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

The ring loss and coupling between waveguides of different widths are studied using ring resonators. A reduction in ring loss from 1.44 to 0.36 dB/cm and an improvement in coupling from 10% to 30% are demonstrated. No evidence of multimode ring is visible. These results show that a broader range of filter designs can be achieved using whispering-gallery-mode rings and current fabrication processes with low loss polarization dependence.