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文中VCSEL的外延结构是利用金属有机化学气相沉积(MOCVD)技术在GaAs衬底上沉积而成。器件结构如图1所示,包括n型GaAs衬底、n型接触层、37对n型Al0.9GaAs∕Al0.12GaAs 分布布拉格反射器(distributed Bragg reflector,DBR)、有源区、22.5对p型Al0.9GaAs∕Al0.12GaAs DBR,以及厚度约60 nm (半对DBR)的p型GaAs接触层。其中,DBR结构采用渐变界面以减小器件的电阻和光损耗,有源区包含3对InGaAs压应变量子阱(QWs),且在有源区上方设置了一层厚度30 nm的高铝组分材料(Al0.98GaAs)用于氧化。笔者在VCSEL表面接触层材料上进行刻蚀来获得表面光栅,用于选择出单一的偏振模式。
文中采用电子束曝光(EBL)工艺定义光栅的掩蔽,并利用电感耦合等离子体刻蚀(ICP)工艺完成表面光栅的刻蚀。所制作的光栅周期约为700 nm,占空比为0.5,刻蚀深度约60 nm,光栅区域的直径为5 μm,如图2所示。这种周期接近激射波长的表面光栅没有可以应用的介质理论公式,只能通过数值仿真来获得光栅对偏振的影响。通过时域有限差分法(FDTD)[16]对平行和垂直光栅方向的偏振的反射率进行了仿真,可以得到两个偏振的反射率随光栅刻蚀深度的变化,再根据公式(1)可以得到反射率对应的镜面损耗,从而得到图3所示的两种偏振的镜面损耗随VCSEL表面刻蚀深度的变化。
图 2 表面光栅的扫描电子显微镜照片。其中,光栅周期约为700 nm,占空比为0.5,刻蚀深度约60 nm,光栅区域直径为5 μm
Figure 2. SEM images of the surface grating, which with a period of 700 nm, a duty cycle of 0.5, and an etching depth of about 60 nm. The diameter of the grating area is 5 μm
$${\alpha _{mirror}} = \frac{1}{{2{L_{eff}}}}\ln \frac{1}{{{R_1}{R_2}}}$$ (1) 式中:αmirror为镜面损耗;Leff为激光器的有效腔长;R1和R2分别为上下DBR的反射率。根据参考文献[17],两个偏振的损耗差达10 cm−1以上时,就可以形成稳定的偏振。由图3可以看出,表面光栅刻蚀深度在44~130 nm范围内镜面损耗差便已超过10 cm−1,因此只要光栅刻蚀深度在此范围内便可获得单一的偏振,无需精确刻蚀。微纳结构的浅刻蚀对于任何刻蚀工艺来说都是一种挑战,而在保证微纳结构形貌的同时实现精确刻蚀将更加困难,这也是此类浅刻蚀光栅制作难度较大的原因。而文中的表面光栅具有较大的工艺容差,大大降低了ICP工艺刻蚀的难度。
图4为偏振测试系统示意图,测试原理如下:激光器经电流源直流(DC)供电,激光光束通过准直透镜进行准直,再经过偏振片后得到单一偏振态,之后利用分束器将光束分为两路,一路经探测器(PD)得到光电流,一路通过光谱仪(OSA)测得此偏振态光谱,最终使用Labview程序读取数据。旋转偏振片对不同偏振态的光电流和光谱进行测量,从而获得偏振P-I曲线和偏振光谱。偏振控制作用的强弱通过OPSR来体现,其计算公式为:
$$ O P S R = 10\log \left( {{P_1}/{P_2}} \right) $$ (2) 式中:P1和P2分别为两偏振态的功率,偏振抑制比也表现为偏振光谱中两偏振态峰值间的差异。
Polarization characteristics of surface grating vertical cavity surface emitting laser
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摘要: 研究了表面光栅结构对垂直腔面发射激光器(VCSEL)的偏振控制作用。引入表面光栅后,对不同刻蚀深度下的偏振相关的镜面损耗进行了仿真,结果表明表面光栅刻蚀深度在44~130 nm范围内均可实现稳定偏振,具有较大的工艺容差。表面光栅VCSEL在基横模工作状态下偏振抑制比(Orthogonal Polarization Suppression Ratio, OPSR)超过20 dB,偏振光谱峰间偏振抑制比达到40 dB,且在多横模状态也实现了有效的偏振控制。为了进一步验证光栅对偏振控制的效果,制作了方向互相垂直的两种表面光栅,具有这两种方向光栅的VCSEL的OPSR均达20 dB以上。测试分析表明表面光栅是VCSEL实现稳定偏振的一种有效手段。Abstract: The polarization control of vertical cavity surface emitting laser (VCSEL) with surface grating structure was studied. After introducing the surface grating, the polarization-dependent mirror loss under different etching depths was simulated. The results show that the etching depth of the surface grating can achieve stable polarization in the range of 44 nm to 130 nm, which has a large fabrication tolerance. For the fundamental transverse mode, the orthogonal polarization suppression ratio (OPSR) of the surface grating VCSEL is more than 20 dB, and the peak-to-peak OPSR of the polarization-resolved spectrum reaches 40 dB. Effective polarization control can also be achieved even for multimode VCSEL. In order to further verify the effect of the grating on polarization control, two surface gratings with mutually perpendicular directions were fabricated. The OPSRs of the VCSEL with the two directional gratings were more than 20 dB. The test results show that surface grating is an effective means for VCSEL to achieve stable polarization.
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图 6 基横模状态下表面光栅VCSEL的偏振特性。(a)远场特性;(b)偏振P-I曲线;(c)偏振光谱;(d)偏振在出光平面上的分布
Figure 6. (a) Far-field characteristics, (b) Polarization-resolved P-I curve, (c) Polarization-resolved spectrum, and (d) The distribution of polarization states in the polar coordinate plane of the basic transverse mode surface grating VCSEL
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