Transformation optics based ultra-compact high-Q micro-ring-resonator with high-precision locking

Achieving ultra-low propagation loss in optical waveguides is essential for enhancing micro-resonator quality factors and enabling large-scale photonic integration. Despite efforts to minimize scattering loss with multimode waveguides, bending radii remain large due to waveguide dimensions. We present an innovative method that combines multimode waveguides with a transformation optics (TO)-based mode converter to achieve ultra-low loss and exceptional bending characteristics in silicon-based multimode micro-ring resonators (MRRs). Our results demonstrate an unprecedented bending radius of just 10 µm for 2-µm-wide waveguides, alongside an intrinsic quality factor (Qᵢ) of (1.13 ± 0.15) × 10⁷ and ultra-low propagation loss of 5.8 ± 0.33 dB/m, a 26-fold reduction compared to conventional waveguides. Additionally, we propose a novel high-precision MRR wavelength locking method based on an optoelectronic oscillator (OEO), achieving a two-order-of-magnitude improvement in resonant wavelength stabilization. These advancements redefine the performance-density trade-off for MRRs, enabling scalable, ultra-dense photonic integration. Our work opens new possibilities for applications in microwave photonics, nonlinear photonics, large-scale integration, and quantum technologies, offering a versatile design framework for ultrahigh-Q multimode MRRs.