Srikanth Raghavan

Intrachannel nonlinear penalties in dispersion-managed transmission systems

We have developed a model for intra-channel impairments in dispersion-managed systems and validated it using numerical simulations. We have shown that intrachannel impairments can be reduced using optimal precompensation. We have studied the intrachannel penalty for different modulation formats, and results show that the optimum precompensation is independent of modulation format. The intrachannel penalty decreases as the spectrum of the modulation format broadens.

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.

Monte Carlo method for computing density of states and quench probability of potential energy and enthalpy landscapes

The thermodynamics and kinetics of a many-body system can be described in terms of a potential energy landscape in multidimensional configuration space. The partition function of such a landscape can be written in terms of a density of states, which can be computed using a variety of Monte Carlo techniques. In this paper, a new self-consistent Monte Carlo method for computing density of states is described that uses importance sampling and a multiplicative update factor to achieve rapid convergence. The technique is then applied to compute the equilibrium quench probability of the various inherent structures (minima) in the landscape. The quench probability depends on both the potential energy of the inherent structure and the volume of its corresponding basin in configuration space. Finally, the methodology is extended to the isothermal-isobaric ensemble in order to compute inherent structure quench probabilities in an enthalpy landscape.

Fiber design considerations for 40 Gb/s systems

In this paper, we review the fundamental advantages and drawbacks of 40-Gb/s systems from a fiber manufacturers perspective. Based on modeling, experimental results, and fundamental understanding, we correlate the fiber design parameters with the expected performance of long-haul systems operating at 40 Gb/s. Nonlinear penalties, dispersion tolerances, modulation formats, polarization-mode dispersion, and Raman amplification are covered. We also present the fiber features required for both metro and submarine networks at this specific data rate.

Experimental Measurements of the Effectiveness of MLSE against Narrowband Optical Filtering Distortion

We experimentally investigate the application of MLSE-EDC to signals transmitted through narrowband optical filters. We find MLSE affords significant OSNR improvement of ~5 dB for NRZ signals, but less or none for duobinary and DPSK.

Impact of transmitter and receiver imperfections on the performance of coherent optical QPSK communication systems

We investigate transmitter and receiver imperfections in coherent QPSK systems, and how their impact can be mitigated using DSP algorithms. Quadrature imbalance was found to be a particularly significant effect that will require compensation.

Analysis of excess scattering in optical fibers

We have systematically analyzed the excess scattering in a relatively large index optical fiber operating in the single mode regime. A mathematical model based on the theory of scattering of plane waves from a randomly perturbed core-clad interface is presented that predicts excess scattering confined within a small angle in the forward direction. An experimental system is developed that can measure the angular distribution of the scattering over ∼0°–180°. Excellent agreement between computed and measured scattering distribution is observed over multiple wavelengths and wide angular range. The spectral and angular distribution of the excess scattering and its response to the perturbation parameters are analyzed.

Nonlinear Electronic Dispersion Compensation Techniques for Fiber-Optic Communication Systems

We show that blind adaptive MLSE and fractionally-spaced DFE receivers based on nonlinear Volterra theory can extend the reach beyond 300 km in uncompensated 10 Gbit/s fiber-optic transmission systems dominated by fiber nonlinearity.

Advanced modulation formats for fiber optic communication systems

Choice of modulation format plays a critical role in the design and performance of fiber optic communication systems. We discuss the basic physics of electro-optic phase and amplitude modulation and derive model transfer functions for ideal and non-ideal Mach-Zehnder modulators. We describe the generation and characteristics of the standard nonreturn-to-zero (NRZ) modulation format, as well as advanced formats such as return-to-zero (RZ), carrier-suppressed RZ (CSRZ), duobinary, modified duobinary, differential phase-shift keyed (DPSK), and return-to-zero DPSK (RZ-DPSK). Finally, we discuss the relative merits of these formats with respect to a variety of system impairments.

Compensation of coherent DQPSK receiver imperfections

This paper studies the impact of imperfections and impairments on the performance of a coherent phase-diversity DQPSK receiver. Experiments demonstrate that quadrature imbalance can be corrected by ellipse fitting and Gram-Schmidt-like orthogonalization. In addition, it is shown that intermediate frequency offset can be estimated using low complexity, feed-forward, least-square-error based algorithms.