可调谐垂直腔面发射激光器支撑结构优化设计

Design Optimization of Mechanical Support Structures in Tunable Vertical Cavity Surface Emitting Lasers

基于微机电系统(MEMS)的850 nm可调谐垂直腔面发射激光器(VCSEL),设计了一种双曲线梁结构,以提升器件机械和调谐特性。通过分析传统等截面梁结构的受力情况,提出了双曲线结构优化设计,将梁结构端面的面积增大从而降低最大应力。理论仿真结果表明:优化后器件上反射镜的最大偏移量基本保持不变,支撑梁上下表面的最大应力分别降低了23.4%和17.0%,谐振频率增大了7.9%;当MEMS-VCSEL分别为半导体腔主导(SCD)结构和空气腔主导(ACD)结构时,波长调谐范围分别为16.6 nm和42 nm。该优化方式的优势在于不需要改变激光器的结构,同时可与其他优化方式兼容,具有一定的应用前景。

Objective Vertical cavity surface emitting lasers(VCSELs) based on micro-electro-mechanical systems(MEMSs) are widely applied in optical communication, optical coherence tomography, and other fields, with the advantages of wide continuous tuning range, fast tuning rate, and low power consumption. The tuning modes of MEMS-VCSEL include electrostatic, piezoelectric and thermoelectric, but the basic principle is to control the mirror shift to change the overall cavity length of the resonant cavity to achieve wavelength tuning. The support structure of the suspended mirror will be subjected to alternating loads during tuning, and the stress concentration will occur at the fixed end position of the structure, leading to the fatigue fracture. The optimization design of the beam structure can improve the reliability of the device at a low cost, which is a high-feasibility optimization method. In this paper, the support beam structure of the suspended mirror is optimized on the micro/nano-scale, and the hyperbolic beam structure is proposed. The maximum stress is reduced, and the resonant frequency is improved without changing the maximum offset and the corresponding bias of the mirror in the MEMS structure. The proposed optimization method can improve the mechanical and tuning characteristics of the device without introducing additional process steps, which is well compatible with other materials and structure optimization methods.Methods A hyperbolic beam structure is designed for 850 nm tunable VCSEL based on MEMS to improve the mechanical and tuning characteristics. First, the Mises stress distribution of the traditional constant cross section(CCS)beam structure is analyzed by the finite-element method. The stress concentration problem will appear at the fixed end surfaces because the section modulus at different positions of the structure is the same with different moments when the CCS beam structure is subjected to uniform load. Then, the maximum stress on the structure can be reduced by increasing the geometry size of the end surface, and the stress distribution can be more uniform. Based on this theory, the hyperbolic beam structure is designed. In addition, the maximum offsets of the two structures before and after optimization are compared, which means that the stress optimization results do not sacrifice the offset of the mirror. Finally, the relationship between the resonant wavelength of MEMS-VCSEL and the applied bias is calculated by the frequency domain analysis method, and the resonant frequencies of the MEMS structure before and after optimization are compared.Results and Discussions The mechanical and tuning characteristics of the two structure beams before and after optimization are compared. The most important parameters of mechanical properties are the maximum stress and offset of the structure. The stress distribution of the traditional CCS beam structure and the hyperbolic beam structure is calculated when the offset is 390 nm. As shown in Figs. 4 and 5, the stress distribution of the two structures is similar. For the traditional CCS beam structure, the maximum stress values of the upper and lower surfaces are 3. 25×107 and 3. 06×107,respectively. For the hyperbolic beam structure, the values are 2. 49×107 and 2. 54×107 respectively, which indicates that the maximum stress is reduced by 23. 4% and 17. 0% after optimization. For hyperbolic beam structure, hyperbolic shapes and stress reduction effects are both different, which means that the best mechanical properties can be obtained by optimizing the size of the hyperbolic beam structure. The wavelength tuning range and tuning rate are the most important parameters to evaluate the tuning characteristics of tunable lasers. The relationship between the offset of the upper mirror and the applied bias in the MEMS structure is calculated, and then the relationship between the cavity length of the laser and the applied bias is obtained. The wavelength tuning range of the device is obtained by the frequency domain analysis method, and the results are shown in Fig. 8. The influence of the air gap on the resonant wavelength is different under different coupling modes of the semiconductor cavity and the air cavity in the laser. For the semiconductor-cavity dominated(SCD) structure, the wavelength tuning range is narrow(16. 6 nm), but it is continuously tuned in the whole range. For the air-cavity dominated(ACD) structures, the wavelength coverage is 42 nm, but "mode hopping" occurs during tuning. The resonant frequencies of the two beam structures will decrease with the increasing applied bias, but the resonant frequency of the hyperbolic beam structure is always increased by 7. 9% compared with the CCS beam structure.Conclusions A hyperbolic beam structure is designed to improve the mechanical and tuning characteristics of MEMSVCSEL devices. The main principle is to reduce the maximum stress and improve the elastic coefficient of the structure by increasing the section size of the large bending moment. The maximum stress of the upper and lower surfaces decreases by 23. 4% and 17. 0% after optimization when the offset is 390 nm. There is little difference between the two structures in the maximum offset and the required applied bias. In addition, the resonant frequency of the hyperbolic beam structure is increased by 7. 9%. The wavelength continuous tuning range is 16. 6 nm when the coupling structure between the two cavities in the laser is SCD structure. The wavelength coverage is about 42 nm for the ACD structure, but there is a "mode hopping" phenomenon during tuning. This optimization method does not need to change the structure of the laser, and is compatible with other optimization methods, with certain application prospects.