Coating design by optimizing field penetration to minimize phase variations for multi-wavelength adaptive optics

Multiwavelength adaptive optic systems can experience phase errors when corrections measured at one wavelength are applied to others. Such errors originate from several sources, but poor optical coating design can play a major role since the phase dependence of any coating varies greatly with wavelength, angle, and thickness. To address these errors, a multilayer coating design is proposed where the effective electric field penetration depth into the coating is similar at all design wavelengths. Using this method, a two-wavelength coating is designed for 1070 and 1550 nm and compared to a standard two-wavelength high-reflectivity coating consisting of stacked Bragg reflectors. The standard stacked Bragg reflector design induces up to 173.0 nm of phase error across the wavelength band that reflects off the more deeply buried part of the multilayer. Alternatively, the two-wavelength equal field penetration design maintains a high reflectivity of 99.999%, while only inducing phase variations of 28.6 nm for both target wavelengths.