Holographic Optical Testing for ELTs

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

The production of a huge number of mirror segments will be key to the construction of extremely large telescopes (such as the E-ELT). It is clear that the optical testing of these mirror segments will be problematic without the further development of new techniques in optical metrology. For example, the mirror segments in the US TMT are likely to be highly aspheric and the E-ELT segments will have very long (100m) focal lengths. One solution to both problems is to generate an aspheric reference wave-front in the interferometer. This can be achieved with fixed conventional optics, but time consuming to set up and often costly. A much more flexible approach uses computer generated holograms (CGH). A CGH can be relatively simply designed such that it produces, in one of its diffracted orders, an exact copy of the desired wavefront. The technique is robust and verifiable in that the generated wave-front is defined by the patterns of binary lines in the CGH. An alternative to either conventional optics or fixed CGH elements would be to use similar computer generated holographic techniques but with a programmable spatial light modulator (SLM) making the technique flexible and reconfigurable. An SLM is essentially an array of electrically controllable pixels, similar in design to an LCD display, which can be switched to modify the intensity, polarisation or phase of light as it passes through or is reflected off them. Displaying a designed CGH pattern on an SLM will produce the same wave-front as the fixed fabricated CGH but with the added advantage that a single element can be programmed to produce a wide variety of different reference wave-fronts. An additional benefit of using SLMs over the fixed approach is that the CGH can be varied in time, allowing the SLM to become the controlling element in a phase stepping interferometer overcoming the limitations of their binary phase modulation. In this project we aim to begin a 3-way collaboration between Durham, Imperial College, and the NPL. Durham and Imperial have already collaborated via an initial STFC-funded Innovative Technology Fund Grant. This work was successful and we demonstrated the basic feasibility of using programmable reference wavefronts in optical metrology and also produced a number of new algorithms and ways of using an SLM in an interferometer. We propose to build on this work via 2 CASE studentships - one at Imperial and one at Durham, both in collaboration with NPL. The overall aims are to produce a traceable programmable holographic interferometer based on our new techniques and to transfer this technology to NPL for verification and exploitation. Their will be strong interaction between all 3 groups, but the Imperial student will be biased towards the algorithms and fundamentals of interferometry using SLMs, and the Durham student will be biased towards testing and verification of the interferometer. More generally, a key aspect in any high quality optics manufacturing process is the ability to measure and verify the form of the optical surfaces being produced. Metrology is the technology that enables a production process or scientific instrument to deliver to a defined specification. Producing optics and other complex surfaces requires cycles of polishing and metrology to converge on final form, followed by traceable metrology to certify the final product. Optical designers are increasingly recognising the benefits of more complex and accurate forms, in order to enhance performance, simplify systems and reduce mass. As a result, these same optical designers are increasingly facing the challenge of building these complex forms when the backup of adequate metrology simply does not exist. Ultimately, this will limit the ambition of optical and system designers, and so throttle the use of aspheres, off-axis aspheres and other complex forms in systems design.

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