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Physical Review A

Squeezed lasing without inversion or light amplification by coherence

Klaus M. Gheri, Daniel F. Walls

Published May 1, 1992• 69 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.45.6675

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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.
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We study a nonlinear semiclassical and fully quantum-mechanical theory of a laser with an active medium of three-level atoms that may be initially prepared in a coherent superposition and interact with a classical external electromagnetic field. In particular, we determine the intracavity and output photon statistics of the laser light in a broad range of parameters. We find an inner sub-Poissonian statistics with a Mandel parameter close to the minimal value -1 when the laser is operating with initially prepared atoms, off resonant, and without inversion of the populations. In another regime, with, however, less favorable intracavity statistics, we find squeezing in the intensity output fluctuations of 80%, which is well above the present state of art noise reduction for this kind of coherently driven laser system.
A recent analysis of the influence of the pump statistics on the laser output fluctuations is applied to the experimentally feasible case of a laser being pumped by light coming from a separate pump field cavity that is driven by a broad-band squeezed vacuum plus a coherent driving field. In contrast to the 'squeezed-reservoir laser', where a reservoir of squeezed modes coming from all directions coupled to the dipole moment of the lasing transition and leads to atomic phase decay inhibition and phase locking of the laser field, here the squeezed light merely pumps the gain atoms, that is gives rise to inversion of the laser transition. In analogy to photodetection theory the photon statistics of the laser output is calculated as a function of the photon statistics of the absorbed squeezed pump light. For the case of a squeezed pump field chosen to display sub-Poissonian statistics, it is demonstrated that the Mandel Q-parameter of the laser cavity output can theoretically reach the optimum value of Q=- 1 / 2 , similar to the case of a deterministic pumping mechanism.
Abstract We investigate the quantum statistical and squeezing properties of radiation generated by a coherently driven three-level laser system coupled to a squeezed vacuum reservoir. The analysis is carried out using c-number Langevin equations for normally ordered operator moments, allowing a transparent description of the interplay among atomic coherence, coherent driving, and reservoir-induced correlations. We show that significant quadrature squeezing can be achieved under appropriate conditions, with the degree of squeezing strongly influenced by the linear gain coefficient of the squeezed-vacuum reservoir, the strength of the coherent driving, and the initial atomic preparation. In particular, we find that maximal initial atomic coherence alone does not lead to squeezing in the absence of coherent driving, highlighting the essential role of phase-sensitive amplification. The results further reveal a shift of optimal squeezing toward lower values of the atomic coherence parameter and higher values of the driving strength as the gain and reservoir squeezing increase. In addition, we demonstrate the coexistence of strong quadrature squeezing with super-Poissonian photon statistics, reflecting the distinct roles of phase-sensitive and intensity fluctuations. The squeezing spectrum is also analyzed to provide frequency-resolved insight into noise suppression, directly relevant to homodyne-detection measurements. These findings clarify the trade-off between coherent excitation, which enhances phase-sensitive correlations, and photon population growth, which tends to increase intensity fluctuations, and they highlight parameter regimes where strong nonclassical effects can be realized under parameter regimes consistent with experimentally demonstrated cavity-QED and squeezed-light implementations.