Spatio‐Temporal Nonlinear Theory in Birefringent Microrings

• Published in: Laser & photonics reviews 2024-08, Vol.18 (8), p.n/a
• Main Authors: Tan, Zelin, Chen, Xingqiao, Liu, Ning, Xu, Jipeng, Zhang, Jianfa, Zhu, Zhihong, Liu, Ken, Qin, Shiqiao
• Format: Article
• Language: English
• Subjects: Azimuth; azimuth‐dependent; birefringence; Coupled modes; Efficiency; Energy conversion efficiency; Frequency ranges; Harmonic generations; Integrated circuits; Light sources; Lithium niobates; microrings; Nonlinear phenomena; nonlinear process; Phase matching; spatio‐temporal coupled‐mode equation; X‐cut lithium niobate
• ISSN: 1863-8880; 1863-8899
• DOI: 10.1002/lpor.202301281

Frequency‐dependent nonlinear process in microresonators is widely acknowledged, but there is no theory available to calculate the conversion efficiency for each resonance of the ring, except for the phase‐matching one. Similarly for azimuth‐dependent nonlinear process in birefringent rings, there is a lack of theory to analysis the conversion efficiency for each azimuth of the ring. Consequently, it leads to old‐fashioned or ill‐considered coupling position and inefficient energy conversion in birefringent microrings. Here, the study introduces spatio‐temporal coupled‐mode equation to describe mode spatial properties in the cavity, compensating for the deficiency of temporal coupled‐mode equation in describing sophisticated responses. By this equation, it is found that over a wide frequency range, the extremely strong second‐harmonic generation of TE modes can be achieved at different azimuth under different pumps in an X‐cut lithium niobate microring, as proved by a 440 000%/W conversion efficiency in a non‐poling ring recently. This theory is important for realizing an efficient entangled quantum light source, for example. Meanwhile, it will provide new ideas and guidelines for design and applications of monolithic birefringent photonic integrated circuits with high energy efficiency. The spatio‐temporal coupled‐mode theory is proposed for the description of frequency‐ and space‐related nonlinear process in birefringent microrings, which is new and can draw the interests from the birefringent platform photonic integrated circuits. This work has implications for the design and application of monolithic birefringent photonic integrated circuits with high efficiency.