Towards the fundamental limits of optical-fiber communications

We review the theoretical limits which restrict transmission over optical fibers. The fundamental limit on channel capacity is 1 nat/photon with a coherent detection receiver or with a thermal-noise-limited receiver. With an ideal photon-counting receiver, the theoretical capacity is infinite. A practical limit of a few nats per photon for direct detection requires a bandwidth expansion consistent with monomode fibers and fast digital circuits and is 35-40 dB better than current direct detection receivers. This limit may be approached by receiver improvements (10 dB with direct detection, 17 dB with optimum coherent detection), by using digital pulse-position modulation (PPM) (10-13 dB) and by using error-correcting codes where constraints on system complexity allow.     

Modified split-step Fourier method for the numerical simulation ofsoliton amplification in erbium-doped fibers with forward-propagatingnoise

Using a modified version of the split-step Fourier method, we analyze the effect of noise on soliton propagation inside erbium-doped fiber amplifiers. In fact, noise from forward-propagating amplified spontaneous emission, associated with a Markov immigration process, is included in the analysis of soliton amplification. Moreover, this algorithm accounts for the real spectral gain profile of the fiber amplifier. The frequency jitter, induced during soliton amplification, is compared with the Gordon-Haus effect where optical amplifiers are considered as noisy point-like devices     

动态自适应分步傅里叶算法及其在光纤传输模型中的应用

在分步傅里叶算法求解非线性薛定谔方程的基础上。发展了一种动态自适应算法,以达到对时间窗口和步长的动态选择。该算法解决了数值计算过程中由于无法事先估计传输后时域范围而可能引起的仿真结果失真,同时通过对步长的动态选择大大提高了计算效率。在非线性光纤光学、光孤子传输等领域应用该算法都能取得较好的效果。     

Suppression of spurious tones induced by the split-step method infiber systems simulation

Numerical approximations involved in solving the nonlinear Schrodinger equation using the split-step method may cause the generation of spurious spectral peaks which can be seen as fictitious four-wave mixing. In this letter, we present a new method for reducing and controlling such simulation artifacts by properly choosing the spatial step size     

非均匀步长分步傅里叶算法的改进

研究了大气湍流中斜程路径上激光波束的传输模拟过程中的步长取值问题。针对分步傅里叶算法中大步长情况下的边界反射问题,首先对已有的吸收边界法进行了数值实验,得到了吸收边界法的步长取值上限。提出了滤波函数法,进一步解放了对步长的限制,利用真空中的波束传输模拟,证明了滤波函数法的有效性及其有效区域。结合这两种方法,给出了一种斜程路径上的相位屏设置方式,模拟了激光在此路径上的传输并统计计算了激光光斑的相干长度和闪烁指数。数值结果与理论吻合,证明了滤波函数法可以有效地增大分步算法中步长的上限,从而提升大气湍流中激光波束远距离斜程传输仿真效率。     

Carrier-induced phase noise in angle-modulated optical-fiber systems

Phase noise in angle-modulated optical-fiber communication systems arising from optical power fluctuations is analyzed. The nonlinear refractive index of silica is the physical mechanism which converts power fluctuations into phase fluctuations. The effects of self-phase modulation (an optical wave acting on itself) and cross-phase modulation in wavelength-division multiplexed (WDM) systems (one optical wave modulating a channel at a different wavelength) have been calculated. The phase noise generated in single-channel systems is negligible for laser fluctuations less than 1-mW rms. In WDM systems containing as few as four channels the phase noise exceeds tolerable levels (0.15 rad) for power fluctuation of 1 mW in each channel.     

Optical-transmission characteristics of optical-fiber cables and installed optical-fiber cable networks for WDM systems

Many cables containing 1.3-/spl mu/m zero-dispersion single-mode (SM) optical fibers are installed in trunk and access networks. Recently, there have been a number of studies on wavelength-division-multiplexing (WDM) systems designed to increase transmission capacity and flexibility. If we can construct WDM systems using SM optical-fiber cable networks designed to transmit using wavelengths in the 1.3-/spl mu/m window (O-band), this will prove very effective in reducing construction costs. It is therefore important to examine the wavelength dependence of the transmission characteristics of SM optical-fiber cables and networks that have already been installed and in which several optical fibers are joined. In this paper, we describe the measured optical characteristics of SM optical-fiber cables and installed optical-fiber cable networks at various wavelengths. The optical characteristics were stable in the 1.46 to 1.625-/spl mu/m wavelength range and we confirmed that the installed SM optical-fiber cable networks could be used for WDM system applications.     

An efficient split-step time-domain dynamic modeling of DFB/DBRlaser diodes

A novel and efficient approach for the numerical solution of time-dependent coupled-wave equations, which are frequently used for the modeling of distributed-feedback, distributed Bragg reflector, and Fabry-Perot laser diodes, is proposed. In this approach, the coupled-wave equations are split into two sets of equations. One of two sets of equations contains only the phase factors and time derivatives, and the other contains only the coupling terms. The separate sets of equations are solved exactly in their split form successively. This new numerical scheme, which we call the split-step time-domain model, is found to require an order of magnitude smaller number of subsections to get more accurate results than previous methods while the computation time for each time step is comparable to previous methods     

Differences between the narrow-angle and wide-angle propagators inthe split-step Fourier solution of the parabolic wave equation

For tropospheric electromagnetic propagation, Maxwell's equations can be reduced to a parabolic wave equation, which is solved by marching over range steps. In each step, the solution is split into a product of three operators. The first and third account for atmospheric and surface variation, while the center operator propagates the field as though in vacuum. This center operator is the object of interest here. Older versions of the method used the narrow-angle propagator, while more recent versions use the wide-angle propagator. It was thought that the wide-angle propagator was entirely superior to the narrow-angle propagator, but some artifacts observed in experiments have led to the present investigation. The two propagators are examined numerically and analytically and found to exhibit subtle differences at large angles from the horizontal. This has required modifications to the way in which sources are created for starting the split-step solution. The narrow- and wide-angle propagators are also compared on two problems with analytic solutions to quantify the improvement of the wide-angle over the narrow-angle     

Development of split-step FDTD method with higher-order spatial accuracy

A split-step FDTD method with higher-order spatial accuracy is presented, which is proved to be unconditionally stable. From the dispersion analysis, it is justified that the method achieves improved accuracy compared with lower-order cases and its dispersion error is comparable with the higher-order ADI-FDTD method.     

The Brownian-bridge method for simulating polarization mode dispersion in optical communications systems

We propose a technique that allows efficient simulation of fibers with a specific value of instantaneous polarization mode dispersion (PMD) at a given optical frequency. The proposed technique is rigorously indistinguishable in the statistical sense from applying the brute-force Monte Carlo method and then selecting the fibers whose PMD vector has the desired value. To demonstrate the capabilities of the proposed method, we calculate the probability density function of the differential group delay (DGD) conditioned on the value of the DGD at an offset frequency.     

Microcells in personal communications systems

Research projects involving fiber-connected microcell base units, measurements and models of propagation in urban microcells, and radio link performance in urban microcells are summarized. Developments in microcell system issues, such as channel assignments and handoff in microcells and architectures for combining microcells with traditional macrocells, are discussed. Research on both indoor radio propagation and wireless indoor systems is reviewed     

Demonstration of fibre impairment compensation using split-step infinite-impulse-response filtering method

Distributed impairment compensation for dispersion and nonlinear effects using backward propagation in a wavelength-division multiplexing (WDM) environment is demonstrated. Dispersion compensation is achieved using complex infinite-impulse-response filtering, which significantly reduces the computational load compared to using finite-impulse-response filtering.     

A hybrid modeling method for mechanical systems

In this paper, a system identification method for hybrid systems switched by the magnitude of velocity and displacement is proposed. First, it is shown that the regression vector space of a mechanical system switched by the magnitude of velocity cannot be separated by a hyperplane. Then a method based on support vector machines with a polynomial kernel is proposed. The effectiveness of the proposed method is shown by simulations and experiments.     

The reliability of silicon avalanche photodiodes for use in optical-fiber transmission systems

Thermal overstress tests on silicon avalanche photodiodes (APD's) from two manufacturers have shown that APD's, despite their complexity, can have adequate reliability for use in optical-fiber transmission systems in telecommunications networks. However, two failure mechanisms were observed on the APD's from one manufacturer, namely increased surface leakage currents and localized premature breakdown, which would have caused either increased error rates or complete system failure. The results indicate that procurement specifications are required to guard against the use of unreliable APD's in transmission systems, and procedures are recommended.     

Nonlinear amplifier effects in communications systems

This paper introduces a figure-of-merit to investigate tradeoffs between amplifiers and modulation waveforms in complex digital communications systems. Class-AB amplifiers are investigated with a variety of modulation schemes to better understand the relations between amplifier efficiency, amplifier distortion, signal in-band and adjacent channel interference, and power consumption. The goal is to better understand the tradeoffs needed to design low-energy communications systems     

High-order accurate split-step FDTD method for solution of maxwell?s equations

The split-step finite difference time domain (SS-FDTD) method characterised by unconditional stability is becoming an important numerical method in computational electromagnetics. Proposed is a new high-order accurate unconditionally stable SS-FDTD method, which is derived from the exponential evolution operator. Compared with the conventional SS-FDTD method, the numerical dispersion of the new method is greatly reduced     

Optimization of the spectral efficiency in MIMO cellular wireless communications

Expressions are derived that make it possible to calculate and optimize the spectral efficiency (SE) of multiple-input-multiple-output (MIMO) cellular wireless communication systems (WCSs) with allowance for the interaction of radiators in the receiving and transmitting antennas of the MIMO systems, the properties of the deterministic radio channel, and the presence of directionally concentrated interference. The effect of the interaction of radiators on the SE of the MIMO systems is studied, and the efficiency of the MIMO technology is analyzed on the basis of the model example of two receiving and two transmitting antennas with a conducting screen placed in between them. It is demonstrated that, in comparison to single-channel-transmission technology, MIMO technology allows an increase the WCS SE only at relatively high values of the signal-to-interference-plus-noise ratio at the receiver input and in the case of nearly equal channel gains. It is shown that the relative positions of the transmitter and receiver at which the MIMO-WCS SE can be significantly increased are dependent on the channel energy and are substantially limited at real powers transmitted under urban conditions.