A VIPA is a special type of Fabry-Perot etalon with three distinct coatings. One surface of a VIPA has an anti-reflection coated section adjacent to a high reflector.
The opposite surface is coated with a partially transmitting mirror. Light is introduced into the VIPA at a small angle on the AR coated area. The VIPA is tilted so that the portion reflected from the partial reflector is fully incident on the high reflectance zone of the input surface.