Square-shaped two-dimensional optical microcavities (mu -cavities) were inv
estigated for possible applications as filters for dense wavelength-divisio
n multiplexing, Multimode cavity resonances were observed in the elastic sc
attering of approximate to 200-mum square-shaped mu -cavities in fused sili
ca. Based on a two-dimensional k-space representation, we accounted for the
multimode spectrum by different normal modes with rays confined by total i
nternal reflection. The cavity-mode trajectories need not be closed after e
ach round trip. Single-mode spectra are expected from smaller square-shaped
mu -cavities. (C) 2001 Optical Society of America.