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Physical Review A

Nonlinear theory of the degenerate quantum-beat laser: Lasing without inversion

Shi-Yao Zhu

Published Nov 1, 1990• 20 citations
Publisher

Abstract

We present the nonlinear theory for lasing without population inversion in the degenerate \ensuremath{\Lambda}-type quantum-beam laser by using the density-operator method. The master equation for the degenerate quantum-beat laser is derived, from which the equation of motion for the mean photon number is obtained. The conditions for lasing without population inversion are discussed, which are the same for linear and nonlinear theories. The corresponding Fokker-Planck equation is obtained, and the possibility of diffusion coefficient reduction is discussed.

DOI
10.1103/physreva.42.5537

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We present the nonlinear theory for lasing without population inversion in the degenerate \ensuremath{\Lambda}-type quantum-beam laser by using the density-operator method. The master equation for the degenerate quantum-beat laser is derived, from which the equation of motion for the mean photon number is obtained. The conditions for lasing without population inversion are discussed, which are the same for linear and nonlinear theories. The corresponding Fokker-Planck equation is obtained, and the possibility of diffusion coefficient reduction is discussed.
A quantum theory of a \ensuremath{\Lambda}-type three-level single-mode quantum-beat laser with injected atomic coherences and with an external microwave field, which coherently drives the lower two nearly degenerate levels, is studied. A master equation for the field-density operator and the photon statistics is derived. The laser operation is analyzed in terms of the coefficients of the Fokker-Planck equation of the laser field. Noninversion lasing is found in both cases: without and with injected atomic coherences. At the same time, in the latter case large quantum-noise reduction is found, and for particular initial parameter choices the laser field is very near to a coherent state with exactly Poissonian photon-number distribution and near-Poissonian phase distribution.
We present the quantum theory of a two-mode Lambda-type three level quantum-beat laser with injected atomic coherences. The lower two levels are coupled by an external classical field. The master equation and the Fokker–Planck equation are derived. Phases of the two modes are locked to a value depending on the phase of the external field, the phase of the injected coherence and the population of the lower two levels. Lasing without inversion and without threshold is found for both modes with and without injected coherences. Applying the appropriate initial atomic coherences one can switch intensities between the two modes, as well as bifurcation and multiple branching can be found when coherences among all levels are present. It is also possible to find phase bistability at a fixed intensity for one of the two modes, when high pump is applied. Strong intensity and phase noise reduction can be found in one of the modes at the expense of increased noises in the other one. Near Poissonian noise can be achieved. This research was supported by a grant of the Office of Naval Research.
We show that the problem of a \ensuremath{\Lambda}-type (and also a V-type) three-level single-mode laser (maser) is isomorphic to that of a two-level single-mode laser (maser) under the conditions of energy-level degeneracy and equal decay rates. This correspondence reveals that lasing without population inversion can occur in the \ensuremath{\Lambda}-type system with or without an initial atomic coherence, and also gives immediately the master equation of the laser (maser) field. We analyze in detail the operational and noise properties of a \ensuremath{\Lambda}-type laser using a Fokker-Planck approach. We also present a vector model to visualize the population trapping and the absorption and emission processes in such a laser (maser) system.
We derive the master equations for the fields in the one-mode \ensuremath{\Lambda}-type micromaser and laser with injected atoms in a superposition of their states. In terms of effective parameters, these equations are equivalent to those for a two-level micromaser and laser. The interference terms in the master equations may cancel the absorption terms independent of statistical properties of the field. Due to the atomic coherence between the two degenerate lower levels, the generation of field states with sub-Poissonian photon statistics is possible in the micromaser without the need for a population inversion. If an equivalent level system is used in a laser operating without population inversion it can produce field states with Poisson distributions of the photon number.