Non-Polarizing Beamsplitter and AR Coating Design

The optical coating design of beamsplitters and anti-reflection coatings that are non-polarizing, those that have the same reflection for both sand p-polarizations at specified angles, is a challenge. This is because the effective indices of refraction for the media and coating layer materials have a different function of the angle of incidence for each polarization. This limits what can be achieved. Specific materials can be used to design reasonable non-polarizing coatings at only certain angles, whereas other materials can be made to serve for other angles. The choice of materials seems to be critical to the success of any particular requirement. Since the range of practical coating materials and substrates is limited for most applications, non-polarizing solutions seem to be quantized in reflectance and angle. This is illustrated with the aid of reflectance amplitude (circle) diagrams, and some design “rules-of-thumb” are provided.