Interferometric Metrology Using Reprogrammable Binary Holograms

Abstract

Interferometric methods for surface metrology have been widely used for many years due to their speed, accuracy and versatility. It is frequently necessary however to produce a known comparison reference surface to minimise the optical path difference and hence enhance the dynamic range. An alternative to this is to use a computer generated hologram to act as the reference wave, or to correct a spherical reference wave to match a highly aspheric optic in order to achieve a null test.
This thesis shall present a novel method of producing such holograms through the use of a binary ferroelectric liquid crystal on silicon spatial light modulator (FLCOS SLM) rather than using the more common lithographically produced plates. One of the primary advantages this could introduce is the ability for arbitrarily reprogrammable holograms to be created upon demand rather than needing to produce a series of holographic plates, saving both time and money in the testing of surfaces. We present results characterising the ability of a FLCOS SLM to produce increasingly large Zernike aberrations as well as quantifying the resulting errors, before using the device to reduce interferometric fringe density allowing us to measure aberrated optics and reveal low amplitude surface variations on the scale of 0.045 waves RMS.