A communications network has a plurality of nodes interconnected by an optical transmission medium. The transmission medium is capable of a carrying a plurality of wavelengths organized into bands. A filter at each node for drops a band associated therewith and passively forwards other bands through the transmission medium. A device is provided at each node for adding a band to the transmission medium. Communication can be established directly between a pair of nodes in the network sharing a common band without the active intervention of any intervening node. This allows the network to be protocol independent. Also, the low losses incurred by the passive filters permit relatively long path lengths without optical amplification.

WDM optical network with passive pass-through at each node

Reconfigurable optical add/drop multiplexers (ROADMs) based on 1 X N wavelength-selective switches (WSS) are evolving to support DWDM networks with higher capacity and increased flexibility in wavelength routing. Different WSS technologies can be employed to provide colorless and steerable functionality for ring, or meshed architectures. Improvements in specifications of WSS modules operating on the 50 GHz wavelength grid have enabled 40 Gb/s transmission rates through extensive ROADM networks. The same ROADMs are also expected to support 100 Gb/s transmission in the near future. In parallel, development of lower-cost WSS technologies is allowing ROADMs to expand into edge networks. In all these network applications, propagation through multiple ROADMs generates transmission penalties for the DWDM channels, which need to be factored into the network design. Such OSNR or Q factor penalties can be induced by passband narrowing, imperfect isolation across the signal bandwidth, insertion loss, PDL, and other effects. The impact of these impairments depend on the transmitter and receiver types (e.g., data rate and modulation format), and on the WSS characteristics (e.g., insertion loss, passband width, shape, isolation magnitude and isolation stopband). Key transmission impairments such as bandpass narrowing, crosstalk, insertion loss, and PDL are estimated based on experiments and numerical simulations for common data rates and modulation formats. Implications of temporal fluctuations during power setting throughout a ROADM network are also discussed.

Transmission Impairments in DWDM Networks With Reconfigurable Optical Add-Drop Multiplexers

A theoretical model is presented for analyzing the propagation of densely spaced WDM optical signals through a cascade of erbium-doped fiber amplifiers and single-mode optical fibers with nonuniform chromatic dispersion. By combining a numerical solution for the EDFA and an analytical expression for FWM components generated through the cascade, the model allows a realistic system analysis which includes gain peaking effect, amplified spontaneous emission accumulation and the effect of dispersion management on the four-wave mixing efficiency. The FWM power distribution at the end of the multi-amplifier transmission link is computed taking into account the phase relation between FWM light amplitudes generated within different sections of the link. The transmission of many WDM channels, evenly spaced around 1547.5 nm, has been analyzed for various dispersion management techniques and propagation distances. Numerical results point out the importance of such a model for a realistic design of WDM optical communication systems and networks. A proper choice of chromatic dispersion, amplifier characteristics, span length, input signal powers and wavelengths, combined with the use of gain equalizing filters, allows to maximize the transmission distance ensuring acceptable signal-to-noise ratio (SNR) and limited SNR variation among channels.

Modeling of four-wave mixing and gain peaking in amplified WDM optical communication systems and networks

A Gaussian approach (GA), which takes into account the influence of arbitrary optical and electrical filters, is proposed to analytically evaluate the performance of optically preamplified receivers. This GA also takes into account the effects of waveform distortion in the signal and in the signal-amplified spontaneous emission beat noise. Exact expressions for the mean and variance of the current at the decision circuit input are derived, which explicitly present the dependence on the optical and electrical filter transfer functions and take into account the presence or absence of a polariser. This GA is computationally much faster and simpler than more rigorous methods and the accuracy of its sensitivity estimates is investigated for different optical and electrical filters. It is shown that the GA predicts the optimal optical filter bandwidth very reasonably. Discrepancies below 1.2 dB are found between its sensitivity estimates and rigorous estimates. The assessment of the optical filter detuning impact on the receiver sensitivity is also investigated with this GA. Discrepancies less than 1 dB are found between the GA estimates and rigorous estimates. The GA sensitivity estimate's accuracy is also investigated in the presence of intersymbol interference (ISI) and very accurate predictions are obtained by the GA for significant ISI.

https://ieeexplore.ieee.org/iel5/2197/20364/00940560.pdf

Gaussian approach for performance evaluation of optically preamplified receivers with arbitrary optical and electrical filters

The relationship between Q penalty (QP) and eye-closure penalty (ECP) is examined for distorted signals in the presence of signal-dependent noise. A simple model is developed to describe the behavior of return-to-zero (RZ) modulation formats and is compared with a model for nonreturn-to-zero (NRZ) formats. The accuracy of the analysis is investigated with extensive simulations, and the numerical results from analysis and simulation are found to be in generally good agreement. Experimental measurements of distortion caused by uncompensated dispersion also show agreement with the simulation results and model predictions. The simplified models allow a means to budget QPs from distortion effects in a straightforward manner during network design for different modulation formats. The analysis predicts a smaller Q penalty as a function of ECP for RZ modulation formats in comparison with NRZ and smaller relative penalties for RZ formats with narrower pulsewidths.

Relationship of Q penalty to eye-closure penalty for NRZ and RZ signals with signal-dependent noise

In this letter, we use computer simulation techniques to evaluate the performance degradation for a multiwavelength optical network with a cascade of 30 randomly misaligned (de)multiplexers. Two types of (de)multiplexers are used in the simulation and modeled as third-order and first-order Butterworth filters, both with a full width half maximum (FWHM) of 125 GHz. For WDM systems at 10 Gb/s per optical channel with 30 uniformly misaligned filters, laser misalignment tolerances (at 0.3 dB distortion-induced-eye-closure penalty) vary from /spl plusmn/30 GHz (/spl plusmn/0.24 nm) for systems using third-order Butterworth filters to /spl plusmn/20 GHz (/spl plusmn/16 mm) for systems using first-order filters. >

Performance degradation of multiwavelength optical networks due to laser and (de)multiplexer misalignments

We use computer simulation techniques to evaluate the limitations on laser misalignment tolerances for multiwavelength optical networks with a cascade of 2 to 100 (de)multiplexers modelled as either first-order filters or third-order Butterworth filters. The results are dependent on the number of (de)multiplexers used, on the bit rate, and on the filter characteristics. We find that both the magnitude and the phase characteristics of the (de)multiplexer transfer function are important in determining the distortion-induced penalties. The allowable laser misalignment tolerances, at 10 Gb/s and for systems using (de)multiplexers modelled as third-order Butterworth filters, vary from /spl plusmn/78 GHz (/spl plusmn/0.63 nm) for systems with a cascade of 2 filters to /spl plusmn/18 GHz (/spl plusmn/0.15 nm) for systems with a cascade of 100 filters.

Performance of cascaded misaligned optical (de)multiplexers in multiwavelength optical networks

This paper introduces a simple optimization method for determining the transmittance characteristics of an MZ filter gain equalizer required for optical networks using 980-nm or 1480-nm pumped EDFA cascades. This approach is shown capable of supporting 10 WDM signals spaced 2 nm apart over a 20-nm optical bandwidth (1540-1560 nm) up to a cascade of 10 amplifiers, allowing a total system gain of more than 150 dB with inter-channel power differences of less than 5 dB.

Multiwavelength fiber-amplifier cascades with equalization employing Mach-Zehnder optical filter

A simple method to optimize individual EDFA performance parameters for multiwavelength operation in an amplifier cascade is presented. The pump power, pump wavelength, erbium-doped fiber length, and input signal levels and their spectral range are each optimized. We find that 980-nm pumping, together with an optimum amplifier length of 9 m, provides adequate performance for six 10 Gb/s WDM channels to be transmitted through a cascade of 27 amplifiers, allowing a total system gain of more than 350 dB without requiring gain equalization methods. >

Performance of erbium-doped fiber amplifier cascades in WDM multiple access lightwave networks

A simple method for optimization of individual amplifier performance parameters for multiple wavelength operation is presented. The method determines the erbium fiber length required to achieve a maximum gain subject to the constraint of minimum spectral gain variations. This simplifies the practical implementation of gain equalizers required to achieve flat gain for multiple wavelength operation. >

A.F. Elrefaie

Papers

Performance degradation of multiwavelength optical networks due to laser and (de)multiplexer misalignments

Performance of cascaded misaligned optical (de)multiplexers in multiwavelength optical networks

Multiwavelength fiber-amplifier cascades with equalization employing Mach-Zehnder optical filter

Performance of erbium-doped fiber amplifier cascades in WDM multiple access lightwave networks

Optimized performance of erbium-doped fiber amplifiers in multiwavelength lightwave systems