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Journal of Optoelectronics·laser

Time Evolution of Atomic Response in an Open V-type System with Two-color Pumps

Fan Xi

Published Jan 1, 2002• 0 citations
OpenAlex

Abstract

The law of time evolution of atomic response in an open V type lasing without inversion (LWI) system with two incoherent pumps is investigated by the numerical results.Following conclusions are shown:the time evolution of the populations of the two levels coupled by the probe field has the same oscillation frequency,however,exists a phase difference π;the initial values of the populations affect mainly the oscillation amplitudes of the gain,coherent item and population difference but dont affect their values in the steady state;the ratio of the atomic injection rates increasing will results in an increase of the gain and a decrease of the population difference in the steady state;the atomic exit rate increasing will make both the gain and population difference in the steady state decreasing.

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ACTA PHOTONICA SINICA•Jan 1, 2008•0 citations
The law of time evolution of atomic response in an open V type lasing without inversion (LWI) system with two incoherent pumps is investigated by the numerical results.Following conclusions are shown:the time evolution of the populations of the two levels coupled by the probe field has the same oscillation frequency,however,exists a phase difference π;the initial values of the populations affect mainly the oscillation amplitudes of the gain,coherent item and population difference but dont affect their values in the steady state;the ratio of the atomic injection rates increasing will results in an increase of the gain and a decrease of the population difference in the steady state;the atomic exit rate increasing will make both the gain and population difference in the steady state decreasing.
The propagation effect in an open V-type three-level atomic system with spontaneously generated coherence is studied by numerical calculation. The results show that changing the relative phase between the probe and driving field has an important effect on variations of gain and intensity of lasing without inversion (LWI) with propagation distance, i.e. spatial evolution; variation of ratio (S) between the atomic injection rate and atomic exit rate (r0) has a remarkable modulation role in phase-dependent spatial evolution. In a certain range of S (r0), with value of S (r0) increasing, LWI gain and intensity increase and the propagation distance needed for reaching an intensity maximum value becomes longer; In a corresponding closed system, the maxima of LWI gain, intensity and the propagation distance along which the gain can be produced are smaller than in an open system. The Doppler effect also has an obvious influence on spatial evolutions of LWI gain and intensity, values of LWI gain and intensity are smaller in the presence of Doppler effect than in the absence of Doppler effect.
In an open V-type system,by numerical results of differential equations,the physical mechanism of laser without inversion(LWI) was analyzed,and the effects of the probe and driving fields detunings, the Rabi frequencies of the driving fields,and the initial values of the populations on LWI transient were discussed.The conclusions show that the populations of the two levels coupled by the driving field have the same oscillation frequency,however,exist a phase differenceπ.The dispersion equals to zero and two- photon coherent item is real number when the driving and the probe fields are both resonant.Oscillation period of the gain,coherent item and population difference reduce with the increase of the Rabi frequencies of the driving field.The initial vaiues of the populations mainly affect the oscillation amplitudes of the gain, coherent item and population difference but don't affect their values in the steady state.
Theoretical model of an open four-level atomic system driven by two fields was presented. The density matrix equations of motion were obtained by the electric dipole and the rotating wave approximations. Under resonance of corresponding fields, the physical mechanism of lasing without inversion (LWI) was discussed and its linear analytical solution was obtained. By numerical results, we studied the effects of the spontaneous decay rates between atomic levels, incoherent pump rate, the injection rates, and the exit rate on the gain without inversion and the population difference of the system. The difference between the linear and nonlinear gain and the population difference of the system were discussed. A LWI gain was got during the region of the spontaneous decay rates between atomic levels. The LWI gain can obtain by adjusting suitably incoherent pump rate, the atomic injection ratio and the exit rate. The curves of the linear gain and the nonlinear gain were similar with the Rabi frequencies of the driving fields. The curves of the linear population difference and the nonlinear population difference were similar too. The linear stable value of the gain was larger than the nonlinear stable value of the gain.
The control role of the relative phase between the probe and driving fields on gain, dispersion and populations in an open V-type three-level system with spontaneously generated coherence is studied. The result shows that by adjusting the value of the relative phase, the transformation between lasing with inversion and lasing without inversion (LWI) can be realized and high dispersion (refractive index) without absorption can be obtained. The shape and value range of the dispersion curve are similar to those of the gain curve, and this similarity is closely related to the relative phase. The effects of the atomic exit and injection rates and the incoherent pump rate on the control role of the relative phase are also analysed. It is found easier to get LWI by adjusting the value of the relative phase using the open system rather than the closed system, and using an incoherent pump rather than without using the incoherent pump. Moreover the open system can give a larger LWI gain than the closed system.
The transient evolution processes and steady outputs of continuous wave lasing without inversion (LWI) and self-pulsing LWI in a resonant open V type three-level system are studied. It was found that the two kinds of LWI have some obvious differences not only from the steady outputs but also from the transient evolution processes. The effects of the unsaturated gain coefficient, cavity loss coefficient, ratio of the atomic injection rates and atomic exit rate on the transient evolution processes and steady outputs are discussed.
The effect of the relative phase Φ between the probe and driving fields on gain and dispersion from different aspects in an open quasi-type four-level system by the numerical solution of the steady state was investigated.The results show that by adjusting the value of the relative phase the inversionless gain and dispersion can be periodically amplified and attenuated.High dispersion (refractive index) without absorption can be achieved.The dispersion curve corresponding to Φ=α is similar to the gain curve corresponding to Φ=α+π/2.When the probe field is on resonance,the largest lasing without inversion (LWI) gain can be obtained by adjusting the relative phase.The effect of the atomic exit and injection rates on the LWI gain for different values of the relative phase is also studied.