Currently mirror metrology relies on Computer Generated Holograms (CGHs), which can typically cost $10k with a lead time of 6 months. A different CGH must be designed for each different test case, and for optics that are significantly affected by temperature changes or gravity sag, or that are imaged during various stages of the polishing process, the departure from the designed CGH may bring the wavefront error beyond the measurement range of a typical interferometer. Free-form optics in particular may have extreme departures from their design CGHs during the early stages of polishing.
BNS proposes to extend the range of an interferometer by providing additional programmable phase control through incorporating an SLM into the beam path. In addition to allowing a single CGH to be used for a range of similar mirrors, or for a single mirror that departs significantly from its design CGH, the SLM interferometer will allow the user to apply additional arbitrary phase. In this way, the SLM interferometer can be used to employ new techniques for retrieving wavefront, gravity sag, and other mirror characteristics, as well as to test wavefronts from simulation.
In Phase II BNS will incorporate an SLM into a commercial interferometer, the 4D PhaseCam 6100, and use this system to quantify performance when measuring a parabolic mirror, a CGH nulling setup, and an off-axis parabolic segment. We will also use Phase II to improve our SLM’s performance, including improved precision/flatness calibration and an upgrade to our new 1536x1536 pixel MacroSLM. This system will be delivered at the end of Phase II.
The addition of an SLM into the reference arm of an interferometer has the effect of extending the interferometer’s range by hundreds of waves. One of the most immediate benefits is the ability to use a single CGH to measure a variety of similar test optics, rather than the single optic for which it was designed.
The SLM interferometer can produce a null by adding the inverse of the test optic’s retrieved wavefront, and add other arbitrary wavefronts for experiments including new methods of wavefront characterization.
The SLM interferometer can save manufacturers of catalog or custom optics, especially free-form optics, time and money during manufacturing, since a single CGH or other reference optic will be usable for a greater range of optics.
The work in Phase 2 will also improve the MacroSLM’s phase calibration, improving diffraction efficiency and consistency for the SLM’s neuroscience users.