A narrow frequency band (20...600 MHz) at frequencies 50...200 GHz and highsuppression (more than 40 dB) are the typical requirements for notch filters used in plasmadiagnostic during experiments with fusion installations. A very narrow frequency bandwidthAf/f=ia3...1a4 in the short millimeter wavelength range implies using of quasi-optical high- quality (Q ?_ 1(1') cavities as elementary cells that constitute a filter, and quasi-optical opentransmission line as a primary waveguide. We considered two possible ways to produce suchcavities (Fig. 1): 1) two-mirror cavity with a standing wave (a), 2) ring cavity with a travelingwave (b). For both cavities coupling is provided by a semi-transparent film or by diffractiongrating. In the first scheme there is a reflected wave, so phase shift (mutual position ofcavities) is important. The second scheme uses only a one-way wave and thus all cells areindependent.
G. G. Denisov,
"Quasi-optical narrow-band notch filters", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 21044Z (30 August 1993); doi: 10.1117/12.2298611; https://doi.org/10.1117/12.2298611