Photonic Ultra-high-Q REsonators (PURE)

Lead Research Organisation: University of Southampton
Department Name: Optoelectronics Research Centre (ORC)


Photonic ring resonators are miniature optical waveguiding structures that enable light to reach very high intensities in closed, circular paths. The loop structure and wave nature of light results in interference of the field such that the system becomes highly resonant with a repeated pattern. Each ring supports a comb of highly defined, specific frequencies of light, the spacing between which depends on the optical path length of the ring. In devices with a high-quality factor (high-Q), the optical circulating power can build up from a small milliwatt input signal to reach kilowatts of circulating power. The small, guided area of these devices results in immense power densities, permitting non-linear optical effects at remarkably low powers, despite the host material having low intrinsic non-linear properties. However, the achievable quality (Q) of such resonators has so far been limited by the losses caused by the absorption and scattering of light by the materials and structures used to fabricate the ring.

The last 20 years have enabled significant progress in integrated photonics (optical circuits that guide and manipulate light analogous to the microchip in electronics), including the reduction of loss. Refined processes using CMOS-based cleanroom techniques have allowed researchers to improve optical transmission from 10% per metre to approximately 99.9% per metre in miniaturised optical chips. This has enabled the fabrication of optical microresonators with ultra-high-Q factors (over 100 million). These wafer-based devices form key components in advanced integrated photonic circuits for narrow linewidth lasers and frequency combs. The first generation of these devices has enabled compact systems for radar as well as for precision timing and navigation.

Despite significant progress in the field, waveguide loss in state-of-the-art integrated photonics devices has plateaued at 100x higher losses than those readily achieved in standard telecoms optical fibre used for long-haul broadband internet. This limit is not fundamental but technological, and if fibre-like losses could also be achieved in an integrated photonics package, this would enable a new generation of applications and improvements in performance. These include compact, robust gyroscopes and low-power frequency combs for navigation and precision timing, ultra-narrow linewidth lasers (mHz to Hz), and advanced photonic components for telecommunication networks.

This proposal seeks to combine the benefits of optical fibre fabrication approaches and material science developed over the past 50 years with the latest state-of-the-art CMOS fabrication techniques used for integrated optics. We aim to develop a manufacturing technique that will produce integrated ring resonator devices with the highest Q ever achieved. Using flame hydrolysis deposition and other standard optical fibre manufacturing techniques, we will develop ultra-pure glass layers to negate absorption losses. In particular, we will focus on high phosphorus and germanium doping, which we have shown can lead to dramatically better uniformity during our recent Caltech-Southampton DARPA seed project. We will use optical fibre manufacturing techniques to reduce loss from absorbed hydrogen and develop diffusion and reflow processes to remove waveguide interface and scattering losses.

Our ambition is to develop the foundations for a scalable manufacturing process for the next generation of ultra-high-Q micro-ring resonators. These devices will enable a range of new technologies, including rugged miniature gyroscopes for navigation, combs for precision timing in data networks and optical sources for quantum technologies.


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