New interferometer measures extreme aspheres without using null optics

June 25, 2008--QED Technologies (Rochester, NY) has just released a new version of its interferometer that is capable of full-aperture measurement of aspheres with more than 200 waves (120 microns) of departure from the best-fit sphere, without the use of null lenses.

June 25, 2008--QED Technologies (Rochester, NY) has just released a new version of its interferometer that is capable of full-aperture measurement of aspheres with more than 200 waves (120 microns) of departure from the best-fit sphere, without the use of null lenses. The metrology product is based on the company's subaperture-stitching technology, and adds to its line of instruments termed SSI-A (Subaperture Stitching Interferometry for Aspheres).

Standard interferometers can handle only a few waves of aspheric departure. One longstanding technique to overcome this limitation has been to fabricate null lenses--special optics that introduce the approximate amount (but opposite sign) of aspheric departure. The null optics are added to the interferometric setup, making the wavefront approximately spherical and thus able to be measured by the interferometer. However, null optics are unique to only a specific optical element or system to be measured, and must be fabricated and aligned extremely precisely in order not to introduce so much wavefront error as to render any interferometric measurements useless.

In contrast, the SSI-A instrument measures a number of subaperture regions and "stitches" them together digitally, eliminating the need for null optics even for measuring extreme aspheres. The new SSI-A incorporates an enhanced stitching engine, says QED. In addition, the SSI-A is capable of taking full-aperture measurements of spheres and flat surfaces.

QED is also known for its MRF (Magnetorheological Finishing) optical-polishing process, which uses a magnetorheological fluid, actively computer-controlled by electromagnetic fields, for polishing. A magnetorheological fluid contains a suspension of extremely small magnetic particles; the application of a magnetic field changes the fluid's viscosity and thus, in the MRF process, the polishing force over that region. QED sells MRF systems designed for the optical-production environment. QED is owned by Cabot Microelectronics Corporation (Aurora, IL), which supplies chemical mechanical planarization (CMP) slurries used in semiconductor and data-storage manufacturing (a CMP slurry polishes both by mechanical abrasion and chemical corrosion).

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