Transformation Optics Enabled Ultra‐Compact High‐Q Micro‐Ring‐Resonator With High‐Precision Locking

Achieving ultra‐low propagation loss is essential for high‐Q microresonators and large‐scale photonic integration. Here, we demonstrate an ultra‐compact, ultra‐low‐loss silicon multimode micro‐ring resonator (MRR) using a transformation‐optics‐based mode converter fully compatible with standard fabrication. The proposed design enables efficient mode conversion and low‐loss multimode propagation while substantially reducing the bending radius. Experimentally, a record‐small bending radius of 10 µm is achieved for 2‐µm‐wide multimode waveguides. The fabricated MRR exhibits an intrinsic Q factor of 1.14 × 10 7 and a propagation loss of 5.8 dB/m. Compared with prior work using the same 2‐µm‐wide waveguide, our design reduces the bending radius by approximately 80% while increasing intrinsic Q by approximately 70%. Furthermore, we employ an optoelectronic oscillator (OEO) to lock the high‐Q MRR. Over an 18‐min measurement period, the free‐running MRR exhibits a frequency drift of 1.34 GHz, whereas the resonant‐frequency variation of the locked MRR is confined within 90 kHz. Meanwhile, the Allan deviation under the locked condition is reduced by approximately four orders of magnitude compared with the free‐running condition.