Synthetic aperture interferometry (SAI) is a novel way of measuring
aspherical optics (aspherics) where a scanning probe measures the optical
path difference (OPD) between light reflected from the front (test) and rear
(reference) surfaces of the aspheric optic. The surface form of the aspherical
surface is subsequently computed assuming that the form of the reference
surface is known. This-method is straightforward to implement, does not
require null compensators and is inherently insensitive to vibration.
Consequently it has the potential to measure aspherics as they are being
polished. When SAI was originally proposed (Tomlinson, Coupland &
Petzing 2003), bare fibres (NA - 0.12) were used to construct the probe,
however this configuration was unable to measure steep aspherics and had
poor light gathering efficiency.
In this thesis, a new probe has been designed to measure the surface
form of steep aspherics by increasing the NA of the probe using
supplementary optics. In addition, the light gathering efficiency of the probe
has been increased by adopting an anamorphical design. A single source and
receive point of the probe was devised and it is shown that this configuration
reduces the computational complexity. Alternative measurement
configurations were investigated and their relative performance compared. A
robust and fast phase evaluation process using a-priori information has been
developed to extract the phase from measured interference pattern. Several
steep surfaces have been measured to assess the feasibility of the SAI
technique. Finally a detailed error analysis has been carried out to identify the
major sources of error in measurement of OPD.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.