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Quantum Electronics

Quantum fluctuations of radiation in a ring Nd : YAG chip laser

E G Lariontsev, Vladimir V. Firsov

Published Jul 28, 2015• 0 citations
Publisher

Abstract

We report theoretical and experimental investigation of intensity fluctuations in a travelling-wave ring Nd : YAG chip laser, caused by the noise of spontaneous emission. In accordance with theory and experiment, quantum intensity fluctuations in the laser under study decrease dramatically with increasing pump over the threshold. As a result of the research performed, the factor is found, which determines the ratio of the rate of spontaneous emission into the generated mode to the total rate of spontaneous emission into all modes. The effect of the relaxation rate from the lower laser level on quantum fluctuations of the radiation intensity is found.

DOI
10.1070/qe2015v045n07abeh015751

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Expressions for the fluctuations and correlations of the light emitted by a ring laser with counter-rotating traveling wave modes are derived theoretically. It is shown that the competition of the two modes for the excited atomic population leads to negative correlations between their intensity fluctuations, whose magnitude depends on the detuning of the laser cavity from the atomic line center. As a result of the mode competition, the relative intensity fluctuations do not die out in general as the working point of the laser is raised above threshold at line center, and the emitted light does not become fully coherent as in a conventional laser. The two-time amplitude and intensity correlation functions of the light are calculated at line center, and are shown to be expressible in terms of the eigenfunctions and eigenvalues of a certain Schr\"odinger equation, in complete analogy with the theory of the single-mode laser. However, the intensity correlation time increases with pump parameter above threshold, unlike that for the single-mode laser.
We consider the problem of reducing the contributions of pump and spontaneous-emission noise to the output noise of a laser field. We consider two models based on an atomic medium, one analytic, the other involving numerical simulation of a sequence of pump cycles. The predictions of both models are shown to agree. For regular pump excitation the intensity fluctuations of a laser well above threshold may be reduced to zero at the cavity resonance frequency if the ratio of the lifetimes of the upper and lower lasing levels is sufficiently large that spontaneous emission is suppressed. When the atomic lifetimes are comparable, spontaneous emission reduces the amount of noise reduction. For example, for equal atomic lifetimes regular pump excitation will result in a maximum noise reduction in the output field of one-half the shot-noise limit.
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