Xie, Zhonghua; Qu, Hongwei; Zhou, Xuyan; Zhang, Jianxin; Sui, Jiatong; Meng, Fansheng; Gong, Kai; Zheng, Meiyin; Wang, Hailing; Wang, Yufei; Qi, Aiyi Source: Zhongguo Jiguang/Chinese Journal of Lasers, v 51, n 6, 2024; Language: Chinese;

Abstract:

Objective A 795 nm vertical cavity surface emitting laser (VCSEL) has the advantages of a low threshold current, single-mode operation, a low power consumption, and high temperature and reliability. It is an ideal light source for quantum precision measurement in devices such as atomic clocks, atomic magnetometers, and atomic gyroscopes. A VCSEL typically uses oxidation limiting structures for mode regulation to achieve electro-optic confinement. A VCSEL with conventional circular oxide apertures has an axisymmetric structure, making it difficult to achieve polarization control in two orthogonal directions and prone to polarization instability with increasing current. The application of a VCSEL in a device such as a chip-level atomic clock requires it to have a stable polarization direction and high polarization suppression ratio. Asymmetric oxidation apertures are introduced to improve the polarization stability of the 795 nm VCSEL used for rubidium atomic clocks. Controlling the oxidation conditions such as the temperature and gas pressure in the wet oxidation process makes it possible to fabricate VCSELs with different oxidation rates and ellipticity values for their apertures and analyze the effects of these on the polarization performance, which assists in achieving a low threshold and high polarization output. Methods A two-dimensional cold-cavity simulation of a 795 nm VCSEL is conducted using the fluctuation optical frequency domain module in COMSOL Multiphysics, and the effects of different oxidation apertures on the resonant light intensity in the active region are simulated. The influence of the oxidation furnace temperature on the oxidation rate and ellipticity is studied using a real-time monitoring wet oxidation system and controlling oxidation parameters such as the temperature and gas pressure in the wet oxidation process. The ellipticity and aperture parameters are obtained by the ellipse fitting of the captured oxidation aperture images. Three types of VCSELs with different long-axis diameters and ellipticity values are prepared using a circular table. The power-current-voltage (P-I-V) curves, mode characteristics, line widths, polarization characteristics, and polarization angle rotation characteristics of the three devices are tested and analyzed. Results and Discussions The influence of the oxidation furnace temperature on the oxidation rate and ellipticity is determined by using the real-time monitoring wet oxidation system and controlling oxidation parameters such as the temperature and gas pressure in the wet oxidation process. Three types of VCSELs with different long-axis diameters and ellipticity values are prepared using a circular tabletop, and their mode characteristics, polarization characteristics, and polarization angle rotation characteristics are analyzed and studied. The experimental results show that a VCSEL with elliptical oxidation holes with a long-axis diameter of 3.7 μm and ellipticity of 1.7 has the best performance (Fig. 7). At 85 ℃, with an injection current of 1.5 mA, the output power is 0.86 mW, laser wavelength is 795.4 nm, side mode suppression ratio (SMSR) is 43 dB, line width is 65 MHz (Fig. 13), and orthogonal polarization suppression ratio (OPSR) is 23.8 dB. The VCSEL remains unchanged in the main polarization direction within the range of 0.6 ‒ 2.7 mA. According to the test results, the three types of VCSELs with different major-axis diameters and ellipticity values show different unidirectional rotation values for the main polarization direction angle with an increase in current (Fig. 14). Conclusions In order to improve the polarization stability of the 795 nm VCSEL used for Rb atomic clocks, this study discusses the effects of different oxidation apertures and ellipticity values on the polarization performance of a VCSEL. The influence of different oxide apertures on the resonant light intensity in the active region is simulated using the fluctuation optical frequency domain module of COMSOL Multiphysics. The results show that the highest resonance intensity in the active region is obtained when the oxidation aperture is 3.5‒4.0 μm. The real-time monitoring wet oxidation system is used to study the effects of the oxidation furnace temperature on the oxidation rate and ellipticity. As the injection current increases, the three types of elliptical oxidation aperture VCSELs exhibit different mode characteristics, polarization characteristics, and polarization angle rotation values. The test results indicate that the VCSEL with an elliptical oxidation aperture with a long-axis diameter of 3.7 μm and an ellipticity of 1.7 has the best performance. At 85 ℃, with an injection current of 1.5 mA, the output power is 0.86 mW, laser wavelength is 795.4 nm, SMSR is 43 dB, line width is 65 MHz, and OPSR is 23.8 dB. The main polarization direction of the VCSEL remains unchanged within the range of 0.6‒2.7 mA.

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