SBIR-STTR Award

Laser interferometers are the state of the art for characterizing large telescope optics, manufacturing custom optics, aspheres, freeform optics, and for semiconductor wafer characterization. For test optics with large surface errors, a reference optic and/or a custom computer-generated hologram (CGH) can be used to bring the errors within the interferometer’s range. However, many situations result in large departures from reference optics. Thermal & gravity sag effects in large optics can cause significant deviations, only some of which may be predictable. Test optics in semiconductor manufacturing may include sharp, irregular steps of many waves. In the early stages of optics polishing, departures from reference can be extreme. Custom optics, including asphere and freeform, can deviate hugely from spherical, and for these a custom CGH (with a typical lead time of 6 months and cost of $10k) is not always economical. We propose to extend the range of an interferometer by providing > 50 waves of programmable phase control using a Spatial Light Modulator (SLM). In addition to extending the range of phase errors that can be characterized, the SLM interferometer can apply additional arbitrary phase. In Phase I we will upgrade a prototype SLM interferometer that we previously used to demonstrate nulling and programmable phase control. Phase I will focus on improving interferometer speed, calibration, and stability, and quantifying performance through the following technical objectives: Design for interferometer upgrade with polarization camera Construction of the SLM interferometer Implementation of single-shot phase shifting interferometry (PSI) 2D Phase/voltage SLM calibration Characterizing range and accuracy Demonstrating key applications Delivery & demonstration of Phase I prototype In Phase II BNS will incorporate an upgraded 1536x1536 pixel MacroSLM into a commercial interferometer and demonstrate its performance in typical use cases. Potential NASA Applications (Limit 1500 characters, approximately 150 words) An extended-range interferometer allows measurement of the phase errors of large telescope optics under conditions for which there is no reference available – such as the presence of thermal changes and gravity sag. This could also allow characterization earlier in the polishing process, lowering their overall cost. The SLM interferometer can also add arbitrary phase, enabling new techniques for retrieving phase error and other mirror characteristics, and testing phase functions from simulation. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words) The SLM interferometer can save optics manufacturers, especially free-form optics, time and money during manufacturing, since a greater interferometer range will mean optics can be characterized earlier in the fabrication process when their deviations from reference can be large. It can also allow spherical references or existing CGHs to be used for a wider range of optic designs.

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. Potential NASA Applications (Limit 1500 characters, approximately 150 words): 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. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words): 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. Duration: 24