Quasioptical

Quasioptical refers to a region of electromagnetic wave propagation that lies between the purely wave-like (diffraction-dominated) and purely ray-like (geometrical optics) regimes. In this regime, the wavelength of the radiation is comparable to or smaller than the characteristic dimensions of the optical elements or structures involved, such as lenses, mirrors, or apertures. This means that diffraction effects cannot be entirely neglected, but the wave phenomena can often be approximated using geometrical optics concepts.

This quasioptical approximation is particularly useful in the design and analysis of microwave and millimeter-wave systems. Components like lenses, horns, and reflectors are often designed using ray tracing techniques, but their performance is then evaluated considering diffraction. For example, when a beam passes through a lens, the geometrical optics prediction of the focal spot size might be refined by accounting for diffraction at the lens aperture. Similarly, the shaping of antennas and beam waveguides often relies on quasioptical principles to achieve desired beam patterns and minimize losses. The validity of the quasioptical approximation generally improves as the wavelength decreases relative to the size of the interacting structures.