Our CGH Cylinder Nulls are cylindrical zone plates with a linear carrier perpendicular to the cylinder axis. They produce a cylindrical test beam from the collimated test beam of your Fizeau or Twyman-Green interferometer.
Each CGH Cylinder Null is surrounded by an annular aperture linear reflective grating which diffracts the collimated test beam back onto itself. By turning adjustment screws to remove the annular tilt fringes, the CGH null is optically aligned to your interferometer test beam.
CGH Cylinder Nulls are switched between positive and negative focal length by reversing the tilt of the collimated input beam. Both diffracted orders exist and the correct wavefront must be selected.
We offer several standard CGH Cylinder Nulls for use with 632.8 nm wavelength interferometers. All models are mastered onto highest grade commercial photomask blanks. Use our selector tool to choose an appropriate model.
Back focal distance (or B.F.D.) is the distance from CGH face to line focus. To avoid mechanical collision, convex cylinder radius of curvature must be less than B.F.D. To suppress a horizontal band of ghosting due to unwanted diffraction orders, the maximum radius of curvature for convex cylinders should be several mm shorter than B.F.D.
|CGH Cylinder Nulls|
|*Note that prices are 10 percent higher outside U.S. and Canada.
†Typical substrate aberration, not a firm spec.
‡H45F1SC works with a 4-inch F/1.5 sphere, producing only a converging cylindrical wavefront.
Test accuracy is limited by CGH pattern, substrate and interferometer errors. Pattern error increases with decreasing F/# because of the smaller minimum grating period. The substrate wavefront distortion of each CGH is measured before and after phase etching. This data is shipped as a software null that can be directly subtracted from your interferograms. Usually, the substrate aberration is mostly power, which displays as toroid after best alignment.
System errors of your interferometer and transmission flat may be compensated in the usual manner—by testing a known flat and subtracting the result from subsequent interferograms.
Our CGH Cylinder Nulls are phase gratings with a nominal diffraction efficiency of 40 percent—16 percent in double pass. Interferogram visibility is suitable for test optic reflectivities ranging from 1 to 100 percent, peaking for a reflectivity of about 20 percent.
For large aperture interferometers, brightness may be reduced when zoomed to an aperture smaller than about Ø30 mm.
We can provide custom CGH Cylinder Nulls for other F/#’s and apertures. We can also provide chrome-type CGH nulls for transmission testing of cylinders with a return flat. Please allow us the opportunity to address your cylinder testing needs.