Nie, Yuwei; Li, Wei; Jiagang, Lü; Pan, Zhipeng; Liu, Suping; Ma, Xiaoyu Source: Zhongguo Jiguang/Chinese Journal of Lasers, v 51, n 6, 2024; Language: Chinese;

Abstract:

Objective The vertical cavity surface- emitting laser (VCSEL) is a type of semiconductor laser that emits light perpendicular to the substrate surface. The VCSEL, as a chip- level atomic clock light source, must possess good single- mode characteristics. Thus the individual frequency required by the atomic clock can be precisely modulated to stimulate the atomic clock s operation, and the atomic clock is ensured not to absorb other signals during the modulation process. The practical design necessitates confining the electric and optical fields of the VCSEL to secure strong single- mode characteristics, while also optimizing the epitaxial structure of the device. This study aims to develop a 795 nm VCSEL device with excellent power characteristics, capable of functioning at high temperatures up to 380 K, and achieving a fundamental mode output in the desired wavelength range. Methods First, the quantum well structure is designed using strain- compensated quantum well band theory and the Kronig- Penney model to determine the material composition and thickness parameters of the quantum well. This ensures that the quantum well material exhibits high gain, with a peak wavelength of 795 nm at the high temperature of 380 K. Next, the distributed Bragg reflector (DBR) is designed using the transfer matrix theory to determine the material compositions of the high and low refractive index layers. The logarithm of the power reflectance for the P-type DBR and N-type DBR is calculated. Following this, the oxide confinement layer is analyzed using the fiber waveguide theory and a thermoelectric coupling model to achieve good single-mode characteristics and thermal properties of the VCSEL. The oxide aperture for the VCSEL in the fundamental mode lasing is calculated. After simulating and calculating the parameters, the devices are fabricated. Different-sized mesa structures are designed in various regions of the layout, and four VCSEL devices with different oxide apertures are fabricated on a single chip. A comparative analysis is performed on these devices to draw conclusions. Results and Discussions The epitaxial wafer results obtained in this study correspond well with the simulation results (Fig. 9). Wet oxidation serves to form the oxide confinement aperture under high-temperature conditions. The oxidation depth is controlled by adjusting the oxidation time, resulting in oxide apertures of 1.9, 3.8, 4.9, and 6.9 μm in a single-chip fabrication process (Fig. 12). Power-current measurements are performed on the fabricated devices. For the device with a 3.8 μm oxide aperture, the threshold current measures at 1 mA, the maximum output power is 2 mW, and the slope efficiency is 0.3 W/A (Fig. 14). When the oxide aperture is 1.9 μm, the device maintains single-mode output throughout the injection current range of 3 ‒ 7 mA, with a side-mode suppression ratio exceeding 35 dB. When the oxide aperture is 3.8 μm, the side-mode suppression ratio surpasses 30 dB. The operating wavelength at room temperature hovers around 790 nm, meeting the requirements for applications (Fig.15). Conclusions This study concentrates on the design of a single-mode 795 nm VCSEL device structure and active region. The gain spectrum of In0.08Ga0.79Al0.13As strained quantum wells is simulated. At room temperature (300 K), the gain peak wavelength is 777 nm. At 380 K, the gain peak wavelength shifts to the desired 795 nm range, with a redshift rate of 0.238 nm/K. A single-mode device is achieved by employing an oxide confinement structure. The device structure and oxide aperture are optimized and designed. The optical and electrical limitations of the oxide aperture are simulated using the fiber waveguide theory and a thermoelectric coupling model, resulting in an oxide aperture of 3.72 μm for the VCSEL in single-mode operation. Moreover, VCSELs with oxide apertures of 1.9, 3.8, 4.9, and 6.9 μm are fabricated in a single-chip process. The fabricated devices are characterized by power-current characteristics and spectral properties. When the oxide aperture is 1.9 μm, the device maintains single-mode output throughout the injection current range of 3‒7 mA, with a side-mode suppression ratio exceeding 35 dB. For the device with a 3.8 μm oxide aperture, it operates in single-mode, with a threshold current of 1 mA at room temperature, a maximum saturated output power of 2 mW, a slope efficiency of 0.3 W/A, and an emitted wavelength of 790 nm under 3 mA injection current, with a side-mode suppression ratio exceeding 30 dB. This study successfully obtains a robust single-mode output device with an emission wavelength of 790 nm, consistent with the design. Considering that atomic clocks need to operate at high temperatures and VCSEL emission wavelength redshifts with temperature, with a temperature coefficient of approximately 0.06 nm/K, the devices fabricated in this study have reserved a wavelength redshift of 5 nm. This ensures the achievement of 795 nm single-mode output under high-temperature conditions for atomic clock applications. It lays the foundation for subsequent high-temperature operation and polarization-selective VCSELs.

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