Ioannis Roudas

10-Gb/s transmission of 1.55-μm directly modulated signal over 100 km of negative dispersion fiber

In this letter, the largest transmission distance (100 km) ever reported for a commercially available 10-Gb/s 1.55-/spl mu/m directly modulated signal over a single fiber link without using any dispersion compensation is demonstrated. The achieved dispersion-length product for a Q-factor greater than 9.4 dB (bit-error rate less than 10/sup -15/) was about 750 ps/nm. The fiber that enabled such long transmission distance with high dispersion tolerance is a nonzero dispersion-shifted fiber that has negative dispersion in the entire usable bandwidth (1280-1620 nm) and is optimized for operation with directly modulated lasers. The excellent single-channel transmission performance that we achieved can be expected also from wavelength-division-multiplexed systems with channels across the erbium-doped fiber amplifier bands.

Demonstration of negative dispersion fibers for DWDM metropolitan area 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.

Performance engineering and topological design of metro WDM optical networks using computer simulation

This paper demonstrates the use of computer simulation for topological design and performance engineering of transparent wavelength-division multiplexing metropolitan-area networks. Engineering of these networks involves the study of various transport-layer impairments such as amplifier noise, component ripple, chirp/dispersion, optical crosstalk, waveform distortion due to filter concatenation, fiber nonlinearities, and polarization effects. A computer simulation methodology composed of three main simulation steps is derived and implemented. This methodology obtains performance estimations by applying efficient wavelength-domain simulations on the entire network topology, followed by time-/frequency-domain simulations on selected paths of the network and finally Q-budgeting on an identified worst case path. The above technique provides an efficient tool for topological design and network performance engineering. Accurate simulation models are presented for each of the performance impairments, and the computer simulation methodology is used for the design and engineering of a number of actual metro network architectures.

Extraction of laser rate equations parameters for representative simulations of metropolitan-area transmission systems and networks

Abstract In this paper we present procedures for the extraction of laser rate equation parameters. The parameters are extracted using fitting of static and dynamic measurements with simple theoretical expressions. In particular, chirp related parameters are extracted directly through measured chirp and power waveforms. The extracted parameters are used in a large-signal rate equation laser model to calculate the power and chirp waveforms. The simulations are compared with experiments and the agreement is excellent. The procedures have been applied to directly modulated lasers having different chirp characteristics and designed for either 2.5 or 10 Gb/s operation. Using the rate equation laser model with the extracted parameters, we performed a simulation study to identify the value of employing a negative dispersion fiber in metro-area networks. It is shown that dispersion shifted fibers with negative dispersion across the entire usable fiber bandwidth (1280–1620 nm) advances the performance of directly modulated lasers used in wavelength division multiplexed metropolitan-area transmission systems and networks. At 2.5 Gb/s transmission over 300 km of negative dispersion fibers is possible for directly modulated lasers across the erbium doped fiber amplifier bandwidth, while at 10 Gb/s an impressive distance of 100 km can be achieved.

Optimal Polarization Demultiplexing for Coherent Optical Communications Systems

Spectrally-efficient optical communications systems employ polarization division multiplexing (PDM) as a practical solution, in order to double the capacity of a fiber link. Polarization demultiplexing can be performed electronically, using polarization-diversity coherent optical receivers. The primary goal of this paper is the optimal design, using the maximum-likelihood criterion, of polarization-diversity coherent optical receivers for polarization-multiplexed optical signals, in the absence of polarization mode dispersion (PMD). It is shown that simultaneous joint estimation of the symbols, over the two received states of polarization, yields optimal performance, in the absence of phase noise and intermediate frequency offset. In contrast, the commonly used zero-forcing polarization demultiplexer, followed by individual demodulation of the polarization-multiplexed tributaries, exhibits inferior performance, and becomes optimal only if the channel transfer matrix is unitary, e.g., in the absence of polarization dependent loss (PDL), and if the noise components at the polarization diversity branches have equal variances. In this special case, the zero-forcing polarization demultiplexer can be implemented by a 2 ? 2 lattice adaptive filter, which is controlled by only two independent real parameters. These parameters can be computed recursively using the constant modulus algorithm (CMA). We evaluate, by simulation, the performance of the aforementioned zero-forcing polarization demultiplexer in coherent optical communication systems using PDM quadrature phase shift keying (QPSK) signals. We show that it is, by far, superior, in terms of convergence accuracy and speed, compared to conventional CMA-based polarization demultiplexers. Finally, we experimentally test the robustness of the proposed constrained CMA polarization demultiplexer to realistic imperfections of polarization-diversity coherent optical receivers. The PMD and PDL tolerance of the proposed demultiplexer can be used as a benchmark in order to compare the performance of more sophisticated adaptive electronic PMD/PDL equalizers.

Cascadability of passband-flattened arrayed waveguide-grating filters in WDM optical networks

The cascadability of passband-flattened arrayed waveguide-grating (AWG) filters is studied using experimental and theoretical transfer functions. The formalism is general and can be used to cascade any type of filter at any channel spacing. For example, modeling indicates that transmission through 100 such AWG (de)multiplexers at 200-GHz channel spacing, assuming 10-Gb/s data streams introduces distortion-induced penalties below the widely acceptable 0.3-dB limit, provided that certain filter design requirements are satisfied. All simulations focus on the filter cascadability and central frequency misalignment effects, and neglect nonlinearities and crosstalk.

Accurate modeling of optical multiplexer/demultiplexer concatenation in transparent multiwavelength optical networks

This paper presents an accurate theoretical model for the study of concatenation of optical multiplexers/demultiplexers (MUXs/DMUXs) in transparent multiwavelength optical networks. The model is based on a semianalytical technique for the evaluation of the error probability of the network topology. The error-probability evaluation takes into account arbitrary pulse shapes, arbitrary optical MUX/DMUX, and electronic low-pass filter transfer functions, and non-Gaussian photocurrent statistics at the output of the direct-detection receiver. To illustrate the model, the cascadability of arrayed waveguide grating (AWG) routers in a transparent network element chain is studied. The performance of the actual network is compared to the performance of a reference network with ideal optical MUXs/DMUXs. The optical power penalty at an error probability of 10/sup -9/ is calculated as a function of the number of cascaded AWG routers, the bandwidth of AWG routers, and the laser carrier frequency offset from the channels nominal frequency.

Simulation of ASE noise accumulation in a wavelength add-drop multiplexer cascade

This letter presents a computer simulation of a cascade of wavelength add-drop multiplexers (WADMs) each consisting of optical amplifiers, a multiplexer/demultiplexer pair, gain equalizing attenuators and 2/spl times/2 optical switches. It is shown that one of the eight wavelengths can propagate through more than 50 equidistant WADMs in a wavelength-division multiplexed (WDM) optical network before its optical signal-to-noise ratio (SNR) drops below acceptable levels. These simulations indicate that a national scale transparent WDM network is feasible.

Nonsingular Constant Modulus Equalizer for PDM-QPSK Coherent Optical Receivers

Adaptive electronic equalizers using the constant modulus algorithm (CMA) algorithm often converge to a singular coefficient matrix that produces the same signal at multiple outputs. We address this issue in the context of optical communications systems with polarization-division multiplexing and coherent receivers. We study, by computer simulation, the performance of multiuser CMA equalizer, an enhanced CMA equalizer initially proposed for use in wireless multiuser and later multiple-input/multiple-output communications systems. We show that the proposed adaptive electronic equalizer does not exhibit singularities and, therefore, is superior to the commonly used CMA equalizer.

Coherent frequency-selective polarimeter for polarization-mode dispersion monitoring

Frequency-selective polarimeters measure the state of polarization of the individual spectral components of a modulated optical signal. They can be used either as stand-alone measuring devices or as parts of adaptive polarization-mode dispersion (PMD) compensators. This paper presents a novel frequency-selective polarimeter based on coherent detection, which has superior accuracy compared to previously proposed direct detection-based counterparts. This is due to the high-frequency resolution and power sensitivity of coherent detection, features that minimize the systematic and random error, respectively, in the measurement of the state of polarization of the individual spectral components of the received optical signal. The accuracy of the measurement is independent of the received signal bit rate and modulation format. The proposed frequency-selective polarimeter is studied both theoretically and experimentally. The primary theoretical contribution of this paper is a unified formalism, which allows the modeling of both direct and coherent detection-based frequency-selective polarimeters. Analytical expressions for the output signal of both types of frequency-selective polarimeters are derived. Based on these expressions, a common algorithm is proposed for the evaluation of the Stokes parameters. In addition, an example error signal is used as a metric in order to test the agreement of the theoretical model with the experimental measurements. The successful operation of the coherent frequency-selective polarimeter is demonstrated experimentally for a 10-Gb/s intensity-modulated nonreturn-to-zero (NRZ) optical signal in the presence of first-order polarization-mode dispersion. There is an excellent agreement between theory and experiment.