Microresonator Frequency Combs Exploiting Quadratic and Cubic Optical Nonlinearities

QuadCOMB致力于探索基于微谐振器的频率梳状源的新概念，该频率梳状源在芯片级器件中结合了二阶(二次)和三阶(三次)光学非线性。在过去的几年里，在具有立方非线性的高Q微谐振器中产生的所谓的克尔梳子从理论和技术角度都引起了人们的极大兴趣。具体地说，这些设备提供具有统一线路功率的宽带频谱和数十或数百吉赫兹的自由频谱范围，同时完全服从芯片级集成。因此，克尔频率梳有可能颠覆一系列高度相关的应用，从光通信中的大规模并行波分复用(WDM)到高精度光学测距和高分辨率光谱分析。然而，尽管有这样的潜力，克尔梳子的广泛应用仍然受到高抽运功率水平和由于相当弱的立方非线性而导致的低功率转换效率的阻碍。此外，目前还缺乏有效的控制克尔梳子载波包络偏移频率的方案，从而阻碍了克尔梳子在高精度光学计量和光学频率标准中的应用。在QuadCOMB中，我们将探索、实施并实验演示在光子集成电路(PIC)中产生频率梳的新方法，该方法通过将克尔类型立方体与二次光学非线性相结合来提高功率转换效率并稳定载波包络偏移频率。我们考虑混合器件的概念，其中立方和二次光学非线性首先在不同的集成平台上实现，然后通过3D打印芯片-芯片连接在封装级别上合并，以及构建在同时具有立方和二次非线性的单个集成平台上的单片集成器件。作为起点，我们将建立一个理论框架，解释三次和二次非线性的同时影响，这将使我们能够研究梳子形成的动力学，并预测由此产生的梳子的特性。在此基础上，我们将设计、制造和表征混合型和单片梳状源，开发具有立方非线性的氮化硅(Si3N4)PIC和同时提供二次和立方非线性的AlGaAs-on-Insulator PIC。在梳状源成功实现的基础上，我们将在与超宽带信号处理和高分辨率光谱学相关的概念验证实验中展示其可行性。QuadCOMB汇集了来自参与的德国和法国小组的领先研究人员，其中包括一位著名的科学家作为墨卡托研究员，QuadCOMB可以依靠在光子集成、非线性光学和芯片级频率梳源领域的国际独特技能组合。

QuadCOMB aims at exploring novel concepts for microresonator-based frequency comb sources that combine second-order (‘quadratic’) and third-order (‘cubic’) optical nonlinearities in a chip-scale device. Over the previous years, so-called Kerr combs, generated in high-Q microresonators with cubic nonlinearities, have raised significant interest both from a theoretical and a technological perspective. Specifically, these devices offer broadband spectra with uniform line powers and free spectral ranges of tens or hundreds of gigahertz, while being fully amenable to chip-scale integration. Kerr frequency combs thus open the potential to disrupt a series of highly relevant applications, ranging from massively parallel wavelength-division multiplexing (WDM) in optical communications to high-precision optical ranging and high-resolution spectroscopy. However, despite this potential, widespread deployment of Kerr combs is still hindered by the high pump power levels and by the low power conversion efficiency via the rather weak cubic nonlinearities. In addition, efficient schemes for controlling the carrier-envelope offset frequency of Kerr combs are still lacking, thereby impeding the application of Kerr combs in high-precision optical metrology and optical frequency standards. Within QuadCOMB, we will explore, implement, and experimentally demonstrate novel approaches for frequency comb generation in photonic integrated circuits (PIC) that offer improved power conversion efficiency along with stabilized carrier-envelope offset frequency by combining Kerr-type cubic with quadratic optical nonlinearities. We consider hybrid device concepts, where cubic and quadratic optical nonlinearities are first realized on distinct integration platforms and then merged on a package-level by 3D-printed chip-chip connections, as well as monolithically integrated devices, built on a single integration platform that simultaneously features cubic and quadratic nonlinearities. As a starting point, we will develop a theoretical framework that accounts for the simultaneous influence of cubic and quadratic nonlinearities and that will allow us to study the dynamics of comb formation and to predict the characteristics of the resulting combs. Based on this, we will design, fabricate and characterize hybrid and monolithic comb sources, exploiting silicon-nitride (Si3N4) PIC with cubic nonlinearities and AlGaAs-on-Insulator PIC offering both quadratic and cubic nonlinearities. Based on successful implementation of the comb sources, we shall demonstrate their viability in proof-of-concept experiments related to ultra-broadband signal processing and high-resolution spectroscopy. Bringing together leading researchers from the participating German and French groups and including a renowned scientist as a Mercator Fellow, QuadCOMB can rely on an internationally unique combination of skills in the field of photonic integration, nonlinear optics and chip-scale frequency comb sources.