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Acta Physica Sinica

TRANSVERSE DIFFRACTION EFFECTS OF LIGHT FIELDS ON THE GAIN WITHOUT POPULATION INVERSION

Hanzhuang Zhang, Gao Jin-Yue, 吉林大学物理系

Published Jan 1, 1997• 0 citations
Publisher

Abstract

We discussed transverse diffraction effects of probe and driving fields on the gain in the model of Raman driven four level atomic system. The calculated results showed that the spectrum of the probe field has different structure compared with that in plane wave approximation due to the transverse diffraction effects of light fields, and the optimum focal position of a focus lens for the maximum gain is in the vicinity of a sample cell.

DOI
10.7498/aps.46.2330

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We discussed transverse diffraction effects of probe and driving fields on the gain in the model of Raman driven four level atomic system. The calculated results showed that the spectrum of the probe field has different structure compared with that in plane wave approximation due to the transverse diffraction effects of light fields, and the optimum focal position of a focus lens for the maximum gain is in the vicinity of a sample cell.
In this paper the effects of the transverse profile of the probe and driving fields on the gain in a Raman-driven four-level system are discussed. The results show that when transverse effects are included, the probe gain spectrum has a different structure compared with that in the plane-wave approximation, the optimum focal position of a focus lens for the maximum gain is not at the center of the sample cell, and the gain is very sensitive to the changes of focal position when the waist of the driving field is small.
We discuss the effect of Doppler broadening on the gain and refractive index experienced by an optical probe when passing through a collection of atoms in the absence of a population inversion. We calculate the gain-absorption spectrum of the medium when the atoms have a Maxwell velocity distribution and compare the atomic response when the pump and probe beams are arranged in copropagating and counterpropagating configurations. The results in these two cases can be qualitatively very different from each other, as we show with the help of numerical calculations. A physical interpretation and the conditions for the validity of the results are provided.
The transverse effect of the fields was studied by densitymatrix equations of motion in an open Vtype system and Hurwitz criteria.The result shows that comparing with the case in the plane wave approximation,the transverse profile of the field leads to an obvious variation of the fashion that the system obtains inversionless lasing through instability of nonlasing solution,and makes the system more easy to obtain inversionless lasing through instability of pitchfork bifurcation;the value of the ratio of the active medium transverse section to the beam waist has a determinant effect on size of the instability region in which inversionless lasing arises,and the Raman inversion region increases with increasing of value of the ratio;the effects of unsaturated gain,loss of laser cavity,the ratio of atomic injection rates and atomic exit rate varying on the instability region in which inversionless lasing arises are much different from that in the plane wave approximation case.
With the density matrix equations of motion under the electric dipole and the rotating wave approximations,the mechanism of LWI gain was analyzed.By numerical results,the effects of the Rabi frequencies of the driving fields and nonlinear term on the gain without inversion and the population difference of the system were studied.Theoretical model of an openⅤ-type four-level atomic system was presented.Through calculation,when both the probing and driving fields are resonant,the dispersion equals to zero.A LWI gain was got during the region of Rabi frequencies of the driving fields.The gain was saturated because of nonlinear term.
A model for an open four-level atomic system driven by two fields is presented.Density matrix equations of motion are obtained by electric dipole and rotating wave approximations.Physical mechanism of lasing without inversion(LWI) is discussed.Effects of detuning on gain without inversion and population difference are studied.Dispersion equals to zero as two driving and probe fields are resonant.As one driving field(probe field) is resonant,the gain and the population difference versus detuning of driving field(probe field)show even symmetry,and the dispersion shows central symmetry.As two or three of driving and probe fields are off-resonance,the symmetries disappear.LWI gain is obtained when the dispersion equals to zero.When both driving and probe fields are off-resonance,the gain,the dispersion and the population difference change greatly.High dispersion without absorption is obtained by adjusting suitable parameters of the system.
Propagation of a weak probe optical beam through a three- level medium pumped with a microwave strong beam is numerically studied. Three various schemes of transitions under the conditions providing the amplification of the probe field without inversion are taken to calculate the atomic polarization which enters the paraxial wave equations. Diffraction and self-action of beams due to nonlinear absorption and refraction are properly taken into account. These effects together with linear absorption and refraction form a complex distribution of gain across the beams. As a result the amplified beam acquires ring transverse patterns and slight asymmetry in the frequency characteristics.