Avner Peleg

Optimized multiemitter beams for free-space optical communications through turbulent atmosphere

Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.

Scintillation index for two Gaussian laser beams with different wavelengths in weak atmospheric turbulence

We study the propagation of the two lowest-order Gaussian laser beams with different wavelengths in weak atmospheric turbulence. Using the Rytov approximation and assuming a slow detector, we calculate the longitudinal and radial components of the scintillation index for a typical free-space laser communication setup. We find the optimal configuration of the two laser beams with respect to the longitudinal scintillation index. We show that the value of the longitudinal scintillation for the optimal two-beam configuration is smaller by more than 50% compared with the value for a single lowest-order Gaussian beam with the same total power. Furthermore, the radial scintillation for the optimal two-beam system is smaller by 35%-40% compared with the radial scintillation in the single-beam case. Further insight into the reduction of intensity fluctuations is gained by analyzing the self- and cross-intensity contributions to the scintillation index.

Phase ordering with a global conservation law: Ostwald ripening and coalescence

Globally conserved phase ordering dynamics is investigated in systems with short range correlations at t=0. A Ginzburg-Landau equation with a global conservation law is employed as the phase field model. The conditions are found under which the sharp-interface limit of this equation is reducible to the area-preserving motion by curvature. Numerical simulations show that, for both critical and off-critical quench, the equal-time pair correlation function exhibits dynamic scaling, and the characteristic coarsening length obeys l(t) approximately t(1/2). For the critical quench, our results are in excellent agreement with earlier results. For off-critical quench (Ostwald ripening) we investigate the dynamics of the size distribution function of the minority phase domains. The simulations show that, at large times, this distribution function has a self-similar form with growth exponent 1/2. The scaled distribution, however, strongly differs from the classical Wagner distribution. We attribute this difference to coalescence of domains. A theory of Ostwald ripening is developed that takes into account binary coalescence events. The theoretical scaled distribution function agrees well with that obtained in the simulations.

Scintillation Reduction by Use of Multiple Gaussian Laser Beams With Different Wavelengths

We study the scintillation index of N partially overlapping collimated lowest order Gaussian laser beams with different wavelengths in weak atmospheric turbulence. Using the Rytov approximation, we calculate the initial beam separation that minimizes the longitudinal scintillation. Further reduction of the longitudinal scintillation is obtained by optimizing with respect to both beam separation and spot size. The longitudinal scintillation of the optimal N-beam configurations is inversely proportional to N, resulting in a 92% reduction for a nine-beam system compared with the single beam value. The radial scintillation values for the optimal N-beam configurations are significantly smaller than the corresponding single beam values

Strongly non-Gaussian statistics of optical soliton parameters due to collisions in the presence of delayed Raman response

We study the effects of a delayed Raman response on soliton collisions in optical fibre transmission systems with multiple frequency channels. We show that the propagation of a given soliton undergoing many collisions with solitons from other frequency channels is described by a perturbed stochastic nonlinear Schrodinger equation, in which the stochastic perturbative terms are due to collision induced amplitude and frequency changes. Using the adiabatic perturbation theory we find that the distribution function of the soliton amplitude is lognormal, i.e. strongly non-Gaussian. The frequency of the soliton is also found to be a random variable that is not self-averaging. The results of our extensive numerical simulations incorporating the technique of importance sampling are in very good agreement with the theoretical predictions.

Log-normal distribution of pulse amplitudes due to Raman cross talk in wavelength division multiplexing soliton transmission.

The effect of delayed Raman response on soliton collisions in wavelength division multiplexing (WDM) transmission systems is investigated. Taking into account the stochastic nature of pulse sequences in different frequency channels and the Raman-induced cross talk, it is shown that the soliton amplitude is a random variable with a log-normal distribution. Moreover, the Raman-induced self-frequency shift and cross-frequency shift are also random variables with log-normal-like distributions. These results imply that fluctuations in soliton amplitude and frequency induced by soliton collisions in the presence of delayed Raman response play an important role in massive WDM transmission.

Deterministic Raman crosstalk effects in amplified wavelength division multiplexing transmission

Abstract We study the deterministic effects of inter-pulse Raman-induced crosstalk in amplified wavelength division multiplexing (WDM) optical fiber transmission lines. We show that the dynamics of pulse amplitudes in an N -channel transmission system is described by an N -dimensional predator–prey model. We find the equilibrium states with non-zero amplitudes and prove their stability by obtaining the Lyapunov function. The stability is independent of the exact details of the approximation for the Raman gain curve. Furthermore, we investigate the impact of cross phase modulation and Raman self and cross frequency shifts on the dynamics and establish the stability of the equilibrium state with respect to these perturbations. Our results provide a quantitative explanation for the robustness of differential-phase-shift-keyed WDM transmission against Raman crosstalk effects.

Intermittent dynamics, strong correlations, and bit-error-rate in multichannel optical fiber communication systems

We investigate the effects of delayed Raman response on pulse dynamics in massive multichannel optical fiber communication systems. Taking into account the stochastic nature of pulse sequences in different frequency channels and the Raman induced energy exchange in pulse collisions we show that the pulse parameters exhibit intermittent dynamic behavior, and that the pulse amplitudes exhibit relatively strong and long-range correlations. Moreover, we find that the Raman-induced cross frequency shift is the main intermittency-related mechanism leading to bit pattern deterioration and evaluate the bit-error-rate of the system.

Inelastic interchannel collisions of pulses in optical fibers in the presence of third-order dispersion

We study the effect of third-order dispersion on the interaction between two solitons from different frequency channels in an optical fiber. The interaction may be viewed as an inelastic collision in which energy is lost to continuous radiation owing to nonzero third-order dispersion. We develop a perturbation theory with two small parameters: the third-order dispersion coefficient d3 and the reciprocal of the interchannel frequency difference 1/Ω. In the leading order the amplitude of the emitted radiation is proportional to d3/Ω2, and the source term for this radiation is identical to the one produced by perturbation of the second-order dispersion coefficient. The only other effects up to the third order are shifts in the soliton’s phase and position. Our results show that the statistical description of soliton propagation in a given channel influenced by interaction with a quasi-random sequence of solitons from other channels is similar to the description of soliton propagation in fibers with weak disorder in the second-order dispersion coefficient.

Dielectronic recombination of Ni-, Cu-, and Ar-like tungsten and barium through the low inner-shell excited configurations including collision processes

Level-by-level relativistic calculations of dielectronic recombination (DR) cross sections and rate coefficients for Ni-, Cu-, and Ar- like tungsten in the ground state were performed. Similar calculations were carried out for Ni-like barium for comparison. The most important low-lying inner-shell excited configuration complexes are taken into account, namely (3p3d)154141′ for Ni-like W and Ba, (3p3d)154s4141′ for Cu-like W, and finally 3p53d91, 3s3p63d71, and 3p54141′ for Ar-like W. These complexes give the dominant contributions to the total DR rate coefficients for kTe < 0.5 keV, and are still expected to give major contributions at higher electron temperature. Configuration mixing is taken into account when significant. The energy levels of the tungsten inner-shell excited DR channels are found to have typical j-j coupling characteristics, whereas those of Ni-like barium do not. The role of non-resonant radiative stabilizations is found to be generally minor for the Ni- and Cu-like ions. In contrast, for Ar-like tungsten these stabilizations are found to be very important. The DR rate coefficients for Cu-like tungsten are found to be very close in magnitude to those of the Ni-like ion. Some partial contributions to the DR rate coefficients are found to rise dramatically at very low electron temperature, a fact that has been observed experimentally for lighter ions. It is found that high-1 3p53d91 configurations, that were previously neglected, may have a significant contribution to the DR of Ar-like tungsten. The effect of electron collisions with the inner-shell excited ions is shown to influence the DR rate coefficient only at very high electron densities, greater than 1020 cm−3 for Ni-like W and greater than 1022 cm−3 for Ar-like W. In the Ni-like case at an electron density of 1023 cm−3 these collisions enhance the DR rate by about 30% at 5 keV, and by as much as a factor of 2 at 200 eV.