A computer model of non-dispersion-limited stimulated Ramanscattering in optical fiber multiple-channel communications

The development of a computer model for non-dispersion-limited stimulated Raman scattering (SRS) in optical fiber multiple-channel communication systems is reported. The model takes into account the pump depletion due to SRS and the different attenuation coefficients at different wavelengths of operation. It can calculate the SRS effects in any fiber, provided the Raman gain profile for that fiber is known. Thus, the model is general in nature and can be used for a number of problems. It can be used to calculate the output power of all the channels, given the input of every channel and its operating wavelength. It can also be used, at least in principle, to determine the input power level if a desired level of output power is required. Moreover, it can be used to estimate the optical amplification of a signal due to SRS. This is possible because the governing equations for all of these processes are similar and therefore can be solved using the same approach     

A simple expression for optical FDM network scale considering fiberfour-wave mixing and optical amplifier noise

Possible transmission distance and number of channels are studied in optical FDM (frequency division multiplexed) systems. Optical amplifier noise and fiber four-wave mixing, which respectively restrict input power to a repeater and input power to a fiber, are taken into account. A simple analytical expression is derived for evaluating FDM network scale by using a Gaussian noise approximation. Calculation examples are also presented     

On the performance of error control coding with diversity formobile channels

Upper bounds on the bit error probability are applied to evaluate the error performance of coded systems over non-interleaved and partially interleaved Rician fading mobile channels. The correlation between successive received symbols is exploited to bound the error performance. The bound allows useful evaluation of coding gains on realistic communication systems without going into lengthy computer simulations. By further defining the maximum energy degradation factors, compact upper bounds are expressed in a similar way as on the fully interleaved or memoryless channels. The maximum energy degradation factors are computed for a wide variety of mobile channel conditions. These factors give an interesting evaluation of the fading conditions and may be used to design coded communication systems on mobile channels. Furthermore, independent space or frequency diversity may be taken into account in these bounds and it is shown that the energy degradation due to correlation is independent of this added independent diversity     

The performance of the maximum ratio combining method in correlated rician-fading channels for antenna-diversity signal combining

The performance of the maximum ratio combining method for the combining of antenna-diversity signals in correlated Rician-fading channels is rigorously studied. The distribution function of the normalized signal-to-noise ratio (SNR) is expanded in terms of a power series and calculated numerically. This power series can easily take into account the signal correlations and antenna gains and can be applied to any number of receiving antennas. An application of the method to dual-antenna diversity systems produces useful distribution curves for the normalized SNR which can be used to find the diversity gain. It is revealed that signal correlation in Rician-fading channels helps to increase the diversity gain rather than to decrease it as in the Rayleigh fading channels. It is also shown that with a relative strong direct signal component, the diversity gain can be much higher than that without a direct signal component.     

Optical time division multiplexing: systems and networks

The recent advances in optical time division multiplexed (OTDM) systems and components research show the technique to be highly suited to the generation and transmission of high capacity data on a single optical carrier. This approach uses a single wavelength to carry capacities of at least 40 Gb/s. Such systems are based on a clock frequency and tributary data rates which are easily accessible using electronic components. Short optical pulses are used in a return-to-zero data transmission format with temporal interleaving to map a number of optical data channels into a single electronic clock cycle. It is an approach that can be used to achieve extremely high data-rate bit interleaved systems. This article summarizes the developments in this field and outlines a possible methodology to evolve transport networks to encompass the potential that both WDM and OTDM have to offer     

Noise and distortions limitations in the design of HFC CATV systems

The paper deals with HFC CATV systems. System parameters determinative in obtaining the desired carrier-to-noise ratio (CNR) and acceptable nonlinear distortions are considered. Formulae for the gain and the maximum number of RF amplifiers in the longest coaxial trunk line are developed. A mathematical model of the optical channel is suggested that makes it possible for the signal minimum level at the optical receiver input to be calculated if the value of CNR and the number of TV channels transmitted are known. Analytical expressions to determine the RF signal voltage in the input and output of the optical line are given that take into consideration the nonlinear distortions in the laser transmitter, the number of TV channels and the losses within the optical fiber and passive elements. Experimental results are given to illustrate the application of the formulae in the design of a HFS system section.     

Nonlinear BER performance of dense optical WDM manhattan street network with deflection routing

The nonlinear bit-error-rate (BER) performance of dense optical wavelength-division-multiplexed (WDM) Manhattan Street Networks with deflection routing was evaluated using an extendable semianalytical process. The results show that nonlinear effects impose significant performance penalties on dense WDM networks, both in terms of maximum hops attainable and average BER, and should be taken into account when modeling such networks. Simple techniques such as optimal amplifier positioning can reduce the nonlinear penalties.     

色散缓变光纤中基于交叉相位调制的不稳定性研究

研究了色散缓变光纤中基于交叉相位调制的不稳定性,得到了同时计及离散和光纤损耗效应时的色散关系式。发现在抽运功率、传输距离、光纤损耗相同的条件下,色散缓变光纤较常规光纤具有较宽的增益谱;研究同时发现,较大的色散缓变参量及两光束较小的离散均会使增益谱的谱宽加宽,振幅的增长速度加快。并用数值方法验证了利用色散缓变光纤更易产生超短脉冲。     

基于等厚干涉原理的液体折射率测量方法

将空气劈尖的等厚干涉原理与CCD图像处理技术相结合,提出了一种对透明液体折射率进行自动测量的新方法.利用一元线性回归方法对CCD的像元序号与所接收到的干涉条纹光强极大值序号之间的线性关系进行拟合,进而由拟合系数与待测液体折射率之间的关系计算出液体的折射率.为了在测量中获得理想的干涉条纹图,对影响干涉条纹图像质量的主要因素进行了详细分析,并给出了具体的背景光消除方法.实验以水为测量对象,测量结果表明,新的测量方法是可行的,测量结果的相对误差为0.09%,另外,新的背景光消除方法,对其他光学实验中如何获得清晰的干涉条纹,也具有一定的参考价值.     

Channel sharing in multi-hop WDM lightwave networks: do we needmore channels?

A local lightwave network can be constructed by employing two-way fibers to connect nodes in a passive-star physical topology, and the available optical bandwidth may be accessed by the nodal transmitters and receivers at electronic rates using wavelength-division multiplexing (WDM). The number of WDM channels, w, in such a network is technology-limited and is less than the number of network nodes, N, especially if the network should support a scalable number of nodes. We describe a general and practical channel sharing method, which requires each node to be equipped with only one transmitter-receiver pair, and in which each WDM channel is shared in a time-division multiplexed fashion; optical fiber LANs are discussed in particular. We also develop a general model for analyzing such a shared-channel, multi-hop, WDM network. Our analysis yields a counterintuitive result: it is sometimes better to employ fewer channels than a larger number of channels. We explore bounds on the ranges of w which admit queueing stability-using too few or too many channels can lead to instability. We also obtain an estimate for the optimal number of channels that minimizes network-wide queueing delay     

PROTON: a media access control protocol for optical networks withstar topology

A new multiple access protocol called PROTON (PROTocol for Optical Networks) is developed for optical local area networks based on a passive star topology. PROTON uses wavelength division multiplexing (WDM) and is highly bandwidth-efficient. One of the available wavelengths is used as a control channel. Time is divided into fixed-sized slots. The size of the slots is the same for the control and the data channels. Before transmitting a packet, a station must compete with others for a slot in a data wavelength, using a collision-free procedure. Transmitting stations and the corresponding wavelengths for their data transmissions are determined at each station by a simple arbitration scheme. The protocol is suitable for networks where the number of users can be much larger than the number of available data channels. In addition to propagation delays, it is considered that transmitter and receiver tuning times as well as the times required to process control packets are not negligible. Whenever possible, and to maximize the throughput of the network, tuning and processing times of transmitters and receivers are overlapped with each other and with data transmission times. Also, data slot requests and packet transmissions are scheduled in a pipeline fashion, thus reducing the detrimental effects on throughput and packet delay of long propagation delays. The paper includes an analysis of the maximum throughput characteristics of PROTON. An analytical model is developed, and several performance measures are obtained     

受光束漂移影响的深空上行激光通信链路信道容量

使用Monte-Carlo仿真方法,基于光强闪烁和光束漂移的联合模型,对使用脉冲编码调制方式（PPM）和雪崩光电二极管（APD）探测器直接探测接收的深空上行激光通信链路的信道容量进行了分析。仿真结果表明,与下行链路中信道容量随着束散角的增加而单调下降的现象不同,对于存在光束漂移作用的上行星地激光链路,存在一个最优发射束散角使得星地激光系统的信道容量最大。在此基础上,对最优束散角与发射光束数目、方位角、热噪声和发射端的光束半径之间的关系进行了分析。所得的结果可以帮助实现星地激光通信系统上行链路的优化设计。     

Optical confinement in laser diodes based on nitrides of Group III elements. Part 2: Analysis of heterostructures on various substrates

Numerical simulation is used to systematically analyze the waveguide properties of laser hetero-structures based on nitrides of Group III elements and formed on substrates made of various materials (sapphire, silicon carbide, and gallium nitride); in the analysis, the birefringence effect both in the nitride structure itself and in the substrate was taken into account. The specific features of optical confinement in typical laser structures and the effect of the substrate material and metallic contacts formed on top of the p-type layers are considered. The coefficients of optical losses for waveguide modes due to free charge carriers and to leakage into the substrate were estimated; these estimates were used to determine the significance of various channels of losses.     

多普勒域上稀疏的双向中继信道估计

信道估计技术作为获得信道衰落信息的方法,是提高无线信道传输接收性能的关键技术。本文针对放大转发双向中继系统的时间选择性平坦衰落信道,利用信道在多普勒域的稀疏性进行压缩信道估计。相比于传统的线性估计方法,压缩信道估计考虑了信道的固有稀疏性,降低了导频的开销,改善了信道估计性能,提高了频谱利用率及系统吞吐量。文中通过对双向中继信道进行多普勒域的稀疏建模,仿真分析了信道估计性能随着导频数量增加、信噪比增加,得到不断改善;而不同的导频分布将影响观测矩阵的相关度,从而对信道估计产生影响。仿真表明,当导频随机分布时,信道估计效果最佳。同时,文中还仿真分析了最大多普勒频移对信道估计性能的影响。      

Impact of Fiber Non-linearity in High Capacity WDM Systems and in Cross-Connected Backbone Networks

The rapidly increasing data traffic volumes will demand for very high transmission capacity and network nodes throughput. Wavelength division multiplexing (WDM) technology will be asked to support many channels on the same fiber, both in point-to-point links and in WDM optical networks. The transmission of a high number of wavelength channels in all these systems is a key issue. This paper analyzes this topic, in both high capacity links and optical networks, highlighting the impact of fiber non-linearity, and addressing the main source of impairments. This is done through the use of a semi-analytical model recently upgraded to account for all the contributions deriving from Kerr effects, particularly four-wave mixing and cross-phase modulation. The analysis reveals that more than one hundred of channels at 2.5 Gbit/s can be transmitted in point-to-point links whose length can span until the order of 1000 km, and 32 channels per fiber, at the previous bit rate, can be handled in WDM networks, without dispersion compensation. For a higher number of channels (e.g., 64) dispersion compensation is needed.     

Fault-Locating Accuracy with an OTDR: Reflective Versus Nonreflective Faults

When using an OTDR, fault-locating accuracy can be improved if the nature of the fiber discontinuity can be taken into account in estimating the true distance to the fault. Basically, two types of faults can be encountered: reflective and nonreflective faults. Recognizing that the slope of the rising part of the back-reflected pulse is a function of the optical reflectivity at the fault, a scheme has been devised for correcting the measured distance on the OTDR to derive the exact location of the fault to within ±1 m.     

An Arrangement of Channels and Transceivers in Optical Packet Switching Networks

Based on a media access and control（MAC）protocol,an arrangement of channels and transceivers in optical packet switching dense wavelength division multiplexing（DWDM）networks is proposed in this paper.In order to reduce the cost of nodes,fixed transmitters and receivers are used instead of tunable transmitters and receivers.Two fixed transmitters and many fixed receivers are used in each node in the scheme.The average waiting delay of this scheme is analyzed through mathematics and computer simulation.The result shows that the property of the scheme is almost the same as using tunable transmitter and receiver.Furthermore,if the tuning time of tunable transmitters is taken into account,the performance of the tunable transmitter scheme is poor than this scheme at the average waiting delay and throughput of the network.     

Design and Characterization of Dielectric-Loaded Plasmonic Directional Couplers

Ultracompact directional couplers (DCs) based on dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs) are analyzed using the effective index method (EIM), with the coupling, both in the parallel interaction region and in- and out-coupling regions, being taken into account. Near-field characterization of fabricated DCs performed with a scanning near-field optical microscope verifies the applicability of the EIM in the analysis and design of DLSPPW-based wavelength-selective DCs. The design approach applicable to a large variety of integrated optical waveguides is developed, enabling the realization of DCs in which optical signals at two different wavelengths are coupled into two separate output channels. The developed approach ensures minimization of the crosstalk and overall DC length via simultaneous adjustment of the waveguide separation and length of the interaction region. As an example, the design of a DLSPPW-based DC for complete separation of telecommunication signals at the wavelengths lambda = 1400 nm and lambda = 1600 nm between two output channels separated by 6 mum is worked out, resulting in the total device length of 52.3 mum. In addition, the design of an ultracompact DLSPPW-based DC waveguide crossing that ensures a very low crosstalk over a large wavelength band in the telecommunication range is considered.     

The influence of cross-phase modulation on optical FDM PSK homodynetransmission systems

A combination of optical frequency division multiplexing (FDM) and phase-shift-keying (PSK) homodyne detection can increase transmission capacity. However, phase sensitive transmission systems, especially repeatered ones, suffer from data-dependent optical amplitude fluctuation that is converted to phase fluctuation by fiber nonlinearity. The authors discuss how this data-dependent amplitude fluctuation affects the error rate performance of optical FDM PSK homodyne detection systems. If only the optical amplitude fluctuation induced by phase modulators is taken into account, the allowable power fluctuation to keep the power penalty at 0.5 dB at a bit error rate (BER) of 10^-10 is below 0.17 mW for BPSK homodyne detection and 0.09 mW for QPSK homodyne detection. However, if only the amplitude fluctuation induced by the fiber chromatic dispersion is taken into account, the allowable number of repeaters to keep a 0.5-dB power penalty due to XPM at a BER of 10^-10 is 1 for BPSK homodyne detection and below 5 for QPSK homodyne detection     

Synchronous Parallel Optical I/O on CMOS: A Case Study of the Uniformity Issue

Short-range parallel optical interconnect between integrated circuits can alleviate bandwidth, power, and packaging density issues that are associated with low-latency high-bandwidth input-output over electrical interconnect. In this paper, we evaluate the option of using true source-synchronous signaling over optical interconnect with a large number of channels, reducing the substantial per-channel clock synchronization circuitry to one instance. We also look into dc-unbalanced signaling to remove the need for data coding. Uniformity across channels is key to the feasibility of such an approach. An actual 64-channel parallel optical interconnect setup at 1.25 Gb/s/channel is examined, and models for the performance and uniformity of the different constituent parts of the interconnect are drawn up. Major attention is given to the statistical modeling of the coupling efficiency between a vertical cavity surface emitting laser array and a multifiber connector. Although derived in the context of a uniformity study, the stochastic models and the modeling approach are valuable in their own right. In our case study, the usage of a common logic threshold across all channels, which is required for dc-unbalanced signaling, appears infeasible after all models are combined. Efficient true source-synchronous signaling turns out to be within reach in carefully designed systems.