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Physical Review A

Lasing without inversion: Gain enhancement through spectrally colored population pumping

G. S. Agarwal, Gautam Vemuri, THOMAS W. MOSSBERG

Published Dec 1, 1993• 25 citations
Publisher

Abstract

We propose the use of spectrally colored rather than white (i.e., incoherent) population pumping in lasing-without-inversion schemes. We show that colored pumping can lead to a significant enhancement in gain by presenting explicit results for the gain in a three-level ladder system. These results are obtained from Monte Carlo simulations of the coupled density-matrix equations, which, for a colored pump, acquire the structure of Langevin equations.

DOI
10.1103/physreva.48.r4055

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We analyze the inversionless gain in a three-level ladder system by replacing the usual coherent coupling field with an incoherent field. Surprisingly, it is found that one can obtain inversionless amplification of a weak probe even in the absence of a coherent field in the model. We conclude that gain is determined by the ensemble average of the product of the two-photon coherence and the ``effective Rabi frequency'' of the field. Thus, even though the incoherent pump reduces the two-photon coherence, gain can be restored by choosing sufficiently high strengths of the incoherent field.

Physical Review A•Oct 1, 1995•40 citations
We propose the use of spectrally colored rather than white (i.e., incoherent) population pumping in lasing-without-inversion schemes. We show that colored pumping can lead to a significant enhancement in gain by presenting explicit results for the gain in a three-level ladder system. These results are obtained from Monte Carlo simulations of the coupled density-matrix equations, which, for a colored pump, acquire the structure of Langevin equations.
Most models of lasing without inversion utilize incoherent pumping of the upper state of the transition on which lasing occurs. We suggest replacing incoherent pumping by a color pump (i.e. laser with a finite bandwidth) for selective pumping of the upper state, where one can also take advantage of the tunability of the laser. A three-level ladder system is considered with the upper two levels providing the lasing transition. We reduce the density matrix equations for this system to a set of eight coupled Langevin equations with multiplicative noise. These equations are solved using Monte-Carlo methods to obtain the gain as a function of various atom and field parameters. The use of the color pump leads to a significant enhancement in gain. Using a dressed states formalism, an explanation of the possible physical mechanism behind this gain enhancement is provided.
We explore the possibility of enhancing the gain in lasing without inversion schemes by replacing the usual incoherent pump with a finite bandwidth (spectrally coloured) pump. Three level atoms in the ladder and lambda configuration are modelled and we find that replacing the incoherent pump with a partially coherent pump amplifies the gain by as much as a factor of 4. The results are obtained via Monte-Carlo simulations of the density matrix equations, which, for a coloured pump, have the form of Langevin equations. We find that for a given set of atom-field parameters, the gain for coherent pumping is larger than for incoherent pumping, and that maximum gain is obtained for a partially coherent pump. The frequency of the coloured pump plays an important role in determining the optimum gain that can be obtained.
Recently it was reported that replacing an incoherent pumping mechanism by a spectrally colored (partially coherent) pump can lead to significant gain enhancement in lasing without inversion schemes [Phys. Rev. A 48, R4055 (1993)]. We extend that study further by considering the effects of the noise parameters (strength and bandwidth) of the pump on the gain in two types of three-level atomic energy-level schemes---the ladder and the \ensuremath{\Lambda} models. The calculations presented in this paper show a way to continuously ``tune'' the bandwidth of the pump and make a direct comparison between the gain obtained for coherent, incoherent, and partially coherent pumping. We find that for a given set of atom-field parameters, the gain for coherent pumping is larger than for incoherent pumping, and that maximum gain is obtained for a partially coherent pump. Increasing the strength of the noise also leads to an increase in the gain. The frequency of the colored pump plays a critical role in determining the maximum gain that can be obtained.
We have studied gain without population inversion in two similar four-level systems by employing semiclassical density-matrix formalism. If the incoherent pumping process is not taken into account, both systems can be visualized as an ideal V-type system. Along with the application of strong pump and weak probe fields, an incoherent pump field is added to each of the systems in a different configuration. It has been shown that the different mechanisms of application of an incoherent pump to the system can appreciably change the population difference between the energy levels of the strongly driven transition and results in the evolution of inversionless gain either at the line center or at the sidebands on probe detuning. The effect of inhomogeneous broadening on the probe gain has also been studied.
We analyze the inversionless gain in a three-level ladder system by replacing the usual coherent coupling field with an incoherent field. Surprisingly, it is found that one can obtain inversionless amplification of a weak probe even in the absence of a coherent field in the model. We conclude that gain is determined by the ensemble average of the product of the two-photon coherence and the ``effective Rabi frequency'' of the field. Thus, even though the incoherent pump reduces the two-photon coherence, gain can be restored by choosing sufficiently high strengths of the incoherent field.