James Webb Space Telescope (JWST) Optical Telescope Element (OTE) mirror coating program has been completed.
The science goals of the JWST mission require a uniform, low stress, durable optical coating with high reflectivity over
the JWST spectral region. The coating has to be environmentally stable, radiation resistant and compatible with the
cryogenic operating environment. The large size, 1.52 m point to point, light weight, beryllium primary mirror (PM)
segments and flawless coating process during the flight mirror coating program that consisted coating of 21 flight
mirrors were among many technical challenges. This paper provides an overview of the JWST telescope mirror coating
program. The paper summarizes the coating development program and performance of the flight mirrors.
The James Webb Space Telescope (JWST) is an on axis three mirror anastigmat telescope with a primary mirror, a
secondary mirror, and a tertiary mirror. The JWST mirrors are constructed from lightweight beryllium substrates and the
primary mirror consists of 18 hexagonal mirror segments each approximately 1.5 meters point to point. Ball Aerospace
and Technologies Corporation leads the mirror manufacturing team and the team utilizes facilities at six locations across
the United States. The fabrication process for each individual mirror assembly takes approximately six years due to
limitations dealing with the number of segments and manufacturing & test facilities. The primary mirror Engineering
Development Unit (EDU) recently completed the manufacturing process with the final cryogenic performance test of the
mirror segment assembly. The 18 flight primary mirrors segments, the secondary mirror, and the tertiary mirror are all
advanced in the mirror production process with many segments through the final polishing process, coating process, final
assembly, vibration testing, and final acceptance testing. Presented here is a status of the progress through the
manufacturing process for all of the flight mirrors.
The demands of the optical communications industry have resulted in a dramatic increase in the performance requirements for thin film optical filters. Complex coatings are now manufactured with a level of control that was almost unthinkable a decade ago. In the area of anti-reflection coatings for optical fibers and laser diodes, not only has there been a push towards much lower reflectance requirements, the measurement methods and terminology are different. AR coatings are expected to have optical return losses (ORLs) of -40dB to -50dB, corresponding to reflectance values of 0.01% to 0.001%. By comparison, an element in a typical precision optics system might have an AR requirement of < 0.1% (-30dB). From a manufacturing standpoint, these requirements pose two problems: how to make such coatings, and how to measure them. In this work we review the progress towards routine manufacture of such ultra low reflectance coatings. We examine both the process control issues and the obstacles that must be overcome in the reflection measurement.
There are a variety of optical coatings needed on laser bar facets to make them functional. There are also several different types of laser bar facet materials to be coated which complicates the problem a little bit. These coatings fall into three types; antireflection coatings, high reflectors and partial reflectors. A wide range of coating designs and materials to be used in the coatings has been studied.
The anti reflection coating (AR) typically used is either a single layer coating, obviously consisting of one material, or dual layer coatings consisting of two materials. There are a limited number of applications that may involve larger number of layers if more than one wavelength or a wavelength band of more than 40 nm needs to be covered. The single layer coatings usually do not provide very low reflectance. Dual layer coatings provide the ability to design coatings with very low reflectance. Manufacturing process limitatins allow for producing AR coatings with a residual reflection in the 0.1%-0.2% range. Although lower reflecting coatings can be designed, process control parameters and optical measurement problems limit the coating manufacturer to this range with R ≤ 0.2% being a standard specification. This paper will discuss the various coating designs for achieving low reflectance on InP and GaAs laser facets and the optical measurements problems.
Flat surfaces of reaction bonded SiC and aspheric surfaces of Si clad reaction bonded SiC were polished to yield rms surface roughnesses less than 30Å. These surfaces were then overcoated with two formulations of a typical front surface protected silver film and their reflectances and roughnesses compared with highly polished fused silica flat surfaces that were silver coated at the same time. Surface roughness and BRDF measurements performed before and after coating with protected silver indicated no significant increase in surface roughness or measured scatter for either silver coating formulation and a barely detectable measured roughness difference between the two coating formulations.
The emission distribution characteristics of an evaporation source can be used to defme the correct geometry
in the vacuum chamber for the production of uniform-thickness coatings.
We first measured the thickness of coatings on test pieces positioned at known radial distances on a single
rotation flat rack in the vacuum evaporation chamber and used these data in a computer program which found the
source emission function, in the form cos° , which provided the best fit to the data. is the emission angle of the
evaporant stream from the source, measured from the vertical. The known emission function was then used to
determine the source offset and calotte curvature which produced the best thickness uniformity over the diameter.
In one example, we found 0 = 131 for A1203 evaporated from an electron beam source. This enabled us to
predict a chamber geometry which yielded coatings across a calotte of diameter 81 cm with a thickness variation of
0 = 1.70, but the uniformity was less excellent (±3%) because this material is difficult to evaporate
controllably. The technique is a powerful one for anyone setting up his coating chamber to produce a large number
of coated substrates.