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随着网络服务的发展趋向多元化，互联网的普及率也在逐年增加[1]。面对市场需求的迅速增长，通信骨干网的速率已得到很大的提升，达到Tbit/s级，基本能满足新兴业务发展的需求，但作为电信网络的末端“最后1km”的接入网系统，速率没有明显的提升，这直接制约着用户最终的接入速率和网络质量，其解决方案成为了日益关注的焦点[2-3]。虽然目前接入网速率可满足5G的1Gbit/s需求，但随着对8K超清视频、医疗健康、自动驾驶等技术需求的提高，接入网可能无法应对未来几年的网络需求[4]。
由于接入网系统对成本的敏感性要求，构建一个简化、低成本的光网络单元成为了决定其大规模部署的必要因素。CHEN团队搭建了一个集成了直接调制激光器和半导体光放大器(semiconductor optical amplifier, SOA)[5]的20km双向传输通信系统，测试结果表明, 该系统具有良好的性能[6]，但直接调制激光器的调制效果在高传输速率下受频率啁啾限制[7]。电吸收调制器(electro-absorption modulator, EAM)具有可集成、体积小、驱动电压低、低频率啁啾等优良特性[8]，使它与激光器的集成光源模块成为接入网系统的理想光源之一[9-11]。为了兼顾接入网的特点和传输速率，本文作者在前期研究基础上[6, 12], 将传输速率提升到40Gbit/s, 并以EAM作为上行发射机的外调制器，搭建了以可集成分布式反馈(distributed feedback, DFB)-EAM器件为上行光发送单元的双向通信系统，分别测试了该系统在直接检测方案和相干检测方案下的传输性能。
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EAM是一种PIN半导体节型器件，它基于Franz-Keldysh效应和量子约束Stark效应[13]的共同作用使材料吸收光谱发生变化，通过改变偏压大小影响材料的吸收波长，从而实现光调制。EAM对光的吸收会随着其反偏电压的增加而呈非线性增大，从而使入射光的透过率与反偏电压呈现出特定关系，可以利用这种特性对光波进行调制输出光脉冲。研究了电吸收调制器在输入光波长为1569.59nm、输入光功率为-5dBm时，EAM反向电压和输出光功率之间的关系，确定了EAM调制信号峰峰值为2V时的反向偏置电压。
EAM产生光脉冲的原理如图 1所示。EAM对入射光的吸收随着反向电压的增加而非线性增加，给EAM施加适当的反向直流电压和驱动电压，使得EAM的透过率随着驱动电压的变化而波动，DFB激光器发出的连续光波(continuous wave, CW)经EAM调制，在反向直流电压与驱动电压作用下可产生光脉冲输出，脉冲输出的重复率等于驱动电压重复率[14]。
Figure 1. Structure diagram of EAM generating optical pulse
图 2是EAM在输入激光功率为-5dBm、波长为1569.59nm的情况下，EAM的输出光功率与其反偏电压之间的变化曲线。横轴表示反向电压值，纵轴表示输出光的功率值。在电压处于0V~2V时，随着偏置电压的增加，输出光功率减小；当电压大于2V，输出光功率很小并且不再变化。当调制信号的峰峰值为2V，直流偏置电压取1V，EAM工作在图 2中所示的透过区，能获得良好的调制效果[15]。
Figure 2. Relation curve between EAM reverse bias voltage and output power
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图 3显示了在40Gbit/s数据速率下，使用伪随机二进制序列(pseudo random binary sequence, PRBS)测量以DFB和EAM为上行发射机的5km双向传输网络性能的实验框架。下行传输采用连续激光器和标准幅度调制器(amplitude modulator, AM)为发射机，PRBS经过标准不归零(non return to zero, NRZ)脉冲发生器转换为脉冲信号，再通过AM加载至1552.12nm波长的连续光波上，成为入纤所需的带有信息的光信号，经过5km的单模光纤(single mode fiber, SMF)传输，在光网络单元(optical network unit, ONU)通过PIN光电二极管将其转换为电信号，再通过均衡滤波器转换成低噪声电信号，最终从误比特率(bit error rate, BER)分析仪检测获得输出信号的误比特率。上行传输采用DFB和EAM作为发射机，光源波长为1569.59nm，其它结构与下行一致。图 3中，ODN(optical distribution network)为光分配网络；C1和C2为循环器; VOA(variable optical attenuator)为可变光衰减器; TIA(trans impedance amplitier)为跨阻抗放大器; OLT(optical line terminal)为光线路终端。
Figure 3. Structure diagram of 40Gbit/s direct detection bidirectional transmission system
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图 4显示了40Gbit/s直接检测单向传输系统在单模光纤长度分别为0km和5km两种情况下传输误比特率和接收光功率的关系。在误比特率为2.4×10-4的标准前向纠错(forward error correction, FEC)下[16]，单向下行链路在背对背(back to back, BTB)的接收灵敏度为-20.55dBm，在光纤长度为5km时接收灵敏度为-19.13dBm。单向上行链路在BTB传输的接收灵敏度为-20.51dBm，在光纤长度为5km距离传输的接收灵敏度为-18.18dBm。
Figure 4. Bit error rate of unidirectional transmission in direct detection system
图 5为双向传输系统在单模光纤为0km和5km两种情况下的系统通信误比特率测试结果。在传输速率为40Gbit/s、前向误比特率为2.4×10-4时，BTB传输的下行链路信号接收灵敏度为-20.41dBm，上行链路信号的接收灵敏度为-20.45dBm；在5km距离的传输中，下行链路信号的接收灵敏度为-18.31dBm，上行链路信号的接收灵敏度为-17.94dBm。
Figure 5. Bit error rate of bidirectional transmission in direct detection system
一般来说，与5km的单向传输系统相比，单纤双向传输系统中的瑞利后向散射会限制接收端的接收灵敏度[17]，但结果表明，在5km的双向传输中，上游功率损耗为0.24dB, 而双向下游的功率损耗为0.82dB，说明瑞利后向散射对系统的影响较小。与BTB双向传输的情况下相比，5km的双向上游功率损耗为2.51dB，双向下游功率损耗为2.10dB，这说明5km的光纤色散对整个系统的影响较大。
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图 6是在40Gbit/s传输速率下，以内差检测[18]方案为上下行接收机的双向传输系统的结构示意图。上下行的发射机的结构与直接检测系统的结构一致，接收端都采用功率为3dB的DFB激光器作为本征光源。在接收端，经过光纤传输的信号光与本征光耦合干涉后，通过PIN光电二极管将其转换为电信号，最后经均衡滤波器和误比特率分析仪得到传输误比特率。
Figure 6. Structure diagram of 40Gbit/s coherent detection bidirectional transmission system
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图 7显示了40Gbit/s相干检测单向传输系统在单模光纤长度分别为0km和5km两种情况下传输误比特率和接收光功率的关系。在误比特率为2.4×10-4的FEC级别下，单向下行链路在BTB和5km距离传输的接收灵敏度分别为-33.42dBm和-32.28dBm，单向上行链路在BTB和5km传输的接收灵敏度分别达到了-33.92dBm和-31.88dBm。
Figure 7. Bit error rate of unidirectional transmission in coherent detection system
图 8显示了40Gbit/s相干检测双向传输系统在单模光纤长度分别为0km和5km两种情况下传输误比特率和接收光功率的关系。在误比特率为2.4×10-4的FEC级别下，双向下行链路在BTB和5km距离传输接收灵敏度分别为-34.35dBm和-32.51dBm，双向上行链路在BTB和5km距离传输接收灵敏度分别达到了-34.30dBm和-29.76dBm。
Figure 8. Bit error rate of bidirectional transmission in coherent detection system
直接检测和相干检测的系统误比特率都随着接收功率的减小呈增加趋势；但在相干检测系统中，由于发射端光源和本征光源相位差具有随机性，误比特率曲线邻点间出现了接收功率减小、误比特率也减小的现象，可在接收端设计锁相环来减少该现象[19]。相比直接检测系统，相干检测增加了系统的复杂度和成本，但其能够减小光纤色散带来的影响显著提升系统的接收灵敏度。在此系统中，相干检测将5km双向下行链路的接收灵敏度提升了14.20dB，5km双向上行链路的接收灵敏度提升了11.82dB，拥有良好的应用前景。
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分析了EAM的反向偏压特性，在调制电压峰峰值为2V、反向偏压为1V时，EAM能获得良好的调制效果。设计并运行了以DFB和EAM为上行发射机的40Gbit/s、5km双向传输网络，结果表明，在前向误比特率为2.4×10-4时，以直接检测方案为接收端的双向网络下行和上行信号的接收灵敏度分别为-18.31dBm和-17.94dBm，分析得出光纤色散对此系统影响较大，而光纤的后向瑞利散射对此系统影响较小。相干检测方案的双向网络下行链路信号和上行链路信号的接收灵敏度分别达到了-32.51dBm和-29.76dBm，并且相较于直接检测方案接收灵敏度分别有14.20dB和11.82dB的提升。
本系统的光网络单元所采用的器件皆为可集成化的半导体器件，为未来用户端光网络单元提供一种高速率可大规模部署的集成化方案。
基于DFB-EAM的40Gbit/s可集成光网络单元全双工测试
Full duplex test of 40Gbit/s integrable optical network unit based on DFB-EAM
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摘要: 为了兼顾用户端光网络单元的特点和传输速率, 在接收端采用可集成化的相干检测方案替代传统的直接检测方案, 分析了电吸收调制器的偏压特性, 搭建了速率为40Gbit/s的5km双向接入网通信系统, 对以分布式反馈激光器和电吸收调制器为上行发射机的可集成光网络单元进行了传输测试。结果表明, 在直接检测方案前向纠错(FEC)下, 双向传输下行信号接收灵敏度和上行信号接收灵敏度分别达到了-18.31dBm和-17.94dBm; 在相干检测方案FEC下, 双向传输下行信号接收灵敏度和上行信号接收灵敏度分别达到了-32.51dBm和-29.76dBm。相干检测方案的上行和下行接收灵敏度相较于直接检测分别有14.20dB和11.82dB的提升。该研究为未来用户端光网络单元提供了一种高速率可大规模部署的集成化方案。Abstract: In order to take into account the characteristics of the optical network unit and the transmission rate of the user end, an integrable coherent detection scheme was used to replace the traditional direct detection scheme at the receiving end. The bias characteristics of the electro-absorption modulator (EAM) was analyzed. A 5km bidirectional access network communication system with a rate of 40Gbit/s was built, and the transmission test of the integrable optical network unit with the distributed feedback (DFB) laser and the EAM as the upstream transmitter was carried out. Under the direct detection scheme and forward error correction (FEC), the receiving sensitivity of downstream signal and upstream signal of bidirectional transmission are -18.31dBm and -17.94dBm respectively. Under the coherent detection scheme and FEC, the receiving sensitivities of downstream and upstream signals are -32.51dBm and -29.76dBm, respectively. The results show that the upstream and downstream receiving sensitivities of the coherent detection scheme are 14.20dB and 11.82dB higher than those of the direct detection scheme, respectively. The result of this work provides a high-speed and large-scale deployment integrated scheme for the optical network unit of the future client.
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Figure 3. Structure diagram of 40Gbit/s direct detection bidirectional transmission system
Figure 4. Bit error rate of unidirectional transmission in direct detection system
a—downstream b—upstream
Figure 5. Bit error rate of bidirectional transmission in direct detection system
a—downstream b—upstream
Figure 6. Structure diagram of 40Gbit/s coherent detection bidirectional transmission system
Figure 7. Bit error rate of unidirectional transmission in coherent detection system
a—downstream b—upstream
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