Impact of U-shaped waveguide structures on optical comb generation in microresonator systems

In the research of soliton microcombs, the pump energy conversion efficiency (ECE) of the microcomb is an important parameter. However, in traditional microresonator systems, the ECE is only a few percent. Researchers have enhanced it by introducing additional feedback coupling structures or composite cavity structures into the microresonator system to suppress the output of pump light in the soliton generation region. The additional structures may affect the output comb characteristics of the system, but most articles do not delve into this issue, which hinders the practical application of high-efficiency soliton microcombs. In this paper, we establish rate equations describing the nonlinear dynamics of a microresonator with a U-shaped waveguide feedback coupling structure (coupled-cavity system, CCS), and numerically simulate and conduct steady-state analysis of the simulation results to obtain the influence of the U-shaped waveguide on the CCS’s output comb characteristics. Then we optimize the structural parameters of the CCS reducing the parametric oscillation threshold of the pump by more than 50%, while increasing the ECE to over 25% when the optical field in the microresonator propagates in the form of a single soliton.