Coupling Ideality Optimization of High-Q Deuterated Silicon Nitride Microresonator

As one of the important photonic integrated platforms, microring resonators based on silicon nitride have great potentials in photonic applications such as filters, sensors, modulators, as well as microcombs generation. They are usually designed with a multi-mode waveguide to guarantee lower loss and desirable dispersion properties. To improve device performance such as quality factor (Q) and extinction ration (ER), the coupling region should be design carefully for restraining the unwanted excess coupling loss induced by the low-Q higher order modes and enhancing the power coupling to high-Q fundamental modes. Coupling ideality investigation is a useful tool to evaluate the coupling loss induced from modal coupling intuitively, which is defined as the ratio of power coupled to the targeted modes divided by the total power coupled to all modes. In this work, we conduct a detailed coupling ideality investigation of the high-Q deuterated silicon nitride (SiN:D) microring resonators based on a low-temperature PECVD platform, of which details of fabrication were reported in our previous work . The ring waveguides have a fixed multimode cross-section of 2 μm wide and 0.75 μm thick while the width of coupling bus waveguides and the coupling gap vary to verify the optimized coupling region design. As shown in Fig. 1 (b), the coupling ideality is generally high except for W
_bus
=1.8 μm that does not fit well with the ideal coupling curve. Transmission spectrum of the device with a coupling gap of 200 nm [Fig.1 (e)] exhibits pure single mode and a higher ER than those with gaps of 100 nm and 300 nm [Figs. 1 (c) and 1 (f)], respectively. With proper choice of W
_bus
and a coupling gap, a high loaded-Q exceeding 1 million close to critical coupling can be achieved. Our experimental results are in accordance with simulation results, and can be beneficial to improve the resonant performance by giving design guidelines.