DEPS Abstract

Guest >> Sign In

DIRECTED ENERGY PROFESSIONAL SOCIETY

Abstract: 25-Symp-155

UNCLASSIFIED, PUBLIC RELEASE

High Reflectivity Coatings to Reduce Phase Variation Errors for Multi-Wavelength Adaptive Optics

The optics used in directed energy systems must often function across a number of wavelength bands. If a system uses adaptive optics, then any wavefront correction measured for one wavelength must apply to all others in the system. Unfortunately, there are a number of factors that can cause a wavefront correction measured at one wavelength to be inappropriate for others. These include turbulence, coating design, chromatic aberration, split beam paths, and others. Previous work has discussed how dielectric coating reflectors may cause multiwavelength correction errors and how coating design can alleviate the issue. In this paper, we present interferometric measurements that support the effectiveness of these designs.
As high reflectivity coatings deposit onto a substrate, small errors in layer thicknesses typically accrue, often with minor center-to-edge thickness variations that are difficult to avoid even in high precision systems. In addition, the phase variation (slope) with wavelength can also vary from wavelength band to wavelength band. The combination of these two effects can cause the coating to produce differences in effective optical curvature or other aberrations between wavelength bands, which translates to different needed wavefront corrections in different bands. To test the effects of this issue on a wavefront, we deposited two plasma-enhanced chemical vapor deposition (PECVD) coatings. One of these coatings was a standard stacked Bragg reflector designed for high reflectivity at 1070nm and 633nm. The second was an optimized coating designed to minimize phase variations from one wavelength band to another by redesigning the electric field penetration into the coating to be approximately equal at both wavelengths. Using an internally-developed two-wavelength interferometer to measure center-to-edge wavefront variations in each coating, we found that the optimized multilayer design outperformed the stacked Bragg reflector in terms of phase variations. Commercial Zygo interferometers indicate that the optimized design shows that the optimized design reduces the phase variation between 633nm and 1070nm by slightly more than a factor of six, which is consistent with theory.

UNCLASSIFIED, PUBLIC RELEASE