'Virtual interferometer' produces surface profiles via gradient measurements

A simple surface-metrology approach developed by researchers at the University of Illinois at Urbana-Champaign allows the user to easily obtain the surface profile of certain optical elements, such as deformable mirrors, that are already buried within existing optical systems, such as adaptive optical systems. The approach relies on a wavefront sensor and at least one lateral translation of the surface under test, and is termed the "virtual interferometer" technique.

Translating the surface in one direction between measurements, and subsequently subtracting the data, produces a map of the surface gradient in that direction. Carrying out the process in a perpendicular direction produces a map of the perpendicular gradient; from these two maps, the 2D surface profile can be reconstructed. A persistent but predictable tilt bias in the setup was removed before calculations were done. The researchers also used the setup to accurately measure an irregular 137 nm root-mean-squared (rms) distortion caused by a microscope slide and then digitally remove the aberration from the setup, leaving a field flat to 24 nm rms. The system resolution was limited by the angular crosstalk of the translation stage. Contact Scott Jobling at [email protected].