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Journal of Optics B Quantum and Semiclassical Optics

Transverse profile effects on lasing without population inversion

Franco Prati, J. Mompart, R. Corbalán

Published Jan 1, 1999• 2 citations
Publisher

Abstract

We analyse the effects of a Gaussian transverse profile for the driving and laser fields on lasing without population inversion, in four different three-level schemes. We show that the lasing domains can be considerably reduced with respect to the plane-wave case in the V and in the cascade configuration driven in the upper transition, while in the and in the cascade configuration driven in the lower transition such domains are always larger in the Gaussian case.

DOI
10.1088/1464-4266/1/1/026

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Annual Meeting Optical Society of America•Jan 1, 1993•0 citations
We analyse the effects of a Gaussian transverse profile for the driving and laser fields on lasing without population inversion, in four different three-level schemes. We show that the lasing domains can be considerably reduced with respect to the plane-wave case in the V and in the cascade configuration driven in the upper transition, while in the and in the cascade configuration driven in the lower transition such domains are always larger in the Gaussian case.
The influence of driving field phase fluctuation on lasing without population inversion in a three-level cascade system was studied. In the rotating wave and slowly varying envelope approximations, the density-matrix motion equations for the three-level cascade system, considered the influence of driving-field phase diffusion is solved, and the exact linear analytical solutions of the three-level cascade system with the driving field having the phase fluctuation in the steady state are obtained. The dependence of LWI gain, dispersion and the populations on the probe field detuning and the strengths of coherent pumping, respectively, are examined and simulated numerically. It was shown from the results of numerical calculation about the steady-state solutions of three-level cascade system that the gain of lasing without population inversion will be decreased due to driving-field phase-fluctuation The effect ran he compensated by increased driving-field intensities. However, the effect that LWI gain is decreased due to driving- field phase-fluctuation can not be always compensated by increased driving field Rabi frequency. Lasing without population inversion is still obtained even if the linewidth due to driving-field phase-fluctuation is large enough. The presence of the linewidth prevents the cascade system from obtaining a high refractive index along with zero absorption. The cascade system can still exhibit a larger refractive index and zero absorption at the lesser linewidth. The linewidth tends to destroy lasing without population inversion and refractive index enhancement. There is no population inversion for the lasing transition and for the driving transition under the given condition. And the condition without population inversion has nothing to do with the variety of linewidth in the steady-state analytical solutions. That is to say, there is not possibility for the cascade system that a change from lasing with population inversion to lasing without population inversion can occur with the linewidth increasing or with the Rabi frequency of driving-field increasing. The conclusion is very different from that obtained in other inversionless lasing system.
Exemplifying the three-level systems in the V configuration, we investigate the role of selective excitation of dressed populations in lasing without inversion. Incoherent driving field transfers population from either of the dressed states to excited state when the level spacing between dressed states produced by the coherent field is much larger than the spectral width of the incoherent driving field. With atomic decay pathways，unidirectional channel of population transfer is formed and selective excitation of dressed populations is established. Employing selective excitation of dressed populations，three severe limits are relaxed. First，it is no longer required that the decay rate of the low-frequency driving transition is larger than that of high-frequency laser transition. Second，the threshold for incoherent driving rate is remarkably lowered. Third，linear gain of the laser without inversion is no longer inversely proportional to the coherent driving field intensity, and the gain is significantly enhanced.
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.
We study propagation effects on lasing without population inversion in a Doppler-broadened V -type three-level system. In particular, we focus our analysis on frequency up-conversion lasing without inversion in atomic rubidium. In an atomic beam configuration, we show that it is possible to increase notably the probe gain per single pass through the active medium by detuning driving and probe fields out of one-photon resonance but maintaining the two-photon resonance condition. In a vapour cell configuration, we show that due to propagation effects the probe gain per single pass depends strongly on the driving field intensity at the entrance of the active medium. In fact, for appropriate parameter values, it is possible to reach values of the probe gain per single pass of around 40%. For this last case, we have considered the feedback of a ring laser cavity.
Considerable attention has been directed recently to the study of lasing without the need of population inversion. The optical coherence and quantum interference associated with the light amplification may lead to interesting statistical properties in inversionless lasers. Here we analyze a laser model that consists of an ensemble of closed multi-level (three or four levels) atoms driven by a single coherent field. Unlike conventional lasers where population inversion is maintained and kept at the threshold value for arbitrary laser output, the multi-level laser analyzed here can start lasing from population inversion and evolves from population inversion into non-inversion with the increasing atomic pumping. In other words, the system makes transition from light amplification by population inversion into light amplification by coherence. On the other hand, the system can be also made to start lasing without population inversion in any state basis. In both cases, initial inversion and non-inversion, which are controlled by the ratio of spontaneous decay rates, the laser well above threshold is always maintained by lasing without inversion (light amplification by coherence), and the laser intensity fluctuations may drop below the shot noise limit. Two factors contribute to the noise reduction: first, the disappearance of the population inversion leads to the depleted atomic population in the upper lasing state, which reduces the spontaneous emission noises; second, the fast coherent cycling of electrons between excited states leads to the highly regulated absorption and emission processes. We found that the combination of these two mechanisms can lead to the noise reduction in the laser output more than 50% below the shot-noise limit.