Stable High Power deep-UV Enhancement Cavity in Ultra High Vacuum with Fluoride Coatings

TL;DR

This paper compares fluoride and oxide coated mirrors in a high power deep-UV enhancement cavity under ultra-high vacuum, demonstrating fluoride coatings' superior stability and performance, with implications for long-term UV laser applications.

Contribution

It provides the first detailed comparison of fluoride versus oxide mirror coatings in high vacuum deep-UV enhancement cavities, highlighting fluoride coatings' durability and recovery limitations.

Findings

Fluoride coatings maintain up to 10 W intracavity power in high vacuum for one hour.

Oxide coatings degrade rapidly at lower powers and cannot be effectively recovered after degradation.

Fluoride coatings can reach 20 W in oxygen-rich environments, outperforming oxide coatings.

Abstract

We demonstrate the superior performance of fluoride coated versus oxide coated mirrors in long term vacuum operation of a high power deep-ultraviolet enhancement cavity. In high vacuum ($10^{-8}$ mbar), the fluoride optics can maintain up to a record-high 10 w of stable intracavity power on one hour time scales, whereas for the oxide optics, we observe rapid degradation at lower intracavity powers with a rate that increases with power. After observing degradation in high vacuum, we can recover the fluoride and oxide optics with oxygen; however, this recovery process becomes ineffective after several applications. For fluoride coatings, we see that initial UV conditioning in an oxygen environment helps to improve the performances of the optics. In oxygen-rich environments from $\sim 10^{-4}$ mbar to 1 mbar, the fluoride optics can stably maintain up to 20 W of intracavity power on several-hour time scales whereas for the oxide optics there is immediate degradation with a rate that increases with decreasing oxygen pressure.