A tunable filter includes a polarizing beam splitter and a half-wave plate that
produce two equally polarized beams from an arbitrarily polarized input light.
The polarized beams are directed toward a tunable component that consists of an
optical substrate coated with a filter and a reflective element on its back and
front surface, respectively. The pass and stop beams emerging from the tunable
component are retro-reflected by roof structures and passed again through the filter.
As a result of this combination of components, the filter's performance is not
dependent on polarization and the filter's output channels are not shifted or deviated
by tuning.
