Photonic Ultra-high-Q REsonators (PURE)

光子环谐振器是一种微型光波导结构，可以使光在封闭的圆形路径中达到非常高的强度。光的环路结构和波的性质导致了场的干涉，使得系统变得与重复的模式高度共振。每个环支持一个高度定义的、特定频率的光梳，它们之间的间距取决于环的光路长度。在具有高质量因数（高q）的器件中，光循环功率可以从一个小毫瓦的输入信号增加到达到千瓦的循环功率。这些器件的小导面积导致了巨大的功率密度，允许在非常低的功率下产生非线性光学效应，尽管宿主材料具有较低的固有非线性特性。然而，到目前为止，这种谐振器的可实现质量(Q)受到用于制造环的材料和结构对光的吸收和散射所造成的损失的限制。在过去的20年里，集成光子学（引导和操纵光的光学电路，类似于电子学中的微芯片）取得了重大进展，包括减少损耗。使用基于cmos的洁净室技术的精细工艺使研究人员能够将小型化光学芯片的光传输从每米10%提高到每米99.9%左右。这使得制造具有超高q因子（超过1亿）的光学微谐振器成为可能。这些基于晶圆的器件构成了用于窄线宽激光器和频率梳的先进集成光子电路的关键部件。第一代这些设备使雷达以及精确定时和导航的紧凑系统成为可能。尽管在该领域取得了重大进展，但最先进的集成光子器件的波导损耗已经稳定在比用于长途宽带互联网的标准电信光纤高100倍的水平。这个限制不是基本的，而是技术上的，如果在集成光子封装中也能实现类似光纤的损耗，这将使新一代的应用和性能的提高成为可能。其中包括用于导航和精确定时的紧凑、坚固的陀螺仪和低功率频率梳，超窄线宽激光器（mHz至Hz），以及用于电信网络的先进光子组件。该提案旨在将过去50年来发展的光纤制造方法和材料科学的优势与用于集成光学的最新最先进的CMOS制造技术相结合。我们的目标是开发一种制造技术，将生产具有最高Q的集成环形谐振器器件。利用火焰水解沉积和其他标准光纤制造技术，我们将开发超纯玻璃层来抵消吸收损失。我们将特别关注高磷和高锗掺杂，在我们最近的加州理工-南安普顿DARPA种子项目中，我们已经证明了高磷和高锗掺杂可以显著提高均匀性。我们将使用光纤制造技术来减少吸收氢的损失，并开发扩散和回流工艺来消除波导界面和散射损失。我们的目标是为下一代超高q微环谐振器的可扩展制造工艺奠定基础。这些设备将使一系列新技术成为可能，包括用于导航的坚固微型陀螺仪、用于数据网络精确定时的梳子和用于量子技术的光源。

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.