Transmission characteristics of meter-long silica-on-silicon optical time delay
Optical time delays with low propagation losses are important for a wide range of applications that require transmission and manipulation of ultra-wide bandwidth optical signals. Fiber-optic based time delays are typically used but they lack the compactness, robustness, manufacturing scalability and precision. An alternative approach is to use lithographically defined planar integrated-optic time delays which offers a high-precision, reduced footprint, and mass-production compatibility. However, planar waveguides suffer from propagation losses that are considerably higher than those of fiber counterparts. In this thesis we reported a detailed investigation on the propagation losses in planar silica-on-silicon waveguides and identified critical factors contributing to such losses. We explored different waveguide designs in combination with post-processing fabrication steps, simulations and high-precision propagation loss measurements. We demonstrate meter-long silica-on-silicon optical time delays with low propagation loss of 0.41 dB/m. Material absorption and scattering due to roughness at the waveguide sidewalls were identified as the dominant mechanisms responsible for high propagation losses in these devices. We found that the propagation losses decrease with the waveguide width and increase with the operation wavelength. We also investigated the ultrafast response of silica-on-silicon optical time delays. Our results indicate the presence of two pulses which were attributed to two different polarization states traversing the waveguides. The separation between the pulses decreases as the heat sink temperature increases. This is attributed to differences in the temperature dependence of the refractive index between core and cladding materials. This was confirmed from independent measurements on ring resonators fabricated in the same wafer.