波长可调谐氮化硅外腔半导体激光器

Wavelength⁃Tunable Silicon Nitride External⁃Cavity Diode Laser

波长可调谐外腔半导体激光器在片上全光通信和片上光子器件互连集成中发挥着至关重要的作用。采用两个半径不同的微环谐振器和两种不同结构的马赫-曾德尔干涉仪,设计了一种基于厚度为700 nm的氮化硅波导的外腔反射镜,并采用大马士革工艺制作了外腔反射镜。结合Ⅲ-Ⅴ族反射型半导体光放大器和可调谐氮化硅外腔反射镜,通过对移相器热电极电压、可调耦合器热电极电压和微环谐振器热电极电压等参数进行调节,获得了最大调谐范围为55 nm、边模抑制比超过40 dB的外腔半导体激光器。

Objective Semiconductor lasers are pivotal light sources in optical communication systems. Owing to their compact size and lightweight, they are beneficial for seamless integration with other devices for monolithic optoelectronic solutions. They play an important role in various fields, including fully integrated optical communication systems, optical sensing, light detection and ranging(LiDAR), and ultra-wideband wavelength division multiplexing(WDM) systems. However, silicon is an indirect bandgap material with a low luminescence efficiency, making it unsuitable as an efficient gain medium for semiconductor lasers. A practical solution involves combining Ⅲ-Ⅴ materials, known for their direct bandgap, with Si, which exhibits low propagation loss. Two-photon and free-carrier absorption are typically negligible because silicon nitride has a wider bandgap than silicon. Therefore, silicon nitride has been gaining significance for the formation of external-cavity structures in semiconductor lasers. Currently, typical external-cavity structures utilize micro-ring resonators or Sagnac loop reflectors as the reflective ends. However, the dispersion effect of the silicon nitride waveguide introduces variability in reflectivity with the operating wavelength, resulting in uncontrollable reflectivity and limiting improvements in laser performance. In this study, we report an external-cavity reflection structure designed to control reflectivity, enabling laser mode selection and the optimization of output characteristics.Methods Figure 1 illustrates the schematic structure of a tunable silicon nitride diode external-cavity laser. Within this structure,the reflective semiconductor optical amplifier(RSOA) and external-cavity reflector collectively form a Fabry–Perot resonant cavity.Light resonates within this cavity and is amplified within the gain medium. If the resulting gain is sufficient to overcome the losses of the resonant optical mode within the cavity, a relatively coherent light is emitted. A spot-size converter is employed as an inverted cone structure to realize efficient coupling between the RSOA and external-cavity mirror. The converter has a height of 700 nm,length of 200 μm, and a width, which linearly varies from 250 nm to 750 nm. The structures of the external mirror are shown in Fig. 2. It comprises three main components: a phase shifter, tunable coupler, and vernier filter. Transmission and reflection spectra of the external-cavity mirror are obtained to determine the optimal device dimensions. A phase shifter with a length of approximately 1 mm is used to adjust the longitudinal mode within the cavity. The tunable coupler consists of a symmetrical Mach-Zehnder interferometer(MZI) with an arm length of 1 mm, allowing for the control of mirror reflectivity through the application of voltage to the MZI. The vernier filter comprises two micro-ring resonators(MRR1 and MRR2) with different radii and an asymmetric MZI(d-MZI). This enhances the wavelength-tuning range and side-mode suppression ratio of the external-cavity diode laser. The radii of MRR1 and MRR2 are 81 μm and 84.3 μm, respectively. The arm length difference in the d-MZI is 264 μm. The total cavity length of the mirror is 24.3 mm, with dimensions of 6.0 mm×1.8 mm.Results and Discussions Simulated results indicate that applying a voltage to the tunable coupler can control the effective reflectance of the mirror from 0 to 1(Fig. 3). The dissymmetry MZI enhances the side-mode suppression ratio of the vernier filter from 0.9 dB to 8.7 dB at 1523 nm and 7.1 dB to 12.8 dB at 1586 nm. The free spectral range of the filter is 55 nm(Fig. 4). The experimental results show that the laser output wavelength is 1523.8 nm without applying additional voltage, with a side-mode suppression ratio exceeding 40 dB(Fig. 7). By controlling the phase shifter, the minimum and maximum wavelength tunings are 4 pm and 20 pm, respectively. A tuning range of 55 nm is realized by adjusting the voltage on the single micro-ring resonator(Fig. 8). The laser output power varies from-20 dBm to-13 dBm with respect to the varying voltage applied to the tunable coupler, which is consistent with the sinusoidal trend in Fig. 3(Fig. 8). Moreover, the tunable coupler can effectively avoid dual-wavelength lasing and allow the selection of the operating wavelength of the laser(Fig. 9).Conclusions An external-cavity reflection structure, comprising two micro-ring resonators and two distinct Mach-Zehnder interferometers, is designed and fabricated using the Damascus process. A semiconductor external-cavity laser, with a side-mode suppression ratio of over 40 dB and a maximum tuning range of 55 nm, which covers the entire communication band, is demonstrated. The laser is realized by combining the Ⅲ-Ⅴ RSOA and a silicon nitride external-cavity reflection structure. In the future, a higher output optical power can be achieved by adopting RSOA chips with a higher gain. A laser mode-hopping phenomenon is observed during the experiment. A mode-hop-free laser output can be further realized by optimizing the ratio of the voltage applied to the phase shifter and micro-ring resonator.