Plasma plated laser end mirrors

Abstract

All-dielectric high reflector coatings produced by plasma plating were evaluated for their applicability as low loss, UV stable intracavity end mirrors. Although ultimately intended for ion laser applications, the design wavelength was set at 633 nm, taking advantage of existing scatter and total loss instrumentation. The total loss using an optoacoustic loss instrument was tracked during exposure to a low pressure mercury lamp simulating the UV environment of an optical cavity. This source has its strongest emission at 248 nm. The reflectors consisted of quarterwave stacks of alternating high and low index materials with halfwave overcoats of low (silica). Four high index materials were examined: titania, zirconia, tantala, and hafnia. Titania/silica stack scatter losses exceeded 2000 ppm. No further testing was attempted with this combination. The zirconia stack scatter averaged 550 ppm, while the tantala and hafnia could be produced with loss averaging ~200 ppm. In selected areas the scatter was under 100 ppm. After annealing at 400°C for 1 h, the total losses closely matched the combined losses from scatter and transmission, with the bake having no measurable effect on scatter. In standard plasma plating deposition conditions, the zirconia, tantala, and hafnia all had similar UV exposure characteristics suggesting a common loss mechanism. Each reached a saturation loss level ~500 ppm higher than its starting value after ~30 min of UV exposure. Unlike conventionally deposited films which degrade rapidly under UV exposure, samples produced omitting the halfwave overcoat had UV-induced losses of only ~100 ppm with a time constant of <2 min.