Silicon nitride waveguides and micro-ring resonators for photonic integrated circuits

This work represents significant progress in the development of a Si3N4 waveguide platform for near infra-red wavelengths as required for 87Rb chip-scale atomic systems. The theoretical framework provided by ray optic and electromagnetic theory have formed a basis for understanding waveguide phenomena. Coupled mode theory has been used to determine the microring resonator coupling coefficient. An analytical model of a micro-ring resonator has been used to show that Q factor can be optimised by the decreasing the coupling coefficient. The Lacey-Payne scattering model suggests that increased waveguide width and reduced index contrast results in decreased loss due to sidewall scattering. Q factor has therefore been optimised by increasing waveguide width, developing a top cladding process, and operating in the undercoupled regime. By this approach, a record high Q factor value of (1.38*0.04)*10^6 has been demonstrated for buried waveguide micro-ring resonators operating around 780 nm corresponding to a propagation loss of 0.261*0.009 dB cm^−1. This high-Q micro-ring resonator could serve as a frequency reference for laser frequency stabilisation but a temperature dependent resonance wavelength shift of 7.34 pmK^−1 is expected and would require active temperature stabilisation to within 25mK. Passive athermalisation of a micro-ring resonator by optimising the thickness of polymer top cladding materials with a negative thermo-optic coeffcient has been investigated by simulation. A significant reduction of the temperature dependent wavelength shift of a micro-ring resonance peak has been found for PMMA and SU8 top cladding layers, 1.20 pmK^−1 and 0.14 pmK^−1 respectively corresponding to a required temperature stability of 167mK and 1.43K. Coupling of light from a Si3N4 waveguide to collimated free-space Gaussian beam has been considered within the context of developing an on-chip interferometer intended to measure the displacement of a MEMS gravimeter proof-mass. The divergence of a free-space coupled Gaussian beam has been determined. Taking into account the use of an SU8 spot-size converter, the Fresnel reflectance of the SU8 waveguide, and the measured reflectance of the silicon proof mass, the beam divergence is still found to be the dominant loss mechanism for the propagation distances involved in this application. Use of a micro-ball lens to collimate the beam was considered using various simulation techniques, determining that the vertical positioning of the lens would pose the greatest fabrication challenge. Processes were developed to produce SU8 waveguides, registration blocks, and micro-ball lens receptacles.