Polarization-insensitive fabry-perot tunable filter

DETAILED DESCRIPTION OF THE INVENTION This invention relates to optical filters that are polarization-insensitive (i.
e.
, polarization-independent) in which spectral filtering is not affected by differences in polarization of input light.
These filters do not require selective polarization of input light and thus in general result in higher levels throughput of a given amount of input light of arbitrary polarization.
These filters provide spectral filtering of unpolarized input light or of light of any arbitrary polarization.
The filters of this invention are Fabry-Perot filters in which selected optical elements are positioned within a FP cavity, i.
e.
, an etalon with reflective surfaces separated by a fixed distance, d, wherein the intracavity light can be treated as an infinite number of partial waves produced by the reflections at the two surfaces.
At optical resonance, the phase delay attributed to one round trip is an integral number of wavelengths, i.
e.
2d=m.
lambda.
/n, where n is the average index of refraction of the cavity and m is an integer.
This resonance condition is satisfied at a series of wavelengths and these transmissions are called FP fringes of order m.
The free spectral range (FSR) is the spacing between these peaks.
The general operation of FP filters, particularly those containing LC optical elements is described, for example, in U.
S.
Pat.
Nos.
5,469,279 and 5,381,253.
FP filters of this invention can be operated in transmission-reflection mode or reflection-only mode.
In transmission-reflection mode, both reflective surfaces of the FP etalon are partially reflective, i.
e.
the reflectivity, R, of each surface is less than one, such that light can enter through one reflective surface, and undergo multiple internal reflections before the portion of light satisfying the resonance condition exits through the other reflective surface.
In reflection mode, one of the reflective surfaces of the etalon is partially reflective and the other is a completely reflective surface, R