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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Ripple growth of laser beam propagating in the multilevel photoionization system

Yasushi TAKIZAWA, R Murakami, Koji Nagasaka, Keiji Nomaru, Yasukazu Izawa

Published Jan 11, 2000• 0 citations
Publisher

Abstract

Spatial near-resonant effects on laser beam propagation in a multilevel photoionization system have been studied by 2D simulation. It was found that the initial amplitude of nonuniformity grows up during the propagation, which degrades the photoionization rate of the target atom. The spatial effects become large for the longer incident laser pulse. A semi-analytical formula to analyze the growth of nonuniformity in the multilevel photoionization system was derived.

DOI
10.1117/12.375091

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Spatial near-resonant effects on laser beam propagation in a multilevel photoionization system have been studied by 2D simulation. It was found that the initial amplitude of nonuniformity grows up during the propagation, which degrades the photoionization rate of the target atom. The spatial effects become large for the longer incident laser pulse. A semi-analytical formula to analyze the growth of nonuniformity in the multilevel photoionization system was derived.
Propagation of laser pulses in a multilevel photoionization system is studied for atomic laser isotope separation. For this purpose a simulation code called cealis-p has been developed. Through numerical solutions of coupled Maxwell–Bloch equations we illustrate the propagation characteristics and the influence on the photoionization efficiency. It is shown that near-resonant interactions with the nontarget isotope will significantly alter the laser-pulse shape, carrier frequency, and propagation velocity, resulting in degradation of the photon-utilization efficiency and ionization efficiency.
A computer simulation code to treat atomic excitation and laser beam propagation simultaneously in an atomic laser isotope separation system has been developed. The three- level Bloch-Maxwell equations are solved numerically to analyze the change of pulse shapes, the modification of laser frequencies and the time-varying atomic populations. The near- resonant effects on the propagation of frequency chirped and non-chirped laser pulses have been analyzed. It was found that there are serious differences in pulse shapes, frequency modifications and propagation velocities between laser pulses with and without chirping.
There are several nonlinear optical processes that can lead to instabilities in a laser beam as it propagates through a nonlinear optical medium. Self focusing effects, for instance, can lead to the growth of new spatial frequency components in a laser beam. A laser beam can also be unstable to the growth of new spectral components. Stimulated Raman scattering is a well-known example of a process that can lead to this sort of instability. In this paper, we present the results of a series of experiments on the somewhat different instabilities that occur for the case of the propagation of a laser beam through an atomic vapor. We find that these instabilities become particularly pronounced when the frequency of the incident laser is near to that of a one- or two-photon resonance of the atom. In either case, we find that the laser beam is unstable to the growth of new frequency components, and that in general these new frequencies are not emitted collinearly with the incident laser. These instabilities are important because they place a limit on the laser intensity that can be used to drive nonlinear optical processes.
A Monte Carlo method has been successfully applied to the resonant multilevel photoionization by finite bandwidth lasers. A system of density matrix or Schrödinger equations is integrated numerically by using a time series of colored laser noises obeying the Langevin equation. Time dependence of the statistically averaged excitation probabilities is obtained by taking ensemble averages of trials with different stochastic initial conditions. It is found that the Monte Carlo results on the resonant two-level photoionization by a chaotic field model are in good agreement with those of Zollar. The results of the computer simulation show that this method is applicable to analyses of complex multilevel systems with statistical broadening such as the case in connection with the atomic laser isotope separation process. The numerical results using stochastic lasers by a modified version of the phase diffusion model, with spectrum falling off faster than a Lorentzian at its wings, are presented.
Among the mainly interesting parameters of an atomic vapor laser isotope separation process is the ionization yield. This parameter can be controlled as long as the laser beam spatial intensity distribution and temporal shape are well defined and not subjected to unexpected disturbing effects such as coherent propagation phenomena, leading to spatial and temporal reshaping with hot spots and pulse lengthening and delays. On the other hand, economical considerations require optically thick atomic columns which can favor such effects. In this work, we study the photoionization yield in atomic thulium vapor when propagation effects occur. We compare our experimental results with those of a coherent propagation code describing a four level system and including inhomogeneous broadening as well as degeneracies. We choose a three step photoionization scheme.