New Probe to Measure Freeform Optics

Demand for freeform optical elements is on the rise, and they have gotten easier to make, but at the moment no commercial systems exist to measure such components. Researchers at ASE Optics Inc. in Rochester, N.Y., have demonstrated that a scanning low-coherence dual-wavelength interferometer can measure the transmitted wavefront of hemispheric dome optics with accuracy by mapping optical thickness as a fiber-optic probe is scanned over the optic to be tested.

Now the research team is looking at more generalized freeform surfaces. In a talk at Optifab 2009 in Rochester, Damon W. Diehl of ASE Optics described recent work on developing new metrology applications for Lumetrics Inc.'s OptiGauge, including a new probe that is better suited for surface metrology: the Quad-Probe.

Recent advances in deterministic polishing have made production of freeform shapes easier, Diehl said, so a new type of metrology tool is needed that can measure a variety of shapes and measure them in situ. And freeform optical elements have advantages over spherical objects: They have reduced aerodynamic drag, for one thing, and they also allow for wavefront coding and wavefront correction.

The OptiGauge is capable of measuring the thickness of all the layers in a multilayer material simultaneously; the Quad-Probe projects four beams rather than one, allowing the instrument to measure both the position and orientation of the surface with respect to the probe as the probe is scanned over the object. The instrument produces a measurement overlap or redundancy; this redundancy enables the removal of some forms of measurement error during post-processing, Diehl noted.

Diehl works on the Quad-Probe with Christopher J. Ditchman and Christopher T. Cotton, all of ASE Optics. The researchers would like to develop finer and coarser beams for the probe, and they are looking to improve upon the error-detection and error-correction algorithms, to find a way to turn the data into null maps for deterministic polishing, and to develop a feedback system for "blind contouring" of unknown objects.

Laura S. Marshall
laura.marshall@laurin.com