CMMN evaluation: the modelers’ perceptions of the main notation elements

Software and Systems Modeling - Case Management Model and Notation (CMMN) has been introduced as a graphical modeling language targeting the modeling of human-centric processes. Despite its growing...     

Multicanonical Monte Carlo Modeling of Wavelength Division Multiplexed Differential Phase Shift Keying Systems

Differential phase shift keying (DPSK) modulation is being considered as a possible candidate for future optical wavelength division multiplexed (WDM) transmission systems. In a single channel link, the balanced interferometric DPSK receiver exhibits increased tolerance against amplified spontaneous emitting (ASE) noise and fiber nonlinear effects. In this paper, a model is presented that can be used to estimate the performance of a multichannel DPSK system taking into account the influence of interchannel phenomena, namely cross-phase modulation (XPM) and four wave mixing (FWM), in the phase noise statistics. The model is based on an approximate solution of the fiber propagation equation and the multicanonical Monte Carlo (MCMC) method. It provides an efficient tool that can be used to investigate the influence of many link design parameters such as channel spacing, launch power, and fiber dispersion. The model is illustrated in the comparison of the performance of multichannel DPSK to on-off keying (OOK) systems. It is verified that, even in the presence of interchannel effects, DPSK modulation greatly enhances the system performance compared to OOK.     

Performance analysis of space time block coding techniques for indoor optical wireless systems

Indoor optical wireless systems provide an attractive alternative for realizing next generation Wireless Local Area Networks (WLANs). In this paper, the performance of diffuse optical wireless systems, employing Space Time Block Coding (STBC) techniques, is numerically investigated, accurately taking into account, the indoor channel impulse response and the characteristics of ambient light and thermal noises at the receiver. Discrete Multitone modulation (DMT) is used to mitigate the effect of intersymbol interference due to the channel?s impulse response. The performance of STBC systems, employing two transmit elements, is compared against Single Input/ Single Output (SISO) and Maximum Ratio Combining (MRC) systems operating with the same total optical transmitter power. It is shown that STBC techniques can be used to increase the capacity of diffuse optical wireless systems, improve their coverage and decrease the required optical power at the transmitter. These results demonstrate the usefulness of Multiple Input Multiple Output (MIMO) techniques in the realization of optical WLANs.     

Impact of Nonlinear LED Transfer Function on Discrete Multitone Modulation: Analytical Approach

Light-emitting diodes constitute a low-cost choice for optical transmitters in medium-bit-rate optical links. An example for the latter is local-area networks. However, one of the disadvantageous properties of light-emitting diodes is their nonlinear characteristic, which may limit the data transmission performance of the system, especially in the case of multiple subcarrier modulation, which is starting to attract attention in various applications, such as visible-light communications and data transmission over polymer optical fibers. In this paper, the influence of the nonlinear transfer function of the light-emitting diodes on discrete multitone modulation is studied. The transfer function describes the dependence of the emitted optical power on the driving current. Analytical expressions for an idealized link were derived, and these equations allow the estimation of the power of the noise-like, nonlinear crosstalk between the orthogonal subcarriers. The crosstalk components of the quadrature and in-phase subcarrier components were found to be independent and approximately normally distributed. Using these results, the influence of light-emitting-diode nonlinearity on the performance of the system was investigated. The main finding was that systems using a small number of subcarriers and/or high QAM level exhibit a large signal-to-noise-ratio penalty due to the nonlinear crosstalk. The model was applied to systems with white and resonant-cavity light-emitting diodes. It is shown that the nonlinearity may severely limit the performance of the system, particularly in the case of resonant-cavity light-emitting diodes, which exhibit a strong nonlinear behavior.     

Frequency dependence of the coupling coefficients and resonant frequency detuning in a nanophotonic waveguide-cavity system

Waveguide-cavity interactions may find important applications in future nanophotonic devices. This paper provides a detailed derivation of the evolution equation of the amplitude of a cavity mode coupled to a waveguide, starting from Maxwell's equations and using the reciprocity relations. The analysis applies to both constant cross-section and periodic waveguides as well. Unlike previous studies, the analysis enables the estimation of the frequency dependence of the coupling coefficients. It is also confirmed that the waveguide-cavity coupling causes a detuning of the resonant frequency of the cavity mode. The detuning is estimated in the case of a photonic crystal waveguide-cavity system and it is shown that it can be significant especially if the structure is intended for filtering applications. The analysis is generalized to the case of a multimode or multiple cavities and provides a useful tool in the analysis of devices based on coupled cavities.     

Accurate estimation of the error probability in the presence of in-band crosstalk noise in WDM networks

In-band crosstalk noise can pose important limitations in wavelength-division-multiplexing (WDM) optical networks. This paper describes a model that takes into account the statistical behavior of in-band crosstalk noise accurately and that can be used to estimate the error probability (EP) in a WDM receiver. It is based on the formulation of the moment-generating function of the decision variable in terms of a double integral, which allows the inclusion of the crosstalk-crosstalk noise, which was previously neglected in the literature. The optical amplifier noise and the electrical receiver noise are also incorporated, and the model is used to assess the implications of the crosstalk-crosstalk contribution. With the aid of some examples, it is shown that the crosstalk-crosstalk noise can influence the value of the EP, change the optimum receiver threshold, and introduce some power penalty.     

New techniques for the suppression of the four-wave mixing-induced distortion in nonzero dispersion fiber WDM systems

The performance of a wavelength-division multiplexing (WDM) optical network can be severely degraded due to fiber nonlinear effects. In the case where nonzero dispersion (NZD) fibers are employed, the four-wave mixing (FWM) effect sets an upper limit on the input power, especially in the case of narrow channel spacing. In order to reduce FWM-induced distortion two new techniques, the hybrid amplitude-/frequency-shift keying (ASK/FSK) modulation and the use of prechirped pulses are investigated. It is shown that both techniques can greatly improve the Q-factor in a 10 Gb/s WDM system. This happens even for very high input powers (/spl sim/10 dBm), where the degradation of the conventional WDM system is prohibitively high. The proposed methods are also applied and tested in higher bit rates (40 Gb/s). It is deduced that although the hybrid ASK/FSK modulation technique marginally improves the system performance, the optical prechirp technique can still be used to greatly increase the maximum allowable input power of the system.     

Evaluation by Monte Carlo simulations of the power limits and bit-error rate degradation in wavelength-division multiplexing networks caused by four-wave mixing

Fiber nonlinearities can degrade the performance of a wavelength-division multiplexing optical network. For high input power, a low chromatic dispersion coefficient, or low channel spacing, the most severe penalties are due to four-wave mixing (FWM). To compute the bit-error rate that is due to FWM noise, one must evaluate accurately the probability-density functions (pdf) of both the space and the mark states. An accurate evaluation of the pdf of the FWM noise in the space state is given, for the first time to the authors' knowledge, by use of Monte Carlo simulations. Additionally, it is shown that the pdf in the mark state is not symmetric as had been assumed in previous studies. Diagrams are presented that permit estimation of the pdf, given the number of channels in the system. The accuracy of the previous models is also investigated, and finally the results of this study are used to estimate the power limits of a wavelength-division multiplexing system.     

A New Formulation of Coupled Propagation Equations in Periodic Nanophotonic Waveguides for the Treatment of Kerr-Induced Nonlinearities

Nonlinear phenomena could be used to implement important signal processing functionalities in future nanophotonic integrated optical devices. In this paper, a semi-analytical model incorporating the influence of Kerr-induced nonlinearity in the propagation of an optical signal inside a periodic nanophotonic waveguide is derived. The approach consists of a system of nonlinear coupled mode propagation equations and is applicable to both single and multimode waveguides. The influence of the mode group velocity on the value of the self-phase modulation coefficient gamma is analyzed and the impact of higher order nonlinear terms is also investigated both at the middle and edge of the guided band. The model is also applied to estimate the nonlinear coupling coefficients of a photonic crystal waveguide coupler and provides an efficient method to analyze the influence of nonlinear phenomena in periodic nanophotonic waveguide devices.     

An efficient technique for the design of an arrayed-waveguidegrating with flat spectral response

The spectral response of the arrayed-waveguide grating (AWG) plays an important role in optical networks. Ideally, the grating should have a rectangular transfer function to reduce the need for accurate wavelength control and achieve low crosstalk. A new technique for designing an AWG with flat spectral response is presented. The problem of the optimization of the transfer function is reduced to that of adjusting the arrayed waveguide lengths and their relative positions on the edge of the free propagating regions in order to minimize a certain error function. As a result, the waveguide lengths and their positions are determined using a rigorous mathematical procedure. The resultant transfer function is flat with low sidelobes