Here, we provide a brief outline of some of the key technologies.
3.1. Monolithic Filters
Interference filter:
The principle relies on a dielectric spacer sandwiched between two
transmitting layers (single cavity). The substrates are commonly fused
silica in the ultraviolet, glass or quartz in the optical, and
water-free silica in the infrared. Between the spacer and the glass,
surface coatings are deposited by evaporation which partly transmit
and reflect an incident ray. Each internally reflected ray shares
a fixed phase relationship to all the other internally reflected
rays. For constructive interference, for a wavelength
to
be transmitted, it must satisfy eqn. 1
where 
=
R is the refracted
angle within the optical spacer,
and the optical gap is the product of the thickness l and
refractive index µ of the spacer. The construction of these
filters
has undergone a revolution through the use of dielectric, multi-layer
thin film coatings, and a proper description
is more involved. All such filters can be tuned through a small
wavelength interval
(
/ =
-R2
/ 2µ2),
which amounts to no more than 2%
in practice. Suffice it to
say, interference filters make for poor tunable devices.