We describe a full field heterodyne interferometry system where the object beam is shifted by a high frequency with respect to the reference beam. The optical path difference between the object and reference beam is encoded in the phase of the envelope of the interference signal which is at the difference frequency. Conventionally, high frequency heterodyne interferometry is restricted to measurement of a single (or a few) point(s) as is the case in displacement measuring interferometers for precision machine tool axis feedback. This is because of the problem of demodulating the signal phase simultaneously for many pixels comprising a full-field image. This problem is overcome here by exploiting the capability of the special pixel structure employed in a Time of Flight (ToF) camera. This structure enables the measurement of the envelope phase of an optical signal at every pixel with respect to an electronic reference at the same frequency. ToF cameras are designed to measure the distance to an object in its field of view by detecting the phase delay, due to time of flight, of reflected light from a modulated source synchronized to the camera. In the described experimental interferometer, a Twyman-Green architecture is used with an acousto-optic modulator to produce interfering beams with a difference frequency of 20MHz. The image detector is a modified ToF camera based on a Texas Instruments OPT8241 sensor. The interferometer directly outputs a wrapped optical phase map with 12-bit resolution (equivalent OPD resolution 0.15 nm) at greater than 50 frames per second with no post-processing. The phase reconstruction is highly insensitive to the reference/object beam intensity ratio or to environmental noise.
The use of autocollimator-based profilometers of the Nanometer Optical measuring Machine (NOM) design has been reported for the evaluation of X-ray optics for some time. We report a related development in the use of a non-contact NOM profilometer for the in situ measurement of base radius of curvature and conic constant for E-ELT primary mirror segments during fabrication. The instrument is unusual in NOM design in that it is deployable onto a CNC polishing machine in an industrial fabrication environment. Whilst the measurement of radius of curvature of spherical surfaces over a single scan has been reported previously, here we report on the use of this instrument to measure optical surfaces with an aspheric departure of 180 micrometers using a grid of multiple scans and bespoke surface fitting software. The repeatability of the measurement has been found to be approximately 1 mm in a measured radius of curvature of approximately 90 m. The absolute accuracy is limited by the accuracy of the calibration of the autocollimator and the <i>in situ</i> calibration of the instrument during operation.