【作者】 周恩波；

【导师】 黄德修； 张新亮；

【作者基本信息】 华中科技大学 ， 电子科学与技术， 2009， 博士

【摘要】 随着光通信系统中单信道容量的迅猛增长，对光信号处理中关键器件性能的要求越来越高。半导体光放大器(SOA)作为广泛应用在光网络节点上的非线性器件，其动态特性直接影响了整个通信网的性能。特别是其应用在超高速的波长转换、复用／解复用以及信号再生等领域中，SOA的载流子恢复时间，反映为增益恢复速度是影响整个系统性能至关重要的因素。近年来随着SOA工艺与理论的发展，各种低维结构的SOA器件实现了增益偏振无关，高增益／微分增益系数以及超快的增益恢复速度。一方面对低维SOA器件的结构设计能从根本上缩短SOA的载流子恢复时间从而提高器件的超快动态性能；另一方面对一个给定的SOA，在应用中对其增益恢复速度进行优化，也能改善器件在高速通信系统中的表现。概括全文的研究成果和贡献，有如下几个方面：(1)全面分析比较了不同物理层面描述SOA动态特性的理论模型。理论研究了SOA在不同量级脉宽的光脉冲入射下其内部各种物理过程以及其产生机理。并进一步分析了不同脉宽的光脉冲作用下在误差允许范围内各模型的适用性。在此基础上提出模型选择的原则应该是从需求出发，需要同时满足时效性与精确性两方面的要求。(2)提出了一种带载流子库的不对称多量子阱结构的SOA。通过采用多量子阱结构的载流子传输模型，数值模拟了这种结构的SOA的超快动态特性。模拟结果表明此种结构设计能有效加速SOA的增益恢复速度。(3)通过分段模型，理论推导了基于SOA中交叉增益调制效应的波长转换中小信号频率响应的解析表达式。通过此模型理论分析了SOA各参数，包括腔长、偏置电流、腔内损耗对频谱响应曲线的影响。同时结果还表明，由于长腔长中SOA更接近饱和，增益色散对小信号调制特性的影响在长腔长波导中比短腔长更为明显。最后模拟结果和实验中测量的小信号频率响应特性曲线进行了比较。(4)首次理论推导了有源增益腔内考虑端面反射时放大的自发辐射噪声(ASE)的频谱特性的解析公式。通过分段模型与频谱切割，数值模拟了ASE在腔内传播的纵向相关的光强分布与有源腔前后端面出射的ASE的频谱特性。以此公式建立的宽带模型采用牛顿迭代与并行计算控制收敛速度，实现了高效的数值模拟。通过此模型，我们理论分析了透明波长辅助光注入和一定端面反射设计对SOA中增益恢复的加速作用。并分别评估了两种方案中与SOA静态特性和动态特性相关的参数。模拟结果表明，两种方案都能有效加速SOA中的增益恢复，并对基于交叉增益调制效应和交叉相位调制效应的波长转换系统中输出信号的质量有明显改善作用。在基于端面反射的设计中，我们提出一种半反射的结构。理论研究表明这种结构的SOA在基于交叉增益调制效应的波长转换方案中采用向下波长转换，而在基于交叉相位调制效应的波长转换方案中采用向上波长转换能得到更好的结果。(5)采用非绝热模型理论分析了脉冲宽度在SOA载流子温度弛豫特征时间量级时，基于SOA和失谐滤波器的瞬态交叉相位调制效应的波长转换中输出波形的非线性码型效应。提出一种采用增益带内的保持光注入的方案来抑制输出波形的非线性码型效应，从而改善输出信号质量。通过数值模拟分析了不同波长的探测光在增益峰值处的泵浦超短光脉冲入射SOA时的增益和SOA载流子温度的超快动态特性。并通过Kramers-Kronig积分关系数值计算了探测光相位的超快动态特性。通过数值计算比较了没有辅助光和辅助光以正向和逆向传播时输出探测光波形的非线性码型效应。理论证实了辅助光入射能有效抑制输出信号的非线性码型效应。同时提出了在向上的波长转换方案中，辅助光逆向传播时，对输出信号的非线性码型效应的抑制最为明显。(6)提出一种基于级联SOA和电吸收调制器(EAM)的波长转换器来降低波长转换输出信号的码型效应。通过采用考虑增益压缩的绝热模型理论分析了此种半导体波导在不同EAM长度时都能通过调整EAM的偏置电压和延时线来优化输出信号质量。同时通过对接收机电路的模拟，理论证明了当调整EAM的偏置电压以改变EAM中的载流子扫除时间时能获得级联SOA-EAM的输出信号相对于单个SOA的最优功率代价。通过对不同速率的波长转换系统的模拟，得出在更高速传输系统中，此波长转换器在理想情况下输出眼图质量仅受限于SOA的超快响应特征时间，一般为1～2个皮秒。实验通过10GHz重复频率的锁模超短脉冲入射，证实了此方案的有效性。同时对此波长转换器使用归零码(RZ)泵浦信号输入，能得到合并了交叉增益调制和交叉吸收调制的超宽带高斯单边信号(Monocycle)和高斯双边信号(Doublet)波形输出。输出波形可以通过调整可调谐延迟器的延时量和EAM的反偏电压来灵活调整。输出信号被调制在单路波长上，并且在C波段内可调。实验测量的信号对应的射频谱完全满足美国联邦通信委员会定义的规范。更多还原

【Abstract】 The demands for key devices with high performance for optical signal processing increase rapidly with the increase of the speed in one wavelength channel in optical networks. The dynamic characteristics of semiconductor optical amplifiers (SOAs) which are extensively used in nodes of optical networks as nonlinear devices determine the performance of the entire optical communication systems directly. The recovery time of the carrier density in SOAs, reflecting the gain recovery speed is a critical parameter to the performance of the entire system, especially for ultra-high speed wavelength conversion, multiplexing/demultiplexing and signal regeneration.With the development of both theory and fabrication technology of SOA, gain polarization independence, high gain as well as differential gain coefficient and ultrafast gain recovery speed are realized in low-dimensional SOAs. On the one hand, the designs of low-dimensional structures are essential to reduce the carrier lifetime in SOAs, so that the improvement of ultrafast dynamics in SOAs can be achieved. On the other hand, optimization for acceleration of gain recovery for a given SOA in its related applications can improve the performance of the device in high speed optical communication system. The activities and contributions of this thesis can be summarized as follows:(1) Several theoretical models for describing dynamic characteristics in SOAs at different physical levels are analyzed and compared. For incidence of optical pulses with pulse width at different time scale level, some kinds of ultrafast physical phenomenon and mechanisms in SOAs are theoretically investigated. Furthermore, for interaction with optical pulses with pulse width at different time scale, the applicabilities of models have been analyzed within the error tolerance. Starting from this principle, the rules for choosing a proper model should be based on the studied objective and satisfy with both the efficiency and accuracy.(2) A type of asymmetry multiple-quantum well (MQW) SOA with carrier reservoir is proposed. A carrier transport model for MQW SOAs is used to numerically simulate the ultrafast dynamic characteristics of this asymmetry MQW SOA. The simulated results show that this structural design can effectively accelerate gain recovery. (3) An analytic form solution is derived by using multisection model to demonstrate small signal frequency response (SSFR) of wavelength conversion based on cross-gain modulation (XGM) in SOAs. The influence of parameters including length of the active region, current injection and cavity loss on the characteristics of SSFR is theoretically analyzed. Also, the results manifest that gain dispersion affects the SSFR more evidently in long SOAs than short ones as a result of deeper saturation in the formers. Finally, the measured SSFR is compared with the simulated results.(4) The analytic form solution for spectral characteristics of amplified spontaneous emission (ASE) noise in consideration of facet reflection of active cavity with gain is derived for the first time. Utilizing multisection model and slicing frequency spectrum, the longitudinal dependence of the optical power distribution of ASE propagating in the cavity and the ASE spectral characteristics emitting from the front and rear facets of the active cavity are numerically simulated. The Newton method and parallel computational algorithm are used to control the convergence speed based on this wideband model. And high efficiency is achieved in the numerical simulation. The acceleration of gain recovery in SOA by injection of assist light near transparency and design of certain reflectivities on two facets are theoretically analyzed using this model. Key parameters related to both static and dynamic characteristics of SOAs are theoretically estimated for the two schemes. Simulated results show that gain recovery can be effectively accelerated by both two schemes. Also the quality of the output signals in wavelength conversion based on cross-gain modulation (XGM) and cross-phase modulation (XPM) can be greatly improved. A half-reflection structure is suggested based on the design of facet reflection. Theoretical investigation manifests that improvement in both wavelength down-conversion based on XGM and wavelength up-conversion based on XPM can be achieved by this design.(5) Nonlinear patterning effect (NLP) of the output waveform in wavelength conversion based on transient XPM by using a single SOA and a detuned optical bandpass filter (OBF) is estimated by using a non-adiabatic model while the pulse width is comparable to the characteristics time of the temperature relaxation of carriers in SOAs. A scheme of incidence of an assist light near the gain peak to reduce the NLP of the output waveform in wavelength conversion and improve the quality of the output signal is proposed. The ultrafast dynamics of the gain of the probe light with different wavelengths and the carrier temperature in the SOA are estimated while the pump light emitted near the gain peak composed of ultrashort pulses is incident into the SOA. Also the ultrafast phase dynamics of the probe light is numerically calculated by Kramers-Kronig integration relation. The NLP of the output probe waveforms in the schemes of without assist light injection, with assist light co-propagating and with assist light counter-propagating is estimated and compared, respectively. Results show that NLP can be effectively reduced in the proposed scheme by using assist light injection. Furthermore, the improvement of NLP of output waveform is most evident in wavelength up-conversion and assist light present in counter-propagating scheme.(6) A wavelength converter based on a cascaded SOA and electroabsorption modulator (EAM) is proposed to mitigate patterning effect in wavelength conversion. An adiabatic model in consideration of the gain compression is used to analyze this semiconductor waveguide. Simulated results show that the optimization of the quality of the output signal can be always achieved for different length of the EAM by changing the reverse voltage applied on the EAM and regulating the time-delay line. It is further theoretically proved that by simulating the receiver electronics, the optimized power penalty estimated at the output attributed to a concatenated SOA-EAM compared to a single SOA can be achieved by changing the sweep-out time of the EAM practically realized by adjusting the reveres bias applied on the EAM. By simulating the wavelength conversion system at different bitrate, it’s demonstrated that for the ideal situation, the quality of the eye-diagram of the output probe signal in even high speed transmission system is only limited by the ultrafast response in SOAs, the time of which is approximate 1～2ps. This scheme is further experimentally proved by incidence of ultrashort optical pulses emitted from a mode locked laser with a repetition of 10GHz. Furthermore, photonic generation of ultrawideband (UWB) monocycle and doublet pulses is experimentally demonstrated by exploiting of XGM in SOA and XAM in EAM by incidence of return-to-zero pump signal into this wavelength converter. The polarities and shapes of UWB monocycle and doublet pulses can be simply controlled by regulating the time-delay line and the reverse voltage applied on the EAM. Also the output signal is carried on one single wavelength which is supposed to be tunable within the whole C band. The corresponding measured radio-frequency (RF) spectra meet the UWB criteria defined by federal communications commission (FCC).更多还原