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Quantum Electronics and Laser Science Conference

Theory of temporal compression of intense laser pulses in a metal vapor

Michael Shaw, John K. Crane

Published May 12, 1991• 0 citations

Abstract

We wish to use atomic group-velocity dispersion to compress energetic copper-vapor laser- pumped dye-laser pulses from 40 ns to approximately 300 ps. This talk focuses on numerical results that illustrate coherent atomic effects on the process. Traditionally, analysis has been based on linear dispersion theory (LDT). This model represents the atoms as a static susceptibility. This susceptibility gives rise to the differential group delay required for compression of frequency-swept pulses. However, LDT is inadequate in analyzing compression in a regime in which the coherent transition rate (Rabi frequency) is comparable to the laser detuning. Here population transfer becomes significant, thus invalidating the concept of a steady-state dipole. An adequate representation of the atomic system requires solution of the coupled Maxwell-Bloch equations. We have developed a code to model near-resonant coherent propagation through inhomogeneously broadened two-level atoms.

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The theory of temporal compression of intense laser pulses in a metal vapor

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We wish to use atomic group-velocity dispersion to compress energetic copper-vapor laser- pumped dye-laser pulses from 40 ns to approximately 300 ps. This talk focuses on numerical results that illustrate coherent atomic effects on the process. Traditionally, analysis has been based on linear dispersion theory (LDT). This model represents the atoms as a static susceptibility. This susceptibility gives rise to the differential group delay required for compression of frequency-swept pulses. However, LDT is inadequate in analyzing compression in a regime in which the coherent transition rate (Rabi frequency) is comparable to the laser detuning. Here population transfer becomes significant, thus invalidating the concept of a steady-state dipole. An adequate representation of the atomic system requires solution of the coupled Maxwell-Bloch equations. We have developed a code to model near-resonant coherent propagation through inhomogeneously broadened two-level atoms.

Numerical simulations of the compression of laser pulses by propagation through a medium of two-level near resonant atoms is presented. The compression of long pulses (>10 ns) whose peak Rabi frequency is comparable to the laser detuning from the atomic transition is addressed. This regime is one in which an accurate description of the physics requires solution of the Maxwell-Bloch equations. A model to investigate nonlinear effects on the compression process has been developed. The analysis begins with a study of the compression of optimally chirped pulses of various fluences. The onset of the nonlinear effects and their ramifications on the compression process are addressed.>

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.

A time-dependent theory for conical emission during near-resonant propagation of laser light in an atomic vapor, which includes full propagation for the laser and frequency sidebands in a nonlinear two-level medium is presented. The density-matrix equations for the dipole moment and population are solved in the dressed atomic frame. The polarization source terms are accurate to order \ensuremath{\gamma}/R, where \ensuremath{\gamma} is a damping constant and R is the generalized Rabi frequency. Analytical plane-wave solutions and numerical, cylindrically symmetric propagation simulations including diffraction are presented. It is shown that the calculations with cylindrically symmetric fields and atomic excitation profiles are incapable of accounting for the high levels of optical gain that are responsible for the intense conical emission observed in experiments. This result is at first surprising, since the model accounts rigorously for all of the physical phenomena that have been previously proposed as being responsible for generating large gains, and the calculation matches the symmetry of the observations. The lack of large calculated gain seems to imply the existence of higher-order

(m>0)

radial modes in the field for the experimental conditions that give rise to cone emission. In the simulations, however, the cylindrically symmetric fields do produce weak red-detuned cones with angular-frequency distributions similar to those seen in experiments.