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Optics and Spectroscopy

Effect of shot noise of the excitation of the medium on the emission fluctuations of a ring gas laser

Yu. M. Golubev, I. V. Sokolov, V. P. Gryaznevich

Published Nov 1, 1981• 0 citations
OpenAlex

Abstract

The linewidth of the beats between oppositely traveling waves in a ring gas laser due to shot fluctuations of the excitation of the active medium is calculated. It is shown that in saturating fields the contributions to the linewidth of the beats from quantum fluctuations of the emission and from shot fluctuations of the excitation are of the same order of magnitude and must be taken into account simultaneously. Shot fluctuations of the phases of the oppositely traveling wave due to fluctuations in the number of working atoms with a certain velocity are anticorrelated close to the center of the gain contour. This leads to additional line broadening of the beats between the oppositely traveling waves compared with the case of independent waves.

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The linewidth of the beats between oppositely traveling waves in a ring gas laser due to shot fluctuations of the excitation of the active medium is calculated. It is shown that in saturating fields the contributions to the linewidth of the beats from quantum fluctuations of the emission and from shot fluctuations of the excitation are of the same order of magnitude and must be taken into account simultaneously. Shot fluctuations of the phases of the oppositely traveling wave due to fluctuations in the number of working atoms with a certain velocity are anticorrelated close to the center of the gain contour. This leads to additional line broadening of the beats between the oppositely traveling waves compared with the case of independent waves.
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.
The role of nonlinearly absorbing cells filled with an excited and unexcited gas in the formation of the statistical characteristics of laser emission, particularly the level of natural noise, and the emission linewidth, is investigated by the kinetic-equation method for a quantized electromagnetic field. The appropriate corrections are calculated. It is shown that these corrections can be of the same order as the quantities themselves under saturation conditions. Moreover, the results depend very much on the model of the medium in the cell.
It is shown theoretically that the relative fluctuations in the numbers of photons in individual modes of a multimode standing-wave laser with a homogeneously broadened gain profile is extremely sensitive to the degree of filling of the resonator with the active medium. If this medium fills the whole resonator, the spatial effects result in strong suppression of fluctuations in the individual modes. In the opposite case of a thin medium the statistical properties of a multimode standing-wave laser are close to those of a traveling-wave laser. The conclusions reached are used in an analysis of the quantitative discrepancy between the results of published experimental studies of the statistics of multimode lasing.
The effects of phase-interrupting and velocity-changing collisions between active and perturber atoms on the spectral profile of a gas laser are studied. Exact analytical results for the density matrix are presented for the case of strong collisions in a unidirectional ring laser, and the laser intensity is then evaluated in the Doppler limit under the condition of self-consistency. The washout of the hole burning in the steady-state population difference and the broadening of the laser line shape are discussed. The results of the third-order perturbation theory are compared with the exact calculations valid for an arbitrarily intense pump.
A report is given of a study of the amplitude-spectral asymmetry of the stimulated emission of superfluorescence from dye solutions pumped longitudinally by nanosecond laser pulses. This amplitude asymmetry is manifested by the difference between the powers of the forward and backward waves, and it is a function of the rate of pumping and of the optical density of the active medium. The asymmetry of stimulated emission from a ring laser is manifested by the appearance of a wave with a traveling-wave ratio of ~90 in the absence of any optical isolators. The spectral asymmetry of a ring laser is manifested by a difference between the spectra of strong and weak waves traveling in opposite directions. A quantitative analysis is made of the effects based on the solution of the transfer equations for radiation traveling in a nonlinearly absorbing active medium.