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Journal of the Optical Society of America B

Numerical study of propagation of laser pulses in a multilevel photoionization system

Y.-W. Chen, Chiyoe Yamanaka, Hideaki Niki, S. Adachi, Yasukazu Izawa, S. Nakai

Published Sep 1, 1994• 12 citations
Publisher

Abstract

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.

DOI
10.1364/josab.11.001585

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Journal of Physics B Atomic Molecular and Optical Physics•Sep 14, 1995•15 citations
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.
The pulse deformation and time delay, which appear when the laser propagates in a thick atom vapor, influence the ionization yield and selectivity of atomic multi-step photoionization process directly. In this paper, we study the propagation of laser pulse and atomic photoionization in a thick atom vapor medium according to the atom vapor laser isotope separation. The process of atomic multi-step photoionization and the propagation of laser in a thick medium are described by density matrix equation and Maxwell equations, respectively. The medium consists of target isotope and non-target isotope, which is non-resonantly excited. Through numerical solution of the coupled equations we illustrate the propagation characteristics of laser and the influences of atom vapor parameters and laser parameters on average ionization yield and average selectivity in a thick medium. The important results of calculation are as follows:when the atom vapor medium is rather thick, the average ionization yield increases while average selectivity decreases with the increase of laser power. When the atom vapor is relatively thin, the average ionization yield and average selectivity increase with the decrease of laser power simultaneously. Besides, there is a positive time delay between two laser pulses in which case the average ionization yield of target isotope reaches its maximum value. Moreover, when the parameters of atom vapor are constant, extending the width of laser pulses as great as possible can not only increase average ionization yield and average selectivity simultaneously, but also loosen the control accuracy of time delay between laser pulses.
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.
The atomic selective multi-step photoionization process is a critical step in laser isotope separation. In this work, we study three-step photoionization processes with non-monochromatic laser fields theoretically based on the semi-classical theory. Firstly, three bandwidth models, including the chaotic field model, de-correlation model, and phase diffusion model, are introduced into the density matrix equations. The numerical results are compared with each other comprehensively. The phase diffusion model is selected for further simulations in terms of the correspondence degree to physical practice. Subsequently, numerical calculations are carried out to identify the influences of systematic parameters, including laser parameters (Rabi frequency, bandwidth, relative time delay, frequency detuning) and atomic Doppler broadening, on photoionization processes. In order to determine the optimal match among different systematic parameters, the ionization yield of resonant isotope, and selectivity factor are adopted as evaluation indexes to guide the design and optimization process. The results in this work can provide a rewarding reference for laser isotope separation.
The medium of the three-level Yb atom corresponding to the three-colour, three-step, three-photon scheme of 6 1 S 0 (0 cm -1 ) to 6 3 P 1 (17992008 cm -1 ) to (7/2, 3/2) 2 4f 13 6s 2 6p (35196.98 cm -1 ) to autoionization state (52353 cm -1 ) is chosen to examine the selective photoionization of 168 Yb. The Maxwell-Bloch equation is numerically solved, where the interacting laser pulses are supposed to be collinear and resonant with the transition lines of 168 Yb, and the effects of Doppler broadening and laser bandwidth are incorporated. Firstly, the thin medium dynamics are examined to determine the optimal energy of laser pulses for the selective photoionization of 168 Yb, and to determine Yb isotopes which have a large influence on the excitation of 168 Yb. Secondly, the thick medium dynamics are investigated, resulting in the fact that the laser pulse for the first transition is prominently deformed due to the near-resonant interaction with 171 Yb and 173 Yb, and the ion yield along the propagation of the laser beam is strongly dependent on the initial energy of pulse for the first transition and the time-delay between input pulses.