between the data gathered in both of these studies. In order to further corroborate these results, experimental data is then presented. These data were gathered using a laboratory model of a COAT system interacting with a real target that produced speckle modulations.

The measured characteristics of two different deformable mirror designs are presented. Each mirror is uncooled and has 37 piezoelectric actuators. The mirrors have comparable frequency response usable to over 20 kHz, but possess quite different actuator influence functions. One exhibits an exp(−αr1.5) characteristic and the other has an exp(−γr2.5) characteristic. The latter mirror is used in an 18-channel multidither adaptive optical system to perform both phase dither and phase correction functions. The measured characteristics of this system’s performance include a 2 ms convergence time and effective turbulence compensation. Evidence is presented of “2Nπ” servo ambiguities, a behavior which reduces an adaptive system’s overall phase compensation performance. The performance of the deformable mirror adaptive system is compared to an equivalent segmented (piston) mirror system.

Multidither coherent optical adaptive techniques (COAT) are at the present the leading adaptive optics system approach for high-energy laser applications. Multidither COAT has been demonstrated effective in compensating turbulence1 and thermal blooming2 with realistic targets. Previous experimental systems,1-3 however, have utilized segmented transmitter arrays that apply the adaptive phase correction in a stepwise manner across the transmitting aperture. Practical high-power coat systems will use continuous-surface deformable mirrors for both the dither and phase-correction functions, since the mirror surfaces require cooling.

The use of multidither coherent optical adaptive techniques (COAT) is rapidly gaining acceptance as a useful tool for removing phase distortions that are introduced on a transmitted laser beam by its associated optical system or by the atmosphere. Earlier multidither COAT system demonstrations1,2 have used planar segmented-aperture piston mirrors for both the phase dither and phase correction functions. High-power laser applications will require continuous-surface mirrors for both the dither and correction functions. The operating characteristics of COAT systems employing deformable mirrors are expected to be significantly different from systems that utilize segmented, pistonlike mirrors. In particular, the deformable mirror can introduce significant coupling between control channels, it can point the entire beam rather than merely changing the transmitted beam phase front in a stepwise manner, and it can cause the servo system to converge with detrimental 2π phase ambiguities in the error signal.

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.

The use of multiple transmitter beams is shown to significantly increase the peak focal-plane irradiance that can be achieved in the presence of thermal blooming. Computer simulation studies of the beam propagation problem show over a factor of 2 increase in the irradiance of a single beam and a factor of 9 increase when three coherent beams are focused on the same target spot. Preliminary experimental results with three mutually noncoherent, nonoverlapping beams are in qualitative agreement with the computer simulation.

The effects of speckle on the performance of adaptive optical (COAT) systems is examined, with particular emphasis on multidither COAT systems. Experimental, analytical, and computer simulation data are presented that are in mutual agreement and that define the performance of multidither systems in the presence of speckle effects. In general, the performance of coherent-light adaptive systems can be degraded by speckle-induced effects. The severity of the degradation depends on the target parameters (surface roughness, glint structure, geometry, motion) and the adaptive system parameters (wavelength, bandwidth, servo details).

: Coherent optical adaptive techniques (COAT) can be applied to overcome the deleterious effects of atmospheric turbulence. The report covers the design phase of an experimental program to design, fabricate and evaluate an eighteen-element, self-adaptive, optical phased array. In addition, a computer simulation program developed to aid in system design and performance prediction is also described. Results are presented on preliminary experiments performed with an existing seven-element COAT system. Further experiments were performed with different piezoelectric ceramic phase-shifter configurations and with improved servo control electronic systems and are described here. Techniques for offset pointing of the phased array are discussed. A flexible phasor matrix structure is described in which radiating array patterns can be easily changed. Atmospheric characterization measurements performed on the 94 meter test range are described. The design of a dynamic multiglint target system is given.

The atmospheric structure constant, CN2, has been measured by use of three instruments, one optical and two thermal. The data show very good correlation (ρxy = 0.88), but are related by a consistent scale factor: CN (optical) = 0.34 × 10−8 cm−2/3 + 1.45CN (thermal). Several effects are discussed-that may explain these results.