Increasing the free spectral range of silica waveguide rings for filter applications

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 $\left(\mathrm{\Delta }\right)$ 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 $\left(\mathrm{\Delta }=0.75\%\right)$. A FSR of 12 GHz with a loss of $0.36 \mathrm{dB}/\mathrm{cm}$ was demonstrated with coupling ratios of 30% (41% optimum).