Ring resonators are promising building blocks for developing compact optical filters with arbitrary functions; however, a major challenge for planar waveguide filter implementations is to overcome the limited free spectral range (FSR) for a given core-to-cladding refractive-index difference (Δ) while maintaining low loss and a large range of coupling ratios. The loss and coupling of rings operating in the whispering-gallery-mode regime were investigated by use of Ge-doped silica waveguides with a low refractive-index difference (Δ=0.75%) . A FSR of 12 GHz with a loss of 0.36 dB/cm was demonstrated with coupling ratios of 30% (41% optimum).
© 1998 Optical Society of America
Christi K. Madsen and Jian H. Zhao, "Increasing the free spectral range of silica waveguide rings for filter applications," Opt. Lett. 23, 186-188 (1998)