High fluence oblique incidence low retardation multilayer dielectric mirror

High reflector coatings are typically constructed of quarter-wave optical thickness (QWOT) multi-layers of alternating high and low refractive index materials. An absentee half-wave optical thickness (HWOT) low refractive index layer, usually SiO₂, is typically used for the outermost layer of the coating to improve laser damage resistance. Out of plane oblique angle mirrors typically have an additional polarization retardation minimization requirement to maintain the polarization purity of the laser. For monochromatic lasers with a small angular incident range, this can be accomplished with precise layer thickness control for proper angular centration of the coating because of the narrow angular range for low polarization retardation. Reducing the SiO₂ overcoat thickness from HWOT to QWOT significantly increases the spectral bandwidth or angular range for low polarization retardation, however, the standingwave electric field is significantly increased, leading to lower laser resistance. The reflectivity is also reduced leading to the need for extra layers. A three-material hybrid QWOT coating design, with a high fluence medium refractive index material, can be fabricated with simultaneous high laser damage resistance and low phase retardation over a moderately wide spectral or angular range. Finite -difference time-domain simulations explore the impact of different size nodular defects on light intensification in each of the different coating materials for this hybrid design approach. A 2-5x increase in LIDT occurred, depending on polarization, for the two wide low retardation coating, with the best LIDT performance occurring for the narrow low retardation coating.