【作者】 张祥伟；

【导师】 宁永强；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 凝聚态物理， 2013， 博士

【摘要】 垂直腔面发射激光器在光互连、光信号处理、泵浦固体激光器、光线耦合输出等方面越来越吸引着人们的注意，主要是因为垂直腔面发射激光器有着比边发射激光器更为优越的性能包括：非常低的阈值电流、单模输出、圆形光斑、非常方便的二位面阵集成等。尤其是高功率垂直腔面发射激光器在最近几年也取得了非常突出的成就，自从长春光机所报道单管输出1.95W以来，单管的功率也在不断的被刷新。拥有稳定偏振结构的高功率VCSEL可以应用到更多的领域，比如：激光显示、通信、探测等等。但是圆柱状VCSEL结构有源区内有着各向同性的增益结构，因此在激光器激射时的阈值电流附近两个相互正交的偏振方向会出现激烈的模式竞争效应，并且随着注入电流大小的改变，两个偏振方向也会发生偏振开关效应。通常采用的垂直腔面发射激光器偏振控制结构主要有:在高晶向衬底上生长芯片结构、外腔光反馈结构、高密度半导体光栅结构、表面浮雕结构等。目前这些控制偏振的结构在小孔径VCSEL上应用的比较广泛，而且其产生的结果也非常令人瞩目，但是在高功率垂直腔面发射激光器的偏振控制方面却产生了难以克服的困难。经过分析，亚波长金属光栅可以在有源区内引入各项异性增益。当光栅周期远小于入射波长时，只有电矢量振动方向平行于光栅条的光才能反射回腔内参与激光的震荡。首先，分析国内外关于垂直腔面发射激光器偏振控制的几种方法的优劣，提出金属光栅是目前控制高功率大孔径VCSEL的非常有效的方法。其次，理论上对垂直腔面发射激光器的偏振模式进行了理论分析，并对亚波长金属光栅垂直腔面发射激光器的输出特性，以及内部特性，包括P面分布布拉格反射镜的反射率、对数；阈值增益；阈值电流和输出功率进行了理论的模拟和计算，对亚波长金属光栅的反射率进行了有限元分析。同时提出了氧化光栅控制VCSEL偏振的新模型，并理论模拟了氧化光栅型VCSEL有源区内部电流密度的分布，通过模拟可以确定氧化光栅性VCSEL可以有效的控制高功率VCSEL的偏振，并在一定程度上能很好的控制高阶模式的激射。本论文通过引入这种办法来实现最大各项增益差的引入，进而实现非常高的偏振选择性的差异。并且此结构还能有效的压缩发散角，压制高阶模式的产生。主要的方法就是，让光栅条的制作方向沿两个偏振方向的其中一个方向，比如<110>。通过降低P-DBRs的对数来增加两个偏振方向的阈值增益，然后通过光栅附加在其中一个偏振光栅的反射率来降低此偏振光的阈值增益，从而达到增大两个偏振光TE和TM阈值增益的目的。设计的亚波长金属光栅参数为，周期186nm，占空比为0.5。为了更好的实现亚波长金属光栅的偏振分束器的作用并且达到VCSEL偏振的目的，我们需要降低P面分布布拉格反射镜的反射率。经过计算我们设计P-DBRs对数为17对，此时P-DBRs对TE理论的反射率为98.6%。对封装好的980nm亚波长金属光栅VCSEL进行了输出特性的测试和讨论，我们的到了250um-650um的单管输出结果。其中550um的器件输出功率达到860mW,偏振比达到3。通过对其光谱的探测，我们可以得到两个正交方向的光都发生了红移现象，但是速率有着明显的差异。当注入电流分别为1.2A、2.2A、3A的时候两个偏振光的中心波长分别是978.5nm，981.5nm；982.5nm,982.8nm；985.2nm，984.5nm。通过对其远场的测试，我们得到两个正交方向远场的发散角分别为12.9°和12°比普通单管器件的16.2°和15.6°有了非常明显的提高。为了进一步的提高两个偏振光的偏振比，我们把亚波长金属光栅结构进一步的优化，这里我们通过刻蚀掉光栅条之间的盖层来增加电流注入的不均匀而在有源区内引入的非均匀增益。通过测试550um的器件我们得到了两个偏振光的偏振比达到4.8、输出功率为780毫瓦。更多还原

【Abstract】 Vertical cavity surface emitting lasers become more and more attractive inoptical interconnection, optical signal processing, pumped solid state laser, lightcoupling output,etc, mainly because vertical cavity surface emitting laser has moresuperior performance than edge emitting laser including： very low thresholdcurrent;single mode output;circular spot;very convenient array integration, etc.Especially, high power vertical cavity surface emitting lasers have made outstandingachievements recent years, the output power of single device was refreshed again andagain,since ciomp reported the output power of single device reached1.95W.High-power VCSELs with stable single polarization are particularly attractive forapplications including laser display, communication and sensing. Strong polarizationselectivity is especially required for intra-cavity frequency doubling to achieve highefficiency and high power operation. Cylindrically symmetrical VCSELs are inclinedto polarization instabilities due to the symmetrical current distribution across theactive region and furthermore the symmetrical gain distribution. Several approacheshave been presented to improve the polarization properties of VCSELs. Theseapproaches can be divided into three categories: external cavity optical feedback,externally applied stress, and non-isotropic gain. However, the approaches above areall applied to small aperture device due to its simple mode characteristics. Apparentlythe output power is limited due to the small aperture of VCSEL. Few investigationshave been reported for the polarization control of large aperture VCSELs because of complex high order transverse mode characteristics, which are resulted from largeaperture and non-uniform current distribution across the active region of VCSELs.Rectangular aperture of non-isotropic gain was used by our group to increase thepolarization selectivity of large aperture VCSELs. An output power up to660mW wasrealized at a current of5A and the highest polarization ratio can be up to2. On theother hand, rectangular aperture of VCSELs has an obvious drawback of noncircularoutput beam, which might limit their application where high power density or highfiber coupling efficiency was required.After analysis，sub-wavelength metal-grating microstructure is an appropriatecandidate to introduce non-isotropic gain. When the period of the grating is muchsmaller than the incident wavelength, the light polarizing perpendicular to the stripesis transmitted through the grating and only the light polarizing along the stripes isreflected back to establish the laser oscillationFirst of all, analyse the merits and demerits of severval methods which cancontrol the polarization of VCSELs. And the metal-graing is a useful method whencontroling the polarization of high power VCSELs.Then analysed the poarization modes of VCSELs, the output and internalcharacteristics of sub-wavelength metal-grating VCSEL theroy in theroy. Putforward a new mathod oxide grating which can not only control the polarization butalso tanserve mode. And the method of oxide grating was simulated by comsolmulti-physics infinit elment software. As a result, oxide grating structure maycontrol the polarization of high power VCSEL well after simulated.In this work, we presented an approach to achieve higher non-isotropic gain andfurther highly selective polarization of large aperture high-power VCSELs, and thehigh-order of large aperture VCSEL can be suppressed to some extent. Here thepolarizations vibrating along and orthogonal to <110>. The pairs of P-side DBRswere reduced to decrease the reflectance of two orthogonal polarizations andaccordingly the threshold current of both orthogonal polarizations was increased. Bycoupling the additional reflectance which was provided by sub-wavelength metal-grating, the threshold gain of TE was decreased remarkably compared withTM.The grating with a period of186nm and a duty ratio of0.5was fabricated on theGaAs-cap layer to provide additional reflectance for H-polarization. The pairs ofP-DBRs were reduced to realize the maximum difference threshold gain of twoorthogonal polarizations. Polarization ratios of3, an output power of860mW atcontinuous-wave operation at room temperature were demonstrated, and thehigh-order modes were suppressed, as a result the far-field beam divergence wassuppressed to12°.We get the result that both two polarizations apear red shift phenomenon fromthe spectrogram. The center wavelength of two polarization are978.5nm，981.5nm；982.5nm,982.8nm；985.2nm，984.5nm respectively when the input current are1.2A、2.2A、3A. The obvious improvement is the far-field beam divergence of12.9°and12°[full-width at half-maximum(FWHM)] for metal-grating VCSEL compared withthe beam divergence of16.2°and15.6for conventional VCSEL device without themetal-grating at a current of5A.To forther improve the polarization ratio of two polarizations, the pairs ofP-DBRs were reduced and the GaAs-cap between grating stripes was etched to forcethe current to be injected linearly along grating stripes to realize the maximumnon-isotropic gain. A polarization ratio of4.8, an output power of780mW and hightemperature performance were demonstrated for a550um aperture device.更多还原