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如图1所示,VCSEL是由上、下布拉格反射镜(DBR)、有源区、电流限制层、上下电极组成,VCSEL DBR反射镜间的有源区光学厚度为λ/2的整数倍,上下DBR镜通常为P型和N型掺杂,光输出方向垂直于晶圆表面。为降低电容,常用的是苯丙环丁烯(BCB)绝缘介质进行填平来制作共面电极[2]。为了进行有效的注入电流控制,常在有源区与DBR间制备氧化孔。
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VCSEL的调制带宽限制主要来自于其内部本征带宽限制和外部寄生效应。VCSEL的小信号调制可通过传递函数来表示:
$$ {{H}}\left({{f}}\right)=\dfrac{{f}_{\rm R}^{2}}{{f}_{\rm R}^{2}-{f}^{2}+jf\gamma }\cdot \dfrac{1}{1+j\cdot \left(\dfrac{f}{{f}_{\rm p}}\right)}{,} $$ (1) 公式(1)是由本征传递函数与电寄生效应引入的传递函数相乘得出的。其中fP
为寄生截止频率,fR为弛豫振荡频率,理想激光器fR的弛豫振荡频率可表示为注入载流子和生成的光子之间相互作用的动力学: $$ {f}_{\rm R}={{D}}\sqrt{{I}_{0}-{I}_{\rm th}}{,} $$ (2) 其中,
$$ {{D}}=\frac{1}{2\pi }{{\left[\dfrac{ \varGamma {{v}_{\rm g}}{{\eta }_{i}}}{q{{V}_{\rm p}}}\dfrac{{}^{\partial g}\!\!\diagup\!\!{}_{\partial n}\;}{\chi }\right]}^{1/2}},$$ (3) 式中:
${}^{\partial g}\!\!\diagup\!\!{}_{\partial n} $ 为量子阱的微分增益;$ \chi $ =1+${\tau }_{\rm s}/{\tau }_{\rm e}$ 为传输系数,${\tau }_{\rm s}$ 为通过腔未掺杂部分进入有源区量子阱时间,τe为有源区热传输发射时间;Vp为光子占据的腔体积;Γ为光限制因子;νg为光子的群速度;q为电荷;Ith为阈值电流。调制频率随电流变化的反应速率(即:D因子)决定了理想激光器的调制速度,由此可以通过增大D因子来增大调制速率。提高D因子的方法主要有:(1)增大光限制因子;(2)减小有源区体积;(3)增加有源区微分增益。电流孔径的减小可减小有源区体积,增大D因子。H(f)本质上是一个二阶低通滤波器,其阻尼谐振位于截止频率附近。在fR之下强度调制一直都与电流调制一致,在弛豫谐振处该响应得到增强。在谐振点之上,该响应急剧降低。根据阻尼的大小,该谐振的实际峰值频率fp略低于fR。
${f}_{3\;{\rm{dB}}}$ 是电功率响应降低到其直流值一半时的频率,在弱阻尼下该频率略高于fR。可知通过增加光子密度或输出功率可以使fR增加。该增加将一直持续到光子密度接近$ \dfrac{1}{\varepsilon } $ 为止,这时由于增益压缩,微分增益略有减小。阻尼因子γ为:$$ \gamma = {{K}}f_{\rm {R}}^2 + {\gamma _0}, $$ (4) 其中,
$$ {{K}}=4{{\pi }^{2}}\left( {{\tau }_{p}}+\dfrac{ \varepsilon { } \chi }{{}^{{{v}_{\rm g}}\partial g}\!\!\diagup\!\!{}_{\partial n}\;} \right), $$ (5) γ0是由于自发辐射对激光模的贡献而引起的偏移。最大的可实现带宽被弛豫振荡阻尼所限制,当fp=0,且
$ {f}_{\rm{R}}={f}_{3\;{\rm{dB}}} $ 时,阻尼最佳且带宽最大。该点由K因子决定,K因子确定了激光器的本征调制带宽能力。减小K因子的方法主要有:(1) 增大有源区介质的微分增益,可以通过使用InGaAs或InGaAlAs量子阱来代替GaAs量子阱实现;(2) 减小光子寿命τp,可用通过调整顶层相位来实现[2-3]。 -
寄生效应是限制VCSEL高速特性的另一个重要因素。寄生主要分为内部寄生与电极寄生两部分。
以典型的氧化限制型VCSEL为例,其横截面示意图与寄生如图2所示。极板电容Cp表示从探头端到金属触点的信号与地之间的电容。Cp的值根据极板的布局和极板之间的材料不同,从几十fF到几百fF不等。现在最常用的填平材料是聚酰亚胺和苯丙环丁烯(BCB),比使用二氧化硅填平所带来的极板电容要小很多。极板电阻Rp是极板损耗的原因。由于它在欧姆范围内,通常相对较小,在小信号模型中有时被忽略。极板电容Cp要尽力优化到最小,而极板电阻Rp要做到最大,这样能够有效阻止电流流经它们[3]。镜面电阻Rmirr包括上下DBR的电阻。它包括了在偏置点处异质结的净微分阻抗。N-DBR由于材料的损耗和电阻性低,所以减小寄生电阻主要是针对P-DBR,可以通过DBR缓变层的各种带宽带隙工程方案来实现低电阻[4-5],对于氧化限制型VCSEL,也可以采用腔内接触的(部分)未掺杂半导体反射镜来实现[6-7]。但腔内接触结构仍然存在很多问题,例如电流注入的均匀性,有源区附近的高掺杂接触层的光损耗和侧向电阻等。Rsheet表示n-接触层中的薄层电阻,Rcont表示所有接触的接触电阻。所有这些电阻中镜面电阻Rmirr占最大部分,在小信号模型中,可以组合成
${R}_{\rm m}$ =${R}_{\rm mirr}$ +${R}_{\rm sheet}$ +${R}_{\rm cont}$ 。台面电容Cmesa是电流孔径下内部区域的串联氧化物电容。台面电容Cmesa取决于台面和孔径的直径以及氧化物和本征半导体层的厚度。电容Cj表示电流流过的孔径区域的二极管结电容。在正常的正向偏置条件下,Cj通常由扩散电容控制,它模拟了存储在固有的分离限制型异质结构中的少数载流子的调制。结果表明,扩散电容不仅与载流子寿命有关,而且与分离限制型异质结构的设计有关[3]。
Advances in the technology of 850 nm high-speed vertical cavity surface emitting lasers (Invited)
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摘要: 垂直腔面发射激光器(VCSEL)具有低成本、低阈值、高速率和低功耗等优点,在短距离光互连中有着重要的应用。随着大数据、超级计算机技术的发展,短距离光互连性能需求越来越高,从而对高速调制的850 nm VCSEL技术提出了更高要求。从带宽限制机理、调制新方法两方面详细回顾了高速850 nm VCSEL技术最新进展,对技术发展趋势进行了总结与展望。Abstract: Vertical-cavity surface-emitting lasers (VCSELs) have important applications in the short-distance optical interconnection attributed to their advantages, such as low cost, low threshold current, high modulation bandwidth and low power consumption. With the development of big-data and supercomputer technology, the performance demand of short-distance optical interconnection is increasing quickly, which also proposes a challenge for high-speed 850 nm VCSEL. In this paper, the latest development of high-speed 850 nm VCSEL technology was reviewed from the aspects of bandwidth-limited factors and new modulation methods, and the growing trend of this technology is prospected and summarized.
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Key words:
- VCSEL /
- high speed /
- PAM modulation
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