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.