Ultralow loss ring resonators using 3.5%25 index-contrast Ge-doped silica waveguides

Ring resonators (RRs) with a round-trip loss below 0.1 dB and a maximum free-spectral range (FSR) of 62.7 GHz were successfully fabricated. Single-mode waveguides of essentially square cross sections (~2.6 x 2.8 mum(2)) were patterned through reactive ion etching on a core layer of high Ge content doped silica deposited using plasma-enhanced chemical vapor deposition; they received a silica overclad. A propagation loss of 0.085 dB/cm was recorded for these waveguides. We strongly believe that our technology makes it possible to fabricate equally ultralow-loss RRs of even smaller bending radius, reaching an FSR of 100 GHz, thus opening the way to the design of filters for wavelength-division-multiplexing systems.