Quasi Optics

a branch of physics concerned with the propagation of electromagnetic waves with a wavelength λ < 1-2 mm (the shortwave part of the millimetric radio wave band —submillimetric waves and the adjacent optical band) under conditions in which the wave propagation conforms to the laws of geometric optics but in which diffraction phenomena also play a substantial role. A result of these studies is the development of quasi-optical devices—open resonators and quasi-optical lines —in which waves of this band may be excited and propagated.

Cavity resonators and wave guides with dimensions on the order of the wavelength λ, which are widely used for centimetric waves, are unsuited in practice for radio waves shorter than 1-2 mm. Ohmic losses at these wavelengths are so great that the wave is almost entirely attenuated in wave guides at distances of ℏ 10-20 cm from the source, and the quality factor of the resonator is low. Open resonators and open transmission channels (refracting and reflecting quasi-optical lines) have therefore been developed.

The simplest open resonator consists of two parallel mirrors placed opposite each other. A beam of light is reflected successively from each of the mirrors and returns to the opposite mirror. The width of the beam is much greater than the wavelength, but since the distance between the mirrors is much greater than the width of the beam, the diffraction divergence of the beam proves to be significant. This phenomenon, along with diffraction at the edges of the mirrors, leads both to nonuniform-ity of the field distribution in the cross section of the beam and to the occurrence of radiation energy losses. Curved mirrors (in particular, a confocal resonator), which focus the beams, are used to reduce losses (and increase the quality factor of the resonator).

Open resonators, although large in comparison with the wavelength λ, have a quite widely spaced (discrete) spectrum of natural frequences. Therefore, they have proved to be a very convenient resonant system for lasers and for all equipment using electromagnetic waves of the optical and submillimetric bands.

In quasi-optical lines the beam (whose widths ≫) passes in sequence through a number of long-focus lenses or slightly curved mirrors (correctors). The correctors focus the beam, compensating for its diffraction expansion upon propagation between them. Such lines may also be used in optical communications systems. Radio wave guides that are wide in comparison with the wavelength λ and that use mirrors, lenses, and prisms may also be used for submillimetric and millimetric waves.