Application of the incident/reflected wave boundary condition to the wave equation for a weakly irregular waveguide leads to a spectral domain analysis of the open cavity (Fig. 1) with a typical maximum-field spectrum shown in Fig. 2. Low Q value due to the diffraction loss and the consequent overlapping of neighboring modes are the two reasons for the broadening of resonant lines. The latter also lowers the spectral domain Q values relative to those based on the commonly employed time domain approach.
K. R. Chu,
"Spectral domain analysis of open cavities", Proc. SPIE 1929, 17th International Conference on Infrared and Millimeter Waves, 19290H (14 December 1992); doi: 10.1117/12.2298134; https://doi.org/10.1117/12.2298134