• Generic 2 Piece Glass Sandwich & 3mm "O" Ring Sealing
• Generic 2 Piece Glass Sandwich with Edge Seal
• Generic 3 Piece Glass Sandwich

• 4 Equivalent Orientations for Symmetric Gratings
• Non-Symmetric Grating Orientations

• The gelatin contains periodic changes of index that were created by the interference of 2 laser beams and photochemically processed.
• An "O" ring seal is created by removing some of the gelatin and applying telecom-grade epoxy over the gelatin and in the "O" ring area.  3mm is typically removed for non-telecom applications for sizes <3"; more than 3mm is removed for gratings >3".
• In the grating diagram above, the physical size is 40 x 30mm with an active area (clear aperture) of 34 x 24mm.
• Gelatin is typically 2-20 microns thick.  The Front View shows gelatin as rainbow colored and the Side View shows the gelatin as white/dark rectangles of index changes.

• An "Edge Seal" is created without removing any gelatin and applying telecom-grade epoxy over the gelatin.  Both the gelatin and epoxy go to the edge of the glass.
• Edge Seals allow a slightly larger clear aperture relative the physical size of the glass, usually 1-2mm less than the physical glass size.  This can be important when physical sizes get smaller or when the there are physical size constraints.

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Purpose:   This 3 piece glass construction is sometimes used to eliminate an extra AR coating run while making prototypes or small production runs.

• The AR coatings are done on glass blanks that can be AR coated at high temperatures and not the assembled gratings which need to be AR coated at low temperatures (e.g. <100C).  AR coating control is somewhat more predictable for coatings that might not otherwise be easy to make.
• The 2 outside pieces of glass are AR coated at the same time and then glued onto the glass/grating.
• This can be a good option if thickness and/or wavefront error are not concerns.