Our tunable etalon filter is formed by placing mirrors on either side of a spacer of a material which has a relatively large rate of change of refractive index by voltage control.

The spacer is comprised of a liquid crystal of which molecular configuration is controlled to change the refractive index. The controllable difference of extraordinary and ordinary refractive indices make the optical path difference through the spacer and the passing wavelength is chosen. This voltage-controlled 

tunable etalon filter gives fine tuning down to sub-nano meter level.