Wan, Haoran; Yang, Yulin; Qiao, Zhongliang; Li, Xiang; Sia, Jia Xu Brian; Yu, Wenjun; Weng, Dengqun; Li, Zaijin; Li, Lin; Chen, Hao; Zhao, Zhibin; Bo, Baoxue; Gao, Xin; Qu, Yi; Liu, Chongyang; Wang, Hong; Zhang, Yu; Niu, Zhichuan Source: Zhongguo Jiguang/Chinese Journal of Lasers, v 51, n 6, 2024; Language: Chinese;

Abstract:

Objective The tunable laser near the 2 μm wavelength has attracted significant attention due to its importance in gas detection and potential as a core component in high-capacity optical communication technologies. The scheme based on the integration of a silicon photonic chip with a III-V gain chip is gaining popularity in the research field of external cavity tunable lasers owing to its narrow linewidth and wide tuning range. While previous studies have primarily explored the ~1.55 μm wavelength range, limited research has been conducted on ~2 μm wavelength external cavity semiconductor lasers (ECDLs). In addition, in the fields of spatial signal transmission and gas detection, ECDLs must meet higher requirements for the output power, side mode suppression ratio (SMSR), and stability. Therefore, further optimization of the external cavity of the ~2 μm tunable laser is needed. Methods Based on the 220 nm silicononinsulator (SOI) platform, we first used the Lumerical mode to simulate and analyze the mode loss characteristics of optical waveguides with different crosssectional sizes and bending radii. We then studied the impact of the microring resonator Ggapmrr on the performance of a vernier filter (including the Q value, side mode suppression ratio, linewidth, and transmission loss). In addition, we analyzed the effect of waveguide termination reflectivity on the stability of the external cavity of a tunable laser and proposed waveguide termination based on the scattering and bending loss characteristics of multimode waveguides to terminate stray light in the waveguide. Finally, a thermal conduction analysis of the vernier filter was performed using Lumerical device, and the thermal analysis data were imported into Lumerical interconnect to study the thermal tuning performance of the tunable laser external cavity. Results Based on the above methodology, Si waveguide widths ranging from 0.28 µm to 0.63 µm facilitate quasi-singlemode transmission within the 1.95‒2.05 µm wavelength range (Fig. 2). Investigations of various Si waveguide thicknesses indicate a stable effective refractiveindex difference between TE00 and TM00 when the waveguide thickness is below 0.24 µm (Fig. 3). Optimizing the waveguide width enables lowloss TE00 mode transmission in straight waveguides (Fig. 4), and a bending radius exceeding 5 µm tends to approach TE00 mode losses of approximately 0 dB/cm (Fig. 5). By exploring the impact of Ggapmrr on the vernier filter performance (as depicted in Fig. 8), it is found that increasing Ggapmrr enhances the SMSR. However, beyond 250 nm, the improvement stabilizes owing to the reduced coupling coefficients, elongating the effective length of the microring resonators. Consequently, the insertion loss increases with increasing Ggapmrr, affecting the efficiency of the filter. The linewidth decreases sharply and levels off, whereas the Q factor exhibits an inverse trend. Above 300 nm, the difference in radius between the microrings significantly influences the Q values and transmission losses. The transmission spectrum of the vernier filter (Fig. 9) displays fine fringes and significant changes in the SMSR and full width at half maxima, owing to reflections from the bus waveguide terminator. To address this issue, the proposed multimode annular waveguide termination (Fig. 10) effectively terminates stray light, with negligible reflected optical power in the waveguide on the order of 10 W. The vernier filter achieves wavelength tuning through nickelchromium alloy microheaters atop the microrings, leveraging thermaloptic effects. Simulations in SOI waveguides reveal changes in the effective refractive index of the TE00 mode with temperature at wavelengths of 1.95, 2, and 2.05 µm (Fig. 11). Utilizing Lumerical device for thermal conduction simulations, the temperature distribution in the circuit under an applied voltage indicates improved efficiency at 4 V, resulting in a temperature increase of 127 K (Fig. 13). The study delves into the broad and finetuning of a siliconbased tunable laser's external cavity, showing both a wide tuning range of 66 nm (1967 ‒ 2033 nm) at a 3.2 V bias when using a single microheater and a precise tuning with a 0.1 nm/K accuracy when using two microheaters simultaneously (Figs. 14 and 15). Conclusions Research on siliconbased external cavity tunable lasers around the ~2 µm wavelength remains limited. Using a 220 nm SOI platform, we simulate and analyze the mode loss characteristics of optical waveguides of various sizes. A designed Si waveguide (600 nm×220 nm) ensures a lowloss TE00 singlemode transmission in a curved waveguide with a radius exceeding 5 µm. To investigate the impact of a single microring resonator on vernier filter performance, optimal coupling distances are discussed for applications with different requirements. We propose a highly processcompatible multimode annular waveguide termination method. Simulations demonstrate a wide 66 nm tuning range and a finetuning accuracy of 0.1 nm/K for the designed siliconbased tunable laser.

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