Research progress on the underwater wireless optical communication system
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摘要: 水下无线光通信作为一种新兴的高速水下无线通信技术，在海洋生态环境监测、资源勘测以及军事作战等方面的作用不可小觑，并已成为全世界竞相争夺的关键性技术。对目前常用的3种水下无线通信方式进行比较，介绍了水下无线光通信的信道特性，并阐述水下无线光通信系统中光源、调制、信道编码以及探测等关键技术的研究进展。总结了水下无线光通信技术的发展趋势，为未来水下无线光通信系统的深入研究和实用化提供了参考。Abstract: As a new high speed underwater wireless communication technology, underwater wireless optical communication (UWOC) plays an important role in marine ecological environment monitoring, resource exploration, and military operations, and has become a key technology for competition all over the world. Three kinds of underwater wireless communication modes were compared, the channel characteristics of UWOC were introduced, and the research progress of key technologies such as light source, modulation, channel coding, and detection in UWOC system were described in detail. At the same time, the development trend of UWOC technology was summarized, which provides a reference for further research and practical application of UWOC system in the future.
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引言
随着通信速率的不断发展，硅基光调制器在全光信号处理和高速光通信光互连上越来越体现出不可或缺的作用[1-2]。基于Ⅳ族材料(硅，锗)的硅光调制器，按其原理分类有热光型[3-4]、载流子色散型[5-6]和光吸收型[7-9]。而其它新兴的材料平台，也引起了学者们广泛的关注，如硅与Ⅲ-Ⅴ族混合集成的调制器[10-11]、结合2维材料(石墨烯)[12]混合集成的调制器[13-15]，以及高性能的电光材料平台，如氮化铝[16-17]、薄膜铌酸锂[18-20]等在集成光调制器中也显示出巨大的潜力。
上述新材料平台在实际器件制作中都存在与传统CMOS工艺兼容的问题，所以选择Ⅳ族的锗硅材料体系作为研究对象。对于多量子阱体系，研究者发现锗硅量子阱材料中显示出了较强的量子限制斯塔克效应(quantum-confined Stark effect, QCSE)[21]。然而，在Ge/SiGe量子阱的相关研究中，探究吸收系数变化从而引起电致折射率变化的相关实验结果很少。
本文作者提出并设计了一种非对称耦合量子阱(coupled quantum well, CQW)结构，来增强电致折射率的变化，从而实现高效的相位调制。通过设计两种不同的量子阱宽度来使临近量子阱相互耦合，可以调控耦合量子阱的波函数以及电致折射率的特性，并且实验制作的器件性能优异，实现了偏置电压为1.5 V时，在1530 nm波长处的2.4×10-3电致折射率变化，其对应的VπLπ在1 V偏置电压下低至0.048 V·cm(Vπ为半波电压, Lπ为器件的作用区长度)。
1. 锗硅非对称耦合量子阱的设计与仿真
1.1 材料设计
由于关注器件的电致折射率调制的性能，仿真发现量子阱层材料结构采用非对称的结构设计可以得到更大的吸收边红移。从物理机理的层面分析，即使在没有外加电场作用的情况下，非对称耦合量子阱中电子态和空穴态天然就呈现非对称的分布，主要分布在较宽的量子阱中。由于中间的薄势垒存在，在很低的外电场作用下，电子e1能态就会明显地向窄势阱移动，从而实现较大的波函数分布变化及波函数重叠因子变化，因此非对称耦合量子阱结构在参数优化后可以实现明显的光学相位调制。仿真优化得到的锗硅量子阱材料的整体结构参数如图 1a所示; 其中关键的设计在于8对耦合量子阱的参数选取，如图 1b所示。窄阱为6 nm宽，宽阱为12 nm宽，两阱中间由1.6 nm宽的Si0.1Ge0.9势垒隔开，而势阱外两边都是宽度为12 nm的Si0.15Ge0.85势垒。非对称量子阱采用上述参数可以使得中间薄势垒两边的量子阱(quantum well, QW)达到较强的耦合作用，实现较好的有源区调制性能。
![图 1 a—锗硅耦合量子阱的整体外延设计 b—锗硅非对称耦合量子阱的示意图]()
图 1 a—锗硅耦合量子阱的整体外延设计 b—锗硅非对称耦合量子阱的示意图Figure 1. a—epitaxy design of Ge/SiGe CQW b—sketch of Ge/SiGe asymmetric CQW1.2 仿真结果
考虑到实际量子阱生长工艺的难度，实现非对称量子阱中心的势垒区厚度的精确控制，工艺上将势垒区厚度从1.6 nm提高到2 nm，势垒区组分由Si0.1Ge0.9修正为Si0.17Ge0.83(硅组分越高，生长速度越慢，厚度更容易调控)。上述参数的微调在理论上只会导致中间势垒两边的量子阱耦合作用轻微下降，从后面的仿真和实验结果也可看出这一调整是合理的。需要强调的是，下面涉及到量子阱参数的仿真和实验分析，都统一基于2 nm厚度Si0.17Ge0.83的结构组分参数。依据实际生长的耦合量子阱结构，结合八能带\boldsymbol{k} \cdot \boldsymbol{p}理论，可以计算得到在量子阱层的光场模式为TE和TM两种情况下的光学吸收谱线，如图 2所示。图 2b是图 2a中虚线框放大图；图 2d是图 2c中虚线框放大图。从图中可以看出，在TE光模式激发下，由激子引起的吸收峰略多于TM模式激发的吸收峰，并且发现激子数量越多，红移越明显，对应的吸收系数变化也就越大。对比图 2b和图 2d可以发现，TE模式的第一吸收带边(约1435 nm)比TM模式更接近长波长。
![图 2 不同电场强度下锗硅非对称耦合量子阱的光吸收谱]()
图 2 不同电场强度下锗硅非对称耦合量子阱的光吸收谱Figure 2. Absorption spectrums of asymmetric Ge/SiGe CQW with different applied electrical field根据Kramers-Kronig(K-K)关系[22], 如下式所示：
\Delta n(\nu)=\frac{c}{{\rm{ \mathsf{ π} }} } \int \frac{\Delta \alpha\left(\nu^{\prime}\right)}{\nu^{\prime 2}-\nu^{2}} \mathrm{~d} \nu^{\prime} (1) 式中, \nu是光波频率; \nu^{\prime}是对应积分\nu变量的范围; c是光速。
从(1)式可知，吸收系数的变化Δα对应电致折射率的变化Δn，可以由上述吸收谱线变换得到折射率的变化曲线，并进一步分析该调制器的相位调制性能。
锗硅非对称耦合量子阱的电致折射率变化谱线如图 3所示。从仿真曲线可以看出，TE光模式情况下的电致折射率变化的峰值波长更靠近长波段，并且随着外加电场强度的增加，电致折射率变化的峰值呈现出先增加后减少的趋势。显然，在入射光波长为1450 nm，电场强度为40 kV/cm的情况下，TE光模式入射导致的最大电致折射率变化可达0.01，并且显著大于通常的锗硅耦合量子阱。由上述仿真数值可以计算得到，当波长大于1450 nm时，该相位调制器的VπLπ可以接近0.01 V·cm量级。值得注意的是，当电场强度达到60 kV/cm时，TE和TM模式在相应第一吸收边带波长范围内都呈现两个局部极值。这一现象在图 2中也有所反映，这是因为在较强的电场强度下，原先的第二激子峰红移很多，接近原第一吸收边带的位置，从而形成两个明显的吸收边带, 第一和第二吸收带边在不同波长处就会导致两个折射率变化峰值。
![图 3 不同电场强度下锗硅非对称耦合量子阱的电致折射率变化谱线]()
图 3 不同电场强度下锗硅非对称耦合量子阱的电致折射率变化谱线Figure 3. Electrorefractive index variation of asymmetric Ge/SiGe CQW with different applied electrical field2. 器件的制备和相位调制测试
锗硅非对称耦合量子阱的整体外延方案如图 1b所示，这里采用减压化学气相沉积(reduced pressure chemical vapour deposition，RPCVD)的方法来逐层外延量子阱层材料。外延得到的基片材料经过波导刻蚀，电极生长，端面刻蚀，划片解理后就可以用于测试，测试的器件示意图和系统装置如图 4所示。这里采用放大自发辐射(amplified spontaneous emission, ASE)宽谱光源来测试调制器的宽谱响应，数字源表(digital source meter, DSM)连接的直流探针用来给调制器施加直流偏压，同时计算机连接光谱仪(optical spectrum analyzer, OSA)直接采集光谱数据。
由于整个器件在直波导两端面正对的方向上可以看作一个法布里-珀罗(Fabry-Pérot, F-P)干涉仪，因此光谱响应呈现出与波长相关的干涉波纹，从干涉峰波长的漂移变化Δλ就可以导出有效折射率Δneff的变化，如以下公式所示：
\Delta n_{\text {eff }}(\lambda)=\frac{\Delta \lambda}{\lambda} n_{\mathrm{g}}(\lambda) (2) n_{\mathrm{g}}\left(\lambda_m\right)=\frac{\lambda_m{ }^2}{2 L \Delta \lambda_m} (3) 式中, n_{\mathrm{g}}为光模式的群折射率; L表示整个器件的长度, 包括被调制部分和两端雉形波导的长度之和, 约为360 \mu \mathrm{m}, L越短, 实验中能观测到的干涉峰波长间隔(纵模间隔) \Delta \lambda_{m}就越明显, \lambda_{m}的下标m表示整数, 对应不同的纵模波长。
如图 5所示，给出了光波长在1520 nm~1540 nm范围内，光谱响应随着施加偏置电压的变化。已将不同曲线的纵坐标基准做适当整体偏移, 以便于比较。随着反偏电压的增加，干涉峰的波长逐步漂移，当电压达到1.5 V附近时，干涉峰的漂移接近一个周期。从波长漂移量可以导出折射率的变化。结果显示，在1.5 V偏压的情况下，波长1530 nm处的电致折射率变化分别是2.4×10-3，该调制器对应的VπLπ低至0.048 V·cm，显示该器件具有高效的光调制性能。由于实验设备的限制，光源波长限定在光通信C波段，而没有给出1450 nm波段附近器件的相位调制性能，但从仿真结果中分析可知，1450 nm波段的电致折射率变化远大于1530 nm处，可以得到更优的相位调制效果。
![图 5 在反向偏置电压变化时，光通信C波段的光谱响应]()
图 5 在反向偏置电压变化时，光通信C波段的光谱响应Figure 5. Spectral response at optical communication C-band under different reverse bias voltage下面将实验结果与前人的工作进行对比，如表 1所示。可以发现，本文中的实验结果相对于普通锗硅耦合量子阱[23]的工作能耗很低，只需要1.5 V偏压就可以带来很明显的调制效果；相对于对称的锗硅耦合量子阱[24]的调制波长范围更广，不仅在1450 nm处有很强的吸收变化，甚至可以将调制波段扩展至光通信C波段，依然能体现出优越的器件性能。
表 1 锗硅调制器性能对比Table 1. Comparison of this work and other reported Ge/SiGe modulatorsstructural composition bias voltage/V wavelength/nm index variation VπLπ/(V·cm) reference [23] Ge/Si0.15Ge0.85 MQW 8 1475 1.3×10-3 0.46 reference [24] 7×[7 nm Ge QW+1.5 nm Si0.15Ge0.85 inner barrier+7 nm Ge QW+26 nm Si0.15Ge0.85 outer barrier] 1.5 1420 2.3×10-3 0.046 our study 8×[6 nm Ge QW+2 nm Si0.17Ge0.83 inner barrier+12 nm Ge QW+12 nm Si0.15Ge0.85 outer barrier] 1.5 1530 2.4×10-3 0.048 3. 结论
提出了一种锗硅非对称耦合量子阱的设计，用于实现高效的电致折射率相位调制，不仅在理论仿真上给出锗硅非对称耦合量子阱调制器的理想调制性能，并且在实验上成功制备了对应的器件，实验结果与理论高度吻合。结果表明，相比于同类型的硅基调制器，设计的锗硅非对称耦合量子阱调制器在1530 nm波段的电致折射率变化达到2.4×10-3，其工作波段范围理论上可以覆盖1450 nm~1530 nm，这给未来硅基光调制器的发展开辟了新的方向。
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表 1 3种水下无线通信方式
参数 声通信 射频通信 光通信 传输速率 kbit/s Mbit/s Gbit/s 传输距离 km 10m 10m~100m 通信容量 小 大 大 时延 大 大 小 衰减 大 大 小(蓝绿光) 带宽 小 小 大 功耗 高 高 低 
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表 2 4种海水类型的相关系数
海水类型 a b c 纯海水 0.053 0.003 0.056 清澈海水 0.114 0.037 0.151 沿海海水 0.179 0.219 0.398 浑浊海水 0.295 1.875 2.17
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表 3 发射端光源的性能比较
参数 LED LD 最小输出光束发散角 约0.5° 约0.01° 调制带宽 小于200MHz 大于1GHz 温度敏感性 不敏感 敏感 使用寿命 长 适中 相干性 不相干 相干 功耗 低 高 安全性 好 差
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表 4 近年高阶QAM-OFDM调制下的水下无线光通信研究成果
时间 调制格式 接收器 传输速率 距离/m 2015年[18] 16-QAM-OFDM APD 4.8Gbit/s 5.4 2015年[61] 64-QAM-OFDM APD 9Gbit/s 5 2016年[62] 16-QAM-OFDM APD 3.2Gbit/s 6.6 2016年[63] 16-QAM-OFDM PIN 7.2Gbit/s 6.8 2017年[63] 32-QAM-OFDM APD 5.5Gbit/s 26 2017年[64] 16-QAM-OFDM APD 14.8Gbit/s 1.7 2018年[52] 16-QAM-OFDM PD 9.6Gbit/s 8 2018年[65] 128-QAM-OFDM APD 2.92Gbit/s 12.5 2019年[66] 32-QAM-OFDM MPPC 312.03Mbit/s 21 2020年[67] 128-QAM-OFDM APD 6.23Gbit/s 3
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表 5 探测器性能比较
参数 PIN APD PMT 灵敏度 低(μW) 适中(nW) 高(pW) 探测面积 小 小 大 响应速度 慢 快 超快 体积 小 小 大 安全性 好 好 易碎 等效噪声功率 高 低 极低 使用成本 低 适中 高
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