Adaptive interferometric null testing for unknown freeform optics metrology
Lei Huang, Heejoo Choi, Wenchuan Zhao, Logan R. Graves, Dae Wook Kim
2016
Optics Letters
We report an adaptive interferometric null testing method for overcoming the dynamic range limitations of conventional null testing approaches during unknown freeform optics metrology or optics manufacturing processes that require not-yet-completed surface measurements to guide the next fabrication process. In the presented adaptive method, a deformable mirror functions as an adaptable null component for an unknown optical surface. The optimal deformable mirror's shape is determined by the
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... astic parallel gradient descent algorithm and controlled by a deflectometry system. An adaptive interferometric null testing setup was constructed, and its metrology data successfully demonstrated superb adaptive capability in measuring an unknown surface. With the rapid development of freeform optics technology, freeform optical elements have been used to realize a wide range of important applications, including imaging systems, mobile displays, light-emitting diodes, and astronomical/space instruments. Advanced design and fabrication technologies have been developed for producing high-quality freeform optical systems. During the optics manufacturing process, the in-process (i.e., not-yet-completed) optical surface must be accurately measured in order to guide the iterative fabrication process [1, 2] . Although a customized null test [e.g., computer generated holograms (CGHs) or null lens interferometry] can provide high accuracy and precision for known optical surface metrology [3] [4] [5] , its application is limited to a null or near-null situation. For example, if the test surface is being manufactured, no fixed null configuration is available because the metrology target is an evolving freeform surface. This intrinsic challenge of measuring in-process freeform optics becomes a critical factor in advanced optical fabrication processes. Other non-null measurement methods, including deflectometry, coordinate measuring machines (CMMs), and profilometers, have been used to measure such in-process optics. However, these methods often have limitations. Due to a delicate calibration process for each configuration, the deflectometry method [6, 7] has practical challenges, especially in guiding loworder (e.g., astigmatic surface error) figuring processes. Contacttype approaches such as CMMs or profilometers often require long measurement times, suffer from insufficient spatial resolution, and/or may cause damage to the optical surface. We report an adaptive interferometric null testing method that overcomes the intrinsic dynamic range limitations of conventional null approaches. In the presented adaptive solution, a deformable mirror (DM) acts as an adaptable null component. The DM's shape is optimized using the stochastic parallel gradient descent (SPGD) algorithm [8] . The updating DM is precisely measured using an in situ deflectometry system (DS), which includes a display screen and camera [7, 9] . This unique in situ adaptive null measurement approach overcomes the limited accuracy/precision issue of a typical DM in the context of adaptive interferometry. The on-demand null condition achieved by the DM and DS enables rapid measurement of unknown freeform surfaces without requiring moving parts (except DM actuators) in the metrology system. A schematic diagram of the adaptive metrology system including three subsystems (interferometer, DM, and DS) is depicted in Fig. 1 . In practice, if the DM's maximum stroke/ deformation (e.g., 20 μm) is not sufficient, a nominal static null component such as CGH can be adopted to compensate for the nominal wavefront deformation (e.g., wavefront deformation from the ideal freeform optic). Note that the DM's surface, not its wavefront, is directly measured by the DS in the presented adaptive interferometric null testing. Unlike other common adaptive optics/interferometry applications using a wavefront sensor, such as the Shack-Hartmann sensor for measuring the wavefront at a limited spatial resolution (e.g., 30 × 30 lenslet array), acquiring a high-resolution DM surface map (e.g., 500 × 500 pixels) is essential for distinguishing the mid-to high-spatial frequency error of the test optic from the raw data. When the test optic is inserted into the adaptive metrology system, the initial interference fringes are acquired. Starting from the initial fringe data, the SPGD algorithm works to drive the DM. An online null condition for the unknown freeform Letter Vol. 41, No. 23 / December 1 2016 / Optics Letters 5539 0146-9592/16/235539-04 Journal
doi:10.1364/ol.41.005539
pmid:27906233
fatcat:t2pnunxzqnbg3kusmaekrgsde4