Ultralow loss high delta silica germania planar waveguides

A plasma enhanced chemical vapor deposition process was developed to deposit SiO2-GeO2 films suitable for high index contrast planar waveguides. These films were deposited in a standard parallel plate reactor from silane, germane, nitrous oxide, and a nitrogen carrier. The germania content of the film was equal to the mole fraction germane of the hydride precursors in the gas stream, and the refractive index of the film varied linearly with the mole fraction of germania. Low loss guides ranging from 1.5-4.0% A were fabricated. With standard photolithographic patterning, a 0.05 dB/cm propagation loss and minimum bend radius of 1.5 mm were measured for 2.0% Delta. Improvements to the photolithographic patterning to reduce sidewall roughness were required to achieve low propagation loss at higher Delta. This reduced the propagation loss for 3.5% Delta cores to 0.086 dB/cm. An average minimum bend radius of 570 mm was measured for 3.5% Delta, but modeling suggests the bend radius could be reduced below 500 mm with offsets to reduce transition loss. Ring resonator, fabricated from 3.5% Delta waveguides, exhibited a free spectral range as large as 62.7 GHz and a very low round trip insertion loss of 0.06 dB.