Imaging Scatterometer for Observing In-Situ Changes to Optical Coatings During Air Annealing

TL;DR

This paper introduces a novel imaging scatterometer that enables real-time, in-situ observation of optical coating changes and damage mechanisms during air annealing, providing valuable insights for manufacturing improvements.

Contribution

The paper presents a new instrument that allows dynamic, in-situ imaging of optical coating changes during annealing, surpassing traditional static observation methods.

Findings

In-situ imaging of titania-doped tantala coatings during annealing.

Detection of crystallization and damage evolution in real-time.

Potential to improve coating manufacturing processes.

Abstract

Annealing of amorphous optical coatings has been shown to generally reduce optical absorption, optical scattering, and mechanical loss, with higher temperature annealing giving better results. The achievable maximum temperatures are limited to the levels at which coating damage, such as crystallization, cracking, or bubbling will occur. Coating damage caused by heating is typically only observed statically, after annealing. An experimental method to dynamically observe how and over what temperature range such damage occurs during annealing is desirable as its results could inform manufacturing and annealing processes to ultimately achieve better coating performance. We developed a new instrument that features an industrial annealing oven with holes cut into its sides for viewports to illuminate optical samples and observe their coating scatter and eventual damage mechanisms in-situ and in real-time during annealing. We present results that demonstrate in-situ observation of changes to titania-doped tantala coatings on fused silica substrates. We obtain a spatial image (mapping) of the evolution of these changes during annealing, an advantage over x-ray diffraction, electron beam, or Raman methods. We infer, based on other experiments in the literature, these changes to be due to crystallization. We further discuss the utility of this apparatus for observing other forms of coating damage such as cracking and blisters.