A diffuser is simply a thin layer that is applied over the glass prior to engraving. The diffuser absorbs some of the laser energy on the outside of the focused laser spot. The absorption on the outside of the focused spot creates a smaller spot with less overlapping and effectively reduces the Heat Affected Zone.
Quite a few materials will act successfully as a diffuser. Paper mask, a thin paper with a light adhesive, works well and is easily removed after engraving. One example is standard 20 lb. copy paper. Copy paper is readily available and inexpensive. The copy paper will need to be dampened to stay in direct contact with the glass. Water works adequately to dampen the copy paper, but glass cleaner works best in my opinion. When applying the glass cleaner to the copy paper, don’t be bashful and use plenty to thoroughly soak the paper. When fully wet, apply the copy paper to the glass and smooth out any air bubbles. Try not to overcompensate the diffuser process; simpler is always the ideal method.
Using a diffuser on glass has many benefits; the largest is that it works well with a wide range of settings and glass types. With the diffuser in place, my settings are similar to those used for deep engraving wood. On a 50-watt laser system, 30% speed with 100% power using a standard engraving resolution (500 or 600 DPI) achieves excellent results on any type of glass.
Note that it is possible to engrave glass awards without any diffuser, and there are other methods, but it requires adjusting the power, speed, and resolution to reduce the Heat Affected Zone. Settings may need to be adjusted for different types of thicknesses of glass when a diffuser is not used.
—Mike Fruciano, Coherent