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Physical Review A

Inversionless lasing and photon statistics in a V-type atomic system

Yifu Zhu, A. I. Rubiera, Min Xiao

Published Feb 1, 1996• 33 citations
Publisher

Abstract

We analyze intensity and statistical properties of lasers without population inversion in a closed three-level V-type system. We derive the threshold condition and examine the intensity dependence on various system parameters. Unlike other inversionless laser systems that generate amplitude-squeezed light, the intensity fluctuation of the inversionless laser from the three-level V-type system is above the shot-noise limit in a wide parameter range studied here. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.1065

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We analyze intensity and statistical properties of lasers without population inversion in a closed three-level V-type system. We derive the threshold condition and examine the intensity dependence on various system parameters. Unlike other inversionless laser systems that generate amplitude-squeezed light, the intensity fluctuation of the inversionless laser from the three-level V-type system is above the shot-noise limit in a wide parameter range studied here. \textcopyright{} 1996 The American Physical Society.
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.
We analyze a laser system consisting of N closed three-level atoms confined in a single-mode cavity and derive the conditions for the onset of lasing without inversion or with inversion. We calculate the steady-state gain and lasing in the system and show that far above the lasing threshold, depending on the pumping and decay rates, the inversionless laser may operate in three different regimes, where the intensity of the laser depends linearly on N, is proportional to \ensuremath{\surd}N , or independent of N.
We show that a closed four-level system can exhibit lasing without population inversion in any state basis. The same system can be also made to exhibit lasing without population inversion in the bare-state basis but with population inversion in the dressed-state basis. We derive the conditions of lasing without inversion and examine the dependence of the gain on various system parameters. The lasing threshold and the steady-state photon numbers are calculated. Comparison with a ssV-type, three-level model is presented.
We present a detailed analysis of the noise properties of a potential lasing scheme employing a coherently driven three-level system in the \ensuremath{\Lambda} configuration inside an optical cavity. We derive expressions for the steady-state laser intensity and the width of the intensity fluctuations and show that the output fields exhibit sub-Poissonian statistics for suitable operating conditions. The fundamental mechanism responsible for noise suppression is identified as increased intensity dissipation inside the cavity through emission and absorption by the atoms, which counteracts the intensity fluctuations.