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题名	垂直腔面发射激光器高阶模式及偏振控制
作者	李秀山
学位类别	博士
答辩日期	2015-05
授予单位	中国科学院大学
导师	宁永强
关键词	垂直腔面发射激光器 横向模式 偏振 光场
其他题名	Polarization and high-order mode control of vertical cavity surface emitting lasers
学位专业	凝聚态物理
中文摘要	相比于边发射半导体激光器，垂直腔面发射半导体激光器（VCSEL）具有如下优点：远场发散角小，腔面损伤阈值高，光纤耦合容易，纵模单一性好，芯片可在片测试等。但是VCSEL较大的横向尺寸，导致激射的横向模式多，输出光谱宽。圆形台面的对称性，引起出射光的偏振方向不稳定。VCSEL的多横模及非稳定偏振方向的特征限制了在光通讯、光储存、激光打印、光谱分析等领域需要VCSEL具有单偏振方向、单横模等领域的应用。针对VCSEL的多横模和偏振不稳定的这两个缺点，本文通过设计新的器件结构，控制VCSEL的高阶模式输出和偏振态，达到横向模式减少和偏振性稳定的目的。下面分别从抑制高阶模式输出和控制偏振态两个方面进行详细分析和研究。 1.VCSEL的高阶模式控制。VCSEL抑制高阶模式输出的基本方法是：增大高阶模式的阈值和降低基模的阈值。针对VCSEL的出光方式有顶面和底面的区别，本文选择顶面出光波长为850nm和底面出光波长980nm的VCSEL分别进行研究和制备。具体研究内容分为以下两个方面： (1)浅面浮雕矩形台面850nm VCSEL控制高阶模式。有源区面积的增大提高了VCSEL输出功率，但是同时降低了注入有源区的电流密度分布均匀性，最终降低了高阶模式阈值，增多了横向模式。为了在增大有源区面积时不影响有源区的电流密度分布的均匀性，我们设计了矩形台面结构VCSEL。采用有限元分析法仿真了圆形台面和矩形台面的有源区的电流密度分布。通过对比两者的有源区电流密度分布，得出矩形台面有源区的电流密度分布均匀性没有因为有源区面积增加而改变。在VCSEL台面出光孔的边缘刻蚀，形成浅面浮雕，可导致高阶模式的反射损耗增大，而基模的反射损耗却不受影响。理论分析出浅面浮雕的边长，深度等最佳参数，得出高阶模式和基模的最大阈值差。采用有限元分析法仿真了具有浮雕和未刻蚀浮雕两种情况下波长的分布，得出浅面浮雕能够有效抑制高阶模式的结论。根据理论参数，制作了矩形台面浅面浮雕VCSEL，测试得到输出功率为5.87mW时，半高全宽为0.106nm，边模抑制比超过30dB的光谱。 (2)介质膜集成衬底底面出光980nm 外腔VECSEL控制高阶模式。衬底对980nm光吸收很小，并且底面出光有利于电流密度分布均匀性改善，针对先前报道的外腔内为空气介质的外腔VECSEL的结构松散和工艺复杂的缺点，提出了多层介质薄膜集成在N型衬底表面的外腔结构VECSEL，N–DBR 和 P-DBR 及介质膜组成复合腔结构,衬底充当外腔，介质薄膜作为外腔镜。采用有限元分析法计算了外腔VECSEL和普通底面出光的各阶模式的Q值，通过比较两者的Q值，得出介质薄膜集成N型衬底外腔VCSEL能够抑制高阶模式激射。实验上制作了氧化孔径为200m，外腔长为250m的介质薄膜集成N型衬底外腔VECSEL，室温下注入3.4A直流电，测试得到在输出功率为260mW时，光谱线宽为0.046nm，远场发散角半角宽度为3.8°。普通底面出光VCSEL输出功率为270mW时，光谱线宽为0.32。通过比较两者的输出特性，得出尽管外腔VECSEL输出功率减小，但是光束质量得到很大改善的结论。 2.非对称电流注入矩形台面VCSEL控制偏振。矩形台面VCSEL实现稳定偏振光输出的原理是：长边比短边注入有源区的电流多，导致有源区在平行于长边方向比平行于短边方向的增益大，输出光的偏振方向平行于长边。为了提高输出光的偏振比，实现偏振光的稳定性，本文在矩形台面的两端刻蚀两个小洞，阻止电流从矩形台面两端注入有源区，增大注入有源区的电流的非对称性。实验研制了氧化孔边长为2μm  15μm的非对称电流注入的矩形台面VCSEL器件，测试得到的最高功率偏振比为17.6dB，为目前矩形台面VCSEL偏振比的最高值。
英文摘要	Compared to conventional edge-emitting lasers (EELs), vertical cavity surface emitting lasers (VCSELs) have the following advantages: small divergence angle, high damage threshold of the cavity surface, efficient fiber coupling; single longitudinal mode output, available on-chip test. But larger lateral size of VCSELs’ cavity results in multiple transverse modes and wide spectrum. Also, symmetric structure of circle mesa causes the unstable polarization direction of the output light. The disadvantage of multimode and unstable polarization of VCSELs limits its application, such as optical communication, optical storage, laser printing, spectral analysis and other fields which need VCSELs with stable polarization direction and single transverse mode. Aiming at these two disadvantages of VCSELs, a new device structure is designed to control high-order modes and polarization of VCSELs. In what follow, we carry out our research in two aspects: suppressing high order modes and controlling polarization respectively. 1. Control of VCSELs high order modes. The principle method of suppressing high order mode of VCSELs is to increase the threshold of high-order mode and reduce the threshold of fundamental mode. Considering the different output way of top emitting and bottom emitting VCSELs, we study top emitting 850nm VCSELs and bottom emitting 980nm VCSELs respectively. The specific research contents are as follows: (1) High order mode control of rectangular shaped VCSELs with shallow surface relief. For the top emitting 850 nm VCSELs, output power will increase when the size of the active area increases but uniformity of the current density distribution of the active region will decrease, and finally the threshold of the high order mode is reduced and the number of transverse mode is increased. In order to solve the problem mentioned above, we have designed VCSELs with rectangular mesa structure. The current density distribution of VCSELs is compared between the circular and rectangular mesa by finite element analysis. From the comparison, we can conclude that in spite of the increased active area of rectangle shape VCSELs, the uniformity of current density distribution has not been decreased. Shallow surface relief is etched on the edge of output aperture, so the reflection loss of high order modes is increased and reflection loss of fundamental mode is not affected. By theoretical computation, optimized parameters of shallow surface relief including etched length and etched depth are analyzed and the maximum difference of threshold between high order mode and fundamental mode is obtained. Intensity distribution of wavelength of VCSELs is simulated by finite element analysis, and it is concluded that relief on the surface of VCSELs can suppress the high order modes effectively, so spectrum can become narrower. We get a 5.87 mW output power with FWHM of 0.106 nm and side mode suppress ratio more than 30 dB from the results of experiments of rectangular shape VCSEL with shallow surface relief. It is proved that shallow surface relief on rectangular shape VCSELs can suppress high order modes from experiment and theory. (2) High-order mode control of the Dielectric film integrated on the substrate of vertical external cavity surface emitting laser. Absorption for 980 nm light of the substrate is very small and uniformity of the current density distribution will be improved for bottom emitting device. Aiming at the problem of complex fabrication and loose structure of the external cavity, dielectric film was integrated on external cavity substrate. Multilayer dielectric insulation films are grown on the surface of substrate, N–DBR and P-DBR compose the coupled cavity structure, and the substrate is served as external cavity of VECSEL. Quality factor is calculated by finite element analysis. Comparing quality factor to traditional VCSEL, this structure of VECSELs can suppress high order mode, so it could optimize the output spectrum and far-field divergence angle. We manufacture a VECSEL with active area of 200μm in diameter and external cavity length of 250μm. The device was tested on continuous DC of 3.4 A at room temperature. The output power is 260 mw, the divergence angle of far field is 3.8 °, and full width at half maximum is 0.046 nm. For traditional bottom emitting VCSELs, the power is 270 mw, the divergence Angle of far field is 3.8 °, and full width at half maximum of spectrum is 0.32 nm. Obviously, compared to conventional bottom emitting VCSEL, the beam quality of VECSEL devices is improved, but the maximum output power is reduced. 2. Oblong-shaped VCSELs with stable polarization control through asymmetric current injection. The principle to realize stable polarization along longer side of rectangle shape VCSELs is that gain along longer side is higher than gain along short side which attributed to more current inject into active area from longer side than shorter side. So a new structure of VCSELs with strong asymmetry of current distribution across the active region is designed by etching the two holes near the shorter ends of the mesa to prevent the current from injecting to active area from the longer sides of the mesa. Oblong shaped VCSEL with a 2μm ×15μm oxide aperture with two holes near the shorter ends of the mesa which ensures asymmetric current injection is fabricated to realize strong polarization control. Experimental tests show a by far highest OPRS of 17.6 dB of oblong shaped VCSEL.
公开日期	2015-12-24
内容类型	学位论文
源URL	[http://ir.ciomp.ac.cn/handle/181722/48865]
专题	长春光学精密机械与物理研究所_中科院长春光机所知识产出
推荐引用方式 GB/T 7714	李秀山. 垂直腔面发射激光器高阶模式及偏振控制[D]. 中国科学院大学. 2015.

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