基于半导体光放大器和光学滤波器的高速全光信号处理

论文中英文摘要

作者姓名：董建绩

论文题目：基于半导体光放大器和光学滤波器的高速全光信号处理

作者简介：董建绩，男，1979年11月出生，2005年09月师从华中科技大学黄德修教授攻读博士学位，于2008年06月获博士学位。

中 文 摘 要

光纤通信网络已无可争辩地成为整个信息网络的核心。

信息业务的需求量仍在指数增长，以密集波分复用（DWDM）为代表的多种复用技术已成功地解决了光纤通信网络中大容量信息业务的传输问题。信息业务的阻塞将日益凸现在光纤通信网络的节点上。因此，变革传统的“光－电－光”信息交换方式，在网络节点上实施全光交换和全光信号处理是本世纪以来的全球研究热点，也是当务之急。全光波长转换、全光逻辑、全光信号再生和全光码型转换等是全光交换得以实现的一些基本功能。

半导体光放大器（SOA）以其高非线性系数、低功耗、高效率、小体积、低成本以及易于和其他半导体光电子器件集成等特点，将成为实现全光信号处理的基本器件单元。本文以SOA和光学滤波器为基本构件，对上述全光信号处理功能开展了较全面的理论与实验研究工作，所取得的创新性成果如下：

(1) 对SOA和失谐滤波器的组合模型进行了深入的理论分析和实验研究。从研究SOA增益介质内电子-光子、光子-光子相互作用的复杂物理过程及其产生的多种物理效应出发，紧密结合由这些效应在光纤通信中的应用潜力，针对波长转换在光纤通信网络中能有效降低信息业务的阻塞和使网络更具灵活性的特点，首次用时域分析方法求解了基于瞬态交叉相位调制（T-XPM）效应的波长转换解析公式，在理论上很好地阐明了滤波器失谐对全光波长转换动态特性的改善作用，成功解释了滤波器失谐在同相波长转换和反相波长转换中的不同作用和机理，即当滤波器失谐量较小时，得到反相的波长转换结果，此时交叉增益调制（XGM）占主导作用，而滤波器的失谐起到加速增益恢复的作用；当滤波器失谐量较大时，得到同相波长转换结果，此时交叉相位调制（XPM）占主导作用，而滤波器起到提取啁啾信息的作用。该研究为所有基于SOA和失谐滤波器组合模型的全光信号处理技术提供了理论依据。同时从实验上分别验证了工作速率为10Gbit/s和40Gbit/s的同相和反相波长转换功能，这两种波长转换的切换仅仅取决于光学滤波器的失谐量大小，该项技术能极大地提高波长转换输出的灵活性和可控性。

(2) 鉴于光纤通信不同的网络层次和规模，综合考虑光谱效率、抗光纤中色散和非线性影响、成本等因素，不同的网络环境有不同的适用码型。为此，基于SOA和失谐滤波器组合模型，本文全面深入地探索了各种全光码型转换的实现方法以及全光2R再生技术。基于SOA的T-XPM效应，首次从理论上验证了从单信道非归零码（NRZ）到双信道归零码（RZ）的码型转换，并对码型转换的性能参数（消光比、转换效率和非线性码型效应等）做了深入的分析。同时从实验上首次报道了从单信道NRZ到双信道伪归零码（PRZ）的转换，以上的双信道信号分别通过滤波器红移和蓝移得到。另外，从实验上证实了基于SOA的自相位调制效应的NRZ到PRZ的转换和NRZ信号的2R再生功能，当滤波器选择蓝移时，则抑制NRZ信号的畸变，从而实现信号的2R再生，反之当滤波器选择红移时，则提取这种过冲脉冲，抑制信号的直流分量，从而实现NRZ到PRZ的转换。以上各种码型转换方案均只使用一个SOA，两个滤波器则实现双信道输出，结构简单，工作速率高。

(3) 通过用光控制光的逻辑器件以产生类似于电子学中的逻辑功能，以求应用于光通信和光计算中，是上世纪70年代半导体激光器实现室温连续工作并用其实现双稳态以来一直所研究的难题。本文全面研究了基于SOA的各种逻辑门及其高级逻辑器件，首次实验报道了利用单个SOA器件实现5种不同功能的全光逻辑门，包括“或非门”、“或门”、“非门”、“与门”和“同或门”，工作速率达到了40Gbit/s，这可大大地降低光纤通信系统的复杂度，增加光纤通信网络的灵活性。本文还从理论上提出了基于单个SOA和两个并联的滤波器组合的全光“半加器”和“全加器”，利用滤波器的不同失谐量提取不同的逻辑输出，分别得到“进位”位和“求和”位，与传统的SOA干涉结构和交叉增益调制的级联结构相比，本文所提出的全光加法器仅需单个SOA，结构简单，无偏振相关性，工作速率高，不失为未来高级全光逻辑器件的优先选择。

在博士学位论文提交后的进一步理论研究发现了基于单个SOA实现任意布尔逻辑运算的可行性，分析了不同逻辑运算的品质因子和引入的功率代价，这种任意布尔逻辑使得SOA作为未来光子计算的基本元器件成为可能。该研究成果发表在2009年5月11日的Optics Express，虽在有效期内，但未在本学位论文中得到反映。

(4) 定义10dB带宽大于500MHz的无线电信号的所谓超宽带（UWB）在成像系统、车载雷达，以及蓬勃发展的移动通信中具有广泛的应用。用光学方法产生UWB是为当前的研究热点之一。本文将全光信号处理技术应用于超宽带射频信号的产生，在国内率先开展了基于SOA的超宽带射频信号全光处理（包括全光产生、传输和调制）的研究工作，特别提出了几种创新型的方案产生超宽带射频的monocycle，doublet和triplet脉冲信号。首次利用SOA和失谐滤波器的组合产生了超宽带monocycle脉冲，通过选择滤波器蓝移和红移分别得到一对极性相反的monocycle脉冲，首次利用SOA的增益饱和效应，直接从反相RZ信号中提取monocycle波形，实验研究了注入电流、输入信号脉宽和波长对超宽带频谱的影响，首次利用单模光纤的色散特性对差分相移键控信号进行微波滤波，从而获得超宽带doublet脉冲和

triplet脉冲。同时首次实验报道了针对多波长混合型的超宽带信号可以通过色散波长技术实现无畸变传输，并利用SOA实现超宽带脉冲的多种全光调制方法。本文提出的超宽带信号的处理是真正意义上的全光操作，与国际上报道的电子技术方法和光电混合方法相比，具有较高的响应和调制速率，并与全光通信网络具有更好的兼容性等优点，尤其是在应用于UWB-over-fiber 技术中避免光-电-光的转换，体现了其优越性，极大简化了系统网络。

(5) 四波混频是不同波长的光子在光学介质中相互作用所产生的一种非线性效应，也是长期以来研究者致力于克服其在密集波分复用系统中对系统性能的损伤，并如何将它应用于全光信号处理的一个重要课题。本文研究了基于SOA的四波混频效应的全光信号处理技术。从理论上提出并实现了一种全光比特误码检测器，该器件是利用两级级联的SOA的逻辑非门和与门来完成的，结果表明该方案对于40Gbit/s的RZ信号和10Gbit/s的NRZ信号均能胜任。其次从实验上研究了四波混频效应实现多种调制速率、多种调制格式的波长转换功能。特别地，对40Gbit/s的NRZ信号实现了单信道到三信道的多波长转换。同时利用四波混频效应实现了40Gbit/s的单信道NRZ码型到双信道RZ信号的码型转换，输出的双信道分别对应四波混频效应产生的两个边带频率，转换光信号的品质因子均大于6。由于四波混频效应具有对比特率和调制格式严格透明的优点，本方案中的波长转换和码型转换都可以较高速率工作，同时多信道的处理功能能够节约器件资源，降低成本。

关键词： 全光信号处理、半导体光放大器、瞬态交叉相位调制、四波混频、超宽带射频产生

High speed all-optical signal processing based on semiconductor optical

amplifiers and optical filtering

Dong Jianji ABSTRACT

Optical fiber communication networks are definitely becoming the core of the whole telecommunication networks.

The demand of information traffic is increasing exponentially. Some multiplexing technologies, especially the dense wavelength division multiplexing (DWDM) technology, have solved the transmission issues of large information capacity in the fiber communications as the information traffic increases explosively. However, the traffic congestion will take place in the nodes of optical fiber telecommunication networks. Therefore it is a world-wide hot topic and urgent affair to implement all-optical switching and all-optical signal processing in optical network nodes since the beginning of this century. And all-optical wavelength conversion, all-optical logics, all-optical regeneration, and all-optical format conversion are some very basic functions to realize all-optical switches.

Semiconductor optical amplifier (SOA) will become the basic unit in all-optical signal processing because of its high nonlinearities, low power consumption, high power efficiency, small footprint, and low cost, and it is easy to be integrated with other semiconductor optoelectronic devices. In this dissertation, some comprehensive theoretical and experimental researches aimed at all-optical signal processing are carried out based on SOA and optical filter. Some research achievements and contributions are summarized as below:

Firstly, comprehensive theoretical analysis and experimental researches are studied in terms of the combination of SOA and detuning optical filter. Several kinds of crucial physical processes and effects on the interaction of electron-photon and photon-photon within the SOA gain medium are explained in details. The application potentials of these physical effects in optical fiber communications are demonstrated, and especially the wavelength conversion can reduce the block of information traffic and improve the network flexibility. An analytical formula for wavelength conversion of transient cross phase modulation (T-XPM) is deduced from the viewpoint of temporal approaches for the first time. The analytical formula can reveal the gain dynamic improvement by detuning optical filter in the wavelength conversion, and explain the different mechanisms of inverted conversion and non-inverted conversion induced by the detuning optical filter. The inverted wavelength conversion is obtained by small filter detuning, which helps to accelerate the gain recovery, and in this condition the cross-gain modulation (XGM) is dominant. And the non-inverted conversion is obtained by large filter detuning, which is used to extract the frequency chirps, and in this condition the cross phase modulation (XPM) is dominant. These research results

become the theory foundation of all-optical signal processing technologies based on the combination of SOA and detuning filter. At the same time, we have experimentally demonstrated both inverted wavelength conversion and non-inverted wavelength conversion at 10Gbit/s and 40Gbit/s. The polarity of wavelength conversion is dependent on the filter detuning, which ensures the output flexibility and controllability.

Secondly, different data formats are widely used in different network environments because several kinds of impacts, such as spectral efficiency, immunity to the dispersion and nonlinearity, as well as cost, need to be considered in terms of various network layers and scales. Hence all-optical format conversion and 2R regeneration techniques are comprehensively investigated based on the SOA and optical filter. Single-to-dual channel NRZ-to-RZ format conversion by T-XPM effect is theoretically demonstrated. Some important parameters, such as extinction ratio, conversion efficiency, and nonlinear format patterning effect, are analyzed. We also experimentally demonstrate single-to-dual channel NRZ-to-PRZ conversion for the first time. The dual-channel converted signals are obtained by the red shift and blue shift of detuning filters, respectively. Moreover, we also experimentally demonstrate NRZ-to-PRZ conversion and 2R regeneration of NRZ signals based on SOA self phase modulation. The 2R regeneration is realized by suppression of NRZ distortion with a blue shifted filter, while the PRZ signal is implemented by extracting the overshoot with a red shifted filter. In all schemes above, only one single SOA is required, so the structure is quite simple and two filtering conditions ensure dual channels’ output. The system can operate at very high modulation speed.

Thirdly, optically-controlled optical logic units, similar to the logic circuits in electronics, are supposed to be applied in optical communications and optical computing, which have been the crucial issues since semiconductor laser operated at room temperature and was trying to be used to flip-flop memories in 1970s. A variety of SOA-based logic gates and the advanced logic circuits are comprehensively investigated in these years. For the first time, we have experimentally demonstrated five logic functions, including NOR, OR, NOT, AND and XNOR, based on single SOA at the bit rate of 40 Gbit/s. With this technique, the system complexity can be reduced and the system flexibility can be improved in the future optical networks. We also theoretically propose the feasibility of arbitrary Boolean logic functions with one SOA. The Q factor and power penalty for different logics are analyzed. The implementation of arbitrary logics makes the SOA one of the promising candidates of future optical computing units. At last, optical half /full adders based on single SOA and several optical bandpass filter are theoretically proposed. Different logic functions, such as the SUM and CARRY bits are implemented by different filter detuning values. The optical adders are very promising in future advanced optical circuits due to the simple structure, polarization independence, and high speed operation.

After the dissertation was submitted, a new idea that the arbitrary Boolean logic functions can be realized with only one SOA was proposed and published on Optics Express on issue of 11th May, 2009.

Fourthly, ultra-wideband (UWB) wireless signals, defined as any wireless signal with 10 dB bandwidth over 500 MHz, are supposed to be widely used in imaging system, vehicle-based radar, and flourish mobile communications, where the optical generation of UWB pulse signals is one of the hot topics all over the world. In this dissertation, all-optical signal processing techniques are applied to the UWB signal generation. We take the lead in developing all-optical UWB radio signal processing nationally, such as optical generation, transmission, and modulation. Especially several novel schemes about UWB monocycle, doublet, and triplet pulses generation are presented. For the first time, a pair of polarity-reversed monocycle pulses is generated using the combination of SOA and detuning filter. We also for the first time demonstrate a simple and compact scheme to generate UWB monocycle pulses utilizing gain saturation of the dark RZ signal in an SOA. The UWB frequency spectra at different injected currents, different input pulsewidths, and different input wavelengths are analyzed. For the first time, UWB doublet and triplet pulses from NRZ-DPSK signals are experimentally demonstrated assisted by the dispersion of single mode fiber. We also firstly achieve that multiple wavelength-mixed UWB pulses can be transmitted without distortion over the single mode fiber and its dispersion compensation fiber counterpart. Several effective all-optical modulation approaches of UWB pulses are demonstrated. Our schemes have fast dynamic response and modulation speed because of all-optical operation, so that they are more compatible to the all-optical networks compared to the electronic approaches or opto-electronic hybrid approaches. For example, the conversion between optics and electronics is avoided in the UWB-over-fiber technology, making the system simpler.

Finally, four-wave mixing (FWM) is a nonlinear effect induced by photon interaction of different wavelengths. Researchers are trying to overcome the harm of FWM effects to the DWDM systems, and also trying to apply FWM effect to all-optical signal processing technologies on the other hand. In this dissertation, all-optical signal processing based on FWM effects of SOAs is systematically investigated. We propose and demonstrate an all-optical bit error monitoring system (BEMS) based on the FWM model. The BEMS is realized by two cascaded SOAs, which implement logic NOT and logic AND, respectively. The results show that this scheme is competent for 40Gbit/s RZ and 10Gbit/s NRZ format signals. And wavelength conversion based on FWM in an SOA for various bit rate and various data formats is experimentally investigated. Especially, for 40Gbit/s NRZ signals, multi-wavelength conversion from single to triple channels is obtained. At last, all-optical single-to-dual channel format conversion from NRZ to RZ using FWM in single SOA is demonstrated. Based on the FWM effect, two sideband components are generated and both channels are RZ formats. And all the converted RZ signals have the Q factor larger than 6. FWM effects are strictly bit rate transparent and data format transparent, so our schemes can operate at high speed and process for multi-channels with lower cost.

Key words: All-optical signal processing; Semiconductor optical amplifier; Transient cross phase modulation; Four-wave mixing; Ultrawideband generation