Adaptive wavefront interferometry for unknown free-form surfaces

Abstract

The primary problem of conventional wavefront interferometers is limited dynamic range. Unknown free-form surface figure error with large amplitude or slope is not measurable for too dense or invisible fringes. To troubleshoot this problem, we propose adaptive wavefront interferometry (AWI). AWI utilizes a wavefront sensor-less adaptive optics (AO) subsystem to intelligently speculate and compensate the unknown free-form surface figure error. In this subsystem, adaptive null optics is utilized to iteratively generate adaptive wavefronts to compensate the unknown severe surface figure error. The adaptive null optics is close-loop controlled (i.e., wavefront sensor-less optimization algorithms are utilized to control it by real time monitoring the compensation effects to guarantee convergence of the iteration). Ultimately, invisible fringes turn into resolvable ones, and null test is further realized. To demonstrate the feasibility of AWI, we designed one spatial light modulator (SLM) based AWI modality as an example. The system is based on a commercial interferometer and is easy to establish. No other elements are required besides the SLM. Principle, simulation, and experiments for the SLM based AWI are demonstrated.

Measurement of Freeform Optical Surfaces: Trade‐Off between Accuracy and Dynamic Range

“…We first proposed to utilize SLM in adaptive null interferometry for measuring in‐process surfaces in 2018. [ 121 ] The SLM can be calibrated to moderate accuracy by using it as a reconfigurable interferogram‐type CGH. [ 122 ] Note this key difference from the traditional SLM control method based on the electrically controlled birefringence effect.…”

“…b) Reproduced with permission. [ 121 ] Copyright 2018, Optical Society of America. c) Reproduced with permission.…”

“…The detailed test procedures of the SLM‐based adaptive null interferometry for in‐process surfaces are illustrated in. [ 121 ] First, the static null is utilized to get the full aperture surface figure error except some local regions. Second, the SLM is inserted in the test system to measure the local regions with severe figure error.…”

“…It includes three major steps to change the fringe states as shown in Figure 14b. [ 121 ] The first step is fringe restoration based on the SPGD. It turns the invisible fringe state into resolvable fringe state.…”

Surface Topography Measurement of Mirror-Finished Surfaces Using Fringe-Patterned Illumination

“…Optical techniques show great advantages in measuring mirror-finished surfaces due to their nature of as a non-contact mechanism that will thus prevent damage and contamination during measurement [15]. Interferometers are widely used both in academy and industry for the surface characterization of mirror surfaces [16]. Extremely high measurement accuracy, down to a sub-nanometer, can be achieved with heterodyne or phase-modulation techniques [17].…”