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Custom CGH Nulls

Photograph of CGH null.

Designing a custom CGH null does not require raytracing of your interferometer or transmission sphere. The raytrace model begins and ends at a point source concentric with your transmission sphere.

We attempt to locate the CGH null at one of the twelve distances from focus accommodated by our standard Alignment CGHs. The CGH must be outside the caustic region where rays cross. We make the CGH aperture somewhat oversized so that your test optic is the only stop in the system. We favor a CGH aperture of 30 to 45 mm as a reasonable compromise between fabrication costs and alignment sensitivity.

Full Documentation

Every CGH Null is shipped with full documentation. We provide an optical prescription of the CGH phase function and test configuration. We include a sensitivity and tolerance analysis which includes all CGH encoding and fabrication errors plus the alignment errors. Click here to view the sample report from a recent H95F3C Cylinder Null.

Phase grating efficiency

Interferogram Visibility

A CGH null reduces the intensity of the test beam, thus altering fringe visibility. For testing of bright mirrors, this will result in improved visibility, but in other cases visibility may be reduced. Diffraction efficiency is typically 10 percent for chrome CGHs and 35 to 40 percent for binary phase CGHs.

Generally, we recommend binary phase CGHs for testing of bare glass surfaces and chrome CGHs for testing of mirror surfaces. Unless your test optic is anti-reflection coated, fringe visibility is adequate.

Ghosts and Other Artifacts

Unwanted diffraction orders may create ghost fringes. These are minimized or eliminated by careful test configuration design.

Cost Effective

Custom CGH nulls are available in about 4 to 6 weeks. The price will depend on your test requirements, but is generally less than for a conventional refractive null.

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Page updated: 17 October 2014

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