Partial reflectors are used in laser cavities or outside the laser to split the beam. In the former case, the coatings will specified for normal incidence and for the latter, the coating is usually for use at 45 degrees angle of incidence. When used at non-normal incidence, the polarisation also needs to be considered, S , P or R (same as AV). The most common beam splitter coating is a 50% for splitting the beam into two equal power beams.
Flat and Curved Partial Reflectors
Dimensional tolerances for standard lenses, windows and mirrors are as follows:
- Diameter: +0/-0.1 mm for most diameters, +0/-0.05 mm for 1.5″ lenses
- Centre thickness: +/-0.1 mm
- Edge thickness variation: most lenses within 0.03 mm and some within 0.05 mm
- Focal length: +/-0.5%. Some long focal singlet scanning lenses can be 1 to 2% due to highly meniscus shape
- Surface form: typically less than 1 visible fringe on a 38 mm diameter test area
- Surface roughness: within 20 Å rms
Tighter tolerances are possible on custom optics. Sometimes they are interrelated if very tight, e.g. the achievable flatness on a window can depend on the thickness (rigidity).