Multiple-quantum-well Mach-Zehnder modulators: comparison of calculated and measured results for modulator properties and system performance

A measurement-based model of a /spl pi/ phase-shift multiple-quantum-well Mach-Zehnder modulator with asymmetric Y-branch waveguides is presented. Using this model, calculated and measured results for the transmission contours, /spl alpha/-parameter, intensity modulation, and time-resolved chirp are compared. The suitability of the model for system performance evaluation is demonstrated by examining the implications of the modulator chirp on 10-Gb/s transmission over nondispersion shifted fiber, and the combined effect of the modulator chirp and group delay ripple of a dispersion compensating fiber Bragg grating on 10-Gb/s dispersion compensated transmission. Calculated and measured results for the system performance are compared.     

Demonstration of negative dispersion fibers for DWDM metropolitanarea networks

We present a detailed experimental and theoretical study, showing that a novel nonzero dispersion-shifted fiber with negative dispersion enhances the capabilities of metropolitan area optical systems, while at the same time, reducing the system cost by eliminating the need of dispersion compensation. The performance of this dispersion-optimized fiber was studied using different types of optical transmitters for both 1310- and 1550-nm wavelength windows and for both 2.5-and 10-Gb/s bit rates. It is shown that this new fiber extends the nonregenerated distance up to 300 km when directly modulated distributed feedback (DFB) laser transmitters at 2.5 Gb/s are used. The negative dispersion characteristics of the fiber also enhance the transmission performance in metropolitan area networks with transmitters that use electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers, which are biased for positive chirp. In the case of 10 Gb/s, externally modulated signals (using either EA-DFBs or external modulated lasers using Mach-Zehnder modulators), we predict that the maximum reach that can be accomplished without dispersion compensation is more than 200 km for both 100- and 200-GHz channel spacing. To our knowledge, this is the first demonstration of the capabilities of a nonzero dispersion-shifted fiber with negative dispersion for metropolitan applications     

Nonlinear Distortion in a Silicon Microring-Based Electro-Optic Modulator for Analog Optical Links

We analyze silicon microring modulators for analog signal generation, in terms of harmonic and intermodulation distortions. Free carrier plasma dispersion effect in silicon, Lorentzian-shaped resonance profile, and cavity photon lifetime are identified as the sources of modulation nonlinearity. Simulations show that the silicon microring modulators exhibit good modulation index and carrier-to-distortion ratio (CDR). For the generation of 10-GHz suboctave analog signals, a high optical-modulation-index of up to 46%, a CDR of 57 dB, and a spurious free dynamic range of 96 dB can be obtained. In fiber transmission, the microring-generated signal CDR can be improved by 10 dB due to the interaction between frequency chirp and chromatic dispersion.      

基于Optisystem的光电色散补偿技术的性能分析

为了实现光信号的有效传输,减少光纤中色散和非线性效应对通信系统的影响,提出一种基于光纤布拉格光栅(fiber bragg grating,FBG)中间补偿的光电色散补偿技术,即在传输链路上采用FBG中间补偿方式,在接收端采用电色散补偿技术,分别实现了对40 Gbit/s,归零(RZ)码信号的单信道和多信道系统的色散补偿.通过与全光色散补偿技术作对比,可以得到在入射功率较高的条件下,光电色散补偿技术的补偿性能明显优于全光色散补偿技术,同时能够保持较高的Q值,为系统以后的升级、扩容做铺垫.     

Silicon-Based Microring Resonator Modulators for Intensity Modulation

We numerically analyze the characteristics of silicon-based microring modulators consisting of a single-ring resonator. Performance of the devices as digital intensity modulators is examined in terms of extinction ratio, pulsewidth, frequency chirp, spectral broadening, and signal quality. Three types of the modulators built in single-waveguide under-/overcoupling and dual-waveguide configurations are discussed. We show that cavity dynamics significantly affect the modulation properties. Data transmission performance over single-mode fibers is also presented. A silicon microring modulator with negative chirp could achieve 0.8 dB power penalty in 80-km fiber transmission without dispersion compensation.      

On the use of electrical precompensation of dispersion in optical single-sideband transmission systems

The use of electrical predistortion in optical single-sideband (OSSB) transmission systems is proposed to extend the tolerance to fiber dispersion impairment. Due to the spectral properties of OSSB signals, a simple electrical dispersive line can be used to mitigate the fiber dispersion, reducing the complexity of electrical predistortion methods proposed recently for optical modulations other than OSSB. Carrier-suppressed and carrier-unsuppressed optical modulations can be compensated by such technique, extending the range compared to previously reported works on fiber dispersion mitigation after direct detection using OSSB signals, where only carrier-unsuppressed modulations can be effectively compensated. The use of electrical precompensation is investigated for three signaling formats: Nonreturn to zero, return to zero, and alternate mark inversion (AMI). Due to distortion occurring in the sideband suppression process, signaling formats with reduced duty cycles present improved performance as a consequence of enhanced tolerance to intersymbol interference. Using such formats with electrical predistortion, the reach limitation arises not from fiber dispersion but from nonlinear impairments and optical noise accumulation. Using AMI and ideal electrical predistortion, distances greater than 1200 km of standard single mode fiber are achieved at 10 Gb/s with bit error ratio (BER) lower than 10/sup -12/, without using optical dispersion compensation. Introducing accurate models for the electrical dispersive lines, the transmission distance is reduced to 720 km for BER lower than 10/sup -12/.     

Adaptive time‐frequency representation for weak chirp signals based on Duffing oscillator stopping oscillation system

To investigate high‐resolution time‐frequency representations for any type of weak chirp signals with a very low signal‐noise ratio, we revisit the inherent deficiencies of conventional Duffing oscillator detection methods and propose a novel Duffing oscillator stopping oscillation detection system. As a result, the detection of chirp signals can be successfully realized, and the influence of nondetection zones and critical thresholds on the detection accuracy is successfully eliminated. Furthermore, we propose an adaptive Duffing oscillator stopping oscillation detection method to measure the instantaneous frequency variation of a highly dynamic chirp signal within a large frequency range. The simulation results indicate that, compared with the conventional Duffing oscillator detection methods and the Choi‐Williams distribution, the proposed method greatly expands the frequency detection range of a single Duffing oscillator and has a lower computing cost and effective real‐time performance in detecting a high‐precision weak chirp signal, which provides a new solution for the time‐frequency representation of weak chirp signals at a lower signal‐noise ration and reveals broad prospects for applications in engineering.     

Theory of double-chirped mirrors

A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated ≈30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses     

High-capacity undersea long-haul systems

This paper reviews technologies and techniques that have been used in deployed long-haul wavelength division multiplexing (WDM) systems and emerging technologies that could be used for the next generation of cost-reduced systems. The overview of current generation technologies starts with a discussion of modulation formats, focusing on the superior properties of the chirped return to zero on-off shift keying (CRZ-OOK) modulation format. The use of 10-Gb/s CRZ-OOK modulation format together with advanced fiber types, more powerful forward error correction (FEC), and broadband erbium-doped fiber amplifiers resulted in the deployment of dense WDM systems with capacities per fiber in terabits per second range and trans-Pacific reach. Demand for the systems with large design capacity led to further development of broadband optical amplifiers. Laboratory demonstrations successfully expanded transmission into the full C-band and later in the C and L transmission bands. The current market conditions dictate the need for reducing the first cost of an installed system rather than reaching record capacity per fiber. Reducing first cost can be achieved by reducing the amount of optical amplifiers in the cable by utilizing an excess performance margin provided by available RZ-OOK technology. Further improvements would be possible if more powerful FEC and modulation formats with better receiver sensitivity are used. For example, the RZ differential phase-shift keying (RZ-DPSK) modulation format with 3-dB better receiver sensitivity and better nonlinear tolerance to large amounts of accumulated dispersion is a very promising technology. This paper will review long-haul transmission results using RZ-DPSK and will compare the transmission properties of RZ-DPSK signals versus RZ-OOK signals. Due to superior receiver sensitivity, the RZ-DPSK modulation format can be an enabling technology for 40-Gb/s per channel transoceanic transmission.     

Planar waveguide optical spectrum analyzer using a UV-inducedgrating

An optical spectrum analyzer is demonstrated using a UV-induced, chirped grating on a planar waveguide and a linear detector array. The grating provides both the dispersion with respect to wavelength and the imaging function. The spectral response has a full-width at half-maximum of 0.15 nm and a bandwidth of 7.8 nm. A chirp of -1.75 nm/cm and a 12-cm focal length were used. The design parameters are discussed in terms of the chirp, focal length and resolution. The wavelength dependence of the optimal chirp limits the resolution for small focal lengths. For longer focal lengths, the spotsize is diffraction limited and high resolution can be achieved with minimal fabrication complexity     

Tunable compensation of channel degrading effects using nonlinearlychirped passive fiber Bragg gratings

Several channel-degrading effects are present in nonstatic and dynamically reconfigurable wavelength-division-multiplexed systems and networks due to various types of dispersion in the optical transmission fiber. These effects must be addressed by tunable methods so that data signals do not fade with time. The relevant effects for which we demonstrate tunable compensation include: chromatic dispersion accumulated in a single channel and in multiple channels, polarization mode dispersion, and periodic RF power fading. We utilize a nonlinearly chirped fiber Bragg grating that provides a dispersive function that can be varied continuously by tuning a single mechanical stretching element     

基于小波包能量谱的电网故障行波定位方法

为减少电网故障行波传播色散特性对行波波头检测和波速测量的影响,提高电网故障行波定位的准确度,提出了基于小波包能量谱的电网故障行波定位方法。该法结合小波包技术与傅立叶变换的谱分析,对行波信号进行分析,提取能量相对集中的故障频带信号进行故障行波定位计算;行波到达时间由该频带相应尺度下的小波包能量时谱提取的行波特征点位置计算;该频带行波传播速度由输电线路两端对外部扰动的实测行波数据计算。大量的电网故障行波定位ATP仿真分析结果和现场实验测试分析表明,该电网故障行波定位方法能有效提取电网行波特征信号,减少行波传播色散特性和线路长度变化的影响,定位误差<200m。     

10kV扩频数字传输系统的研制

介绍了Intellon公司的线性调频收发器SSCP485，并基于该芯片和声码器AMBE－1000以及单片机研制出了一种新型的10kV扩频数字传输系统，描述了其硬件设计和软件构成，还进行了调试和性能分析。     

Design of amplifier- and modulator-integrated laser diode for 10-Gb/s 80-km transmission

We present the amplifier- and modulator-integrated laser diode designed for 10-Gb/s transmission over 80-km distance. An antireflection window structure is implemented at the tilt facet of the amplifier to reduce optical feedback by 40 dB. A monitor photodiode is monolithically integrated in the antireflection window region for accurate power regulation. To better control the amplifier input power and to reduce the feedback of amplified spontaneous emission, an attenuation inner-window is incorporated by removing a section of the waveguide. The reproducibility of the chip fabrication process is tested to confirm the design margin. Through amplifying the modulated signals and reducing modulator chirp by amplifier saturation, high-power optical transmission is demonstrated.     

基于多峰谱线插值的间谐波检测新方法

插值FFT算法根据检测到的频率分布修正FFT结果,而实际中间谐波频率很难确定,当间谐波信号附近含幅值较大的谐波信号时,FFT的栅栏效应可能降低检测精度。利用频率识别能力较强的线性调频Z变换（CZT）,用检测得到的与估计值邻近的4条离散频谱幅值估计间谐波参数,并根据多项式逼近推导了插值修正公式,在CZT谱频率分布指导下进行插值修正,使各次谐波和间谐波参数的检测精度得到一定程度提高,仿真证明了该方法的可行性和正确性。     

光传输系统中色度色散的计算

广东电力通信传输网经过多年的统一规划和分期建设,已经逐步形成了由省干传输网和21个地市级传输网组成的全覆盖光纤通信网络。随着近年来电网业务的不断拓展,传输系统的速率越来越高、距离越来越长。如何在高速率、长距离的情况下,利用现存的资源结构配置光设备并保证信号传输的质量,成为通信技术人员面临的问题。文章探讨了光纤的色散问题,介绍了几种色散补偿技术,给出了简化的计算模型,以便设计人员选择合适的色散补偿产品。     

Reducing interchannel crosstalk in long-haul DWDM systems

Nonlinear interaction between signal channels may result in significant performance degradation in transoceanic dense wavelength-division-multiplexing (WDM) transmission systems operated at high bit rates. In this paper, we report that the nonlinear crosstalk for systems with high spectral efficiency can be suppressed if WDM channels are launched with orthogonal relative states of polarization. It is shown that for adjacent channels (that experience the strongest nonlinear interaction) the launched relative state of polarization is maintained over the large distance of low polarization-mode dispersion fiber typically used in undersea systems. Numerical studies, as well as experimental results, are presented for standard and dispersion slope-matched dispersion maps     

基于遗传算法的正交多相码产生技术研究

正交信号设计是MIMO背景下的雷达信号产生的关键技术之一,为了降低正交多相码设计的复杂度,提高雷达的相关性能,本文采用了一种基于遗传算法的正交多相码设计方法。该方法利用Hadamard矩阵的严格正交性,首先构造一类Hadamard矩阵,从中选取初始码序列;然后把“和函数”作为代价函数,采用遗传算法求解出近似最优的信号组。整个实验通过MATLAB仿真实现。仿真结果表明,该方法在保证低自相关旁瓣量和低互相关量的同时,能够保证设计出的信号符合严格正交性。     

High bit-rate OTDM transmission over standard fiber using mid-spanspectral inversion and its limitations

An accurate modeling of very high bit-rate transmission systems over standard-fiber using mid-span spectral inversion (MSSI) is reported. As for the pulsewidth, comparison of simulations with experimental data in an 80-Gb/s transmission system over 106 km shows that second-order group velocity dispersion (GVD) is the limiting factor in this transmission scheme. Additionally, it is shown that even though first-order GVD can be compensated easily second-order GVD, is still remaining. It is pointed out that the performance of the system is very sensitive to the length of the different fiber spans     

利用仿真技术研究色散对光纤通信的影响

本文主要研究光纤色散对高速光纤传输系统造成的影响.首先对光纤传输系统中各部件尤其是光纤的色散进行了仿真建模分析,在此基础上,提出了一种针对色散研究的高速光纤传输系统仿真模型,利用该模型详细研究讨论了在多种情况下色散对高速光纤通信的影响.仿真结果与理论分析的一致性证明,该仿真模型是有效的.