CGH Frequently Asked Questions

How can I clean a CGH null?

The face of a phase type CGH is fused silica with a surface-relief squarewave structure. The face of a chrome-type CGH is of durable chrome-on-silica. The backside has a multi-layer AR coating of which the top layer is silica. Alignment CGHs are chrome-on-silica with an overcoat of chrome. All these surfaces are quite durable and you may clean them alcohol followed by acetone or with acetone alone. They will tolerate some fairly vigorous scrubbing. Don't use anything that will leave a residue when you clean a phase-type CGH because it will be difficult to clean it out of the surface relief grooves. You should not immerse a CGH in acetone because the adhesive around the chrome steel balls is somewhat soluble, as is the adhesive which bonds a CGH into a 50R frame.

Do CGH nulls need periodic recalibration?

The short answer is, no.

CGH nulls are comprised of stable and durable fused silica. There is not much that you could do to it that would change its optical properties without also causing obvious cosmetic damage. You can remeasure the zero-order transmitted wavefront of a chrome type CGH (e.g. to confirm Diffraction International's measurements). This cannot be done for a phase type CGH (after the chrome pattern has been removed) because the zero-order wavefront is nominally zero and because the measurement would be corrupted by local variations in grating duty cycle. For diffraction orders other than zero, the wavefront phase is unaffected by grating duty cycle variations so long as such variations are symmetric for leading and trailing grating edges.

The accuracy of CGH framing can be degraded by rough handling. This would critical only if the null test depends on mechanical transfer of alignment from an Alignment CGH to a custom CGH null. Diffraction International can recertify the in-plane alignment of a CGH. The out-of-plane alignment can be checked too, although sliding of the CGH on a flat surface can abrade the chrome steel balls, possibly corrupting this measurement. The points at which the chrome steel balls contact the carbide rods of a CGH positioner are highly unlikely to be subject to wear and tear except by contact with the carbide rods. In Diffraction International's judgement, a CGH is unlikely to be attached and detached from its positioner a sufficient number of times to result in significant wear to the balls.

CGH nulls have been known to become de-bonded from their frames. This would result in perceptible motion between the CGH and its frame. The corners of a 6025-format CGH will not become entirely separated from the CGH because the CGH is also attached to the corner cover plate by an elastomeric adhesive.

Can I obtain a replacement for a lost technical report?

For recent reports, Diffraction International can immediately email you a PDF summary and/or ship you a CDROM or DVD containing the complete report. For older CGHs, portions of the archival record exist in paper form only and report pages that can be regenerated from the OSLO models are not retained. We are gradually working to scan key pages of older reports and to simply discard those pages that can be readily regenerated from the OSLO modes.

Since 1993, Diffraction International's CGH technical reports have much evolved. If you want an up-to-date replacement for an older, paper report, Diffraction International can regenerate portions of the report from the OSLO raytrace models. For very early CGHs, a separate lens files was created for each alignment feature. We will need to combine these into a single multi-configuration OSLO file before we can generate the replacement report. We can do this fairly efficiently, but there is labor involved and we will therefore charge you a nominal fee for the replacement report.

How should I be handling CGHs?

A 6025-format CGH should be handled by its metal corners. Each corner includes "dimples" on both sides to help you get a secure hold. To remove a CGH null from an M6025 positioner, use one of the two releases buttons to release a corner and then the other 2 corners will release easily.

A 50R-format CGH should be handled by its frame. The portion that includes the logo is intended to be used as a handle.