A Precision Interferometric Optical Heterogeneity Mapping System

Gerry Edgar
Ph.D Thesis

Engineering- Physics Group
Aberdeen University 1990

Abstract

The object of this work was to design and build an instrument for mapping optical heterogeneities in fused silica blocks,
with a sensitivity at least an order of magnitude better than any system currently available (prompted by a requirement of
the Stanford Gravity Probe B project).

The sources of inhomogeneities and the environment for containing these blocks to achieve measurements at this level are
discussed with the implications for the instrument design. After a review of homogeneity testing methods, it was decided
to develop an instrument system for use with several measurement methods. The most sensitive of these methods was
selected to be fully developed in order to realize the measurement aim, the others being facilitated in the design.

The design and development of a novel mechanically scanned heterodyne interferometric mapping system is described -
following an error budget driven methodology. The critical 1mK thermally stable environment required for the liquid
immersion measurement method employed, was exceeded by the design and development of a large computer controlled
thermal enclosure to house the whole instrument system - sub-millidegree stability achieved over periods of hours. Also
described is the computer integration and software for execution and co-ordination of all measurements.

The developed system meets the initial aim, being capable of an optical heterogeneity measurement sensitivity of 5x10-8 in
refractive index in a 50mm thick sample. This was calibrated indirectly in a comparison with a Talystep surface profiling
instrument. Further development work suggested could enhance this sensitivity by another factor of two.



Acknowledgment

Many thanks to Stanford University for the Ph.D Studentship to undertake the project at Aberdeen.

All feedback on this site to the developers - Dr. Peter Flett and Dr. Gerry Edgar Last updated: 09 February, 2007