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Soviet Journal of Quantum Electronics

Quantum intensity fluctuations in multimode cw lasers and maximum sensitivity of intracavity laser spectroscopy

Sergey A. Kovalenko

Published Jun 30, 1981• 30 citations
Publisher

Abstract

A model using rate equations which allow for quantum noise is used to make a theoretical analysis of the steady-state lasing regime of a cw dye laser. Equations describing the laser performance are derived in the adiabatic approximation neglecting spatial inhomogeneities in the burnup of the active medium. An investigation is made of the stability of steady-state multimode laser action and it is found that this is weak in certain modes. Calculations are made of correlation functions describing fluctuations in the numbers of photons in the modes and fluctuations in the total intensity. Although the fluctuations in individual modes are large, the integrated intensity fluctuates negligibly. An analysis is made of the influence of quantum intensity fluctuations on the attainment of maximum sensitivity in intracavity laser spectroscopy.

DOI
10.1070/qe1981v011n06abeh007044

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A model using rate equations which allow for quantum noise is used to make a theoretical analysis of the steady-state lasing regime of a cw dye laser. Equations describing the laser performance are derived in the adiabatic approximation neglecting spatial inhomogeneities in the burnup of the active medium. An investigation is made of the stability of steady-state multimode laser action and it is found that this is weak in certain modes. Calculations are made of correlation functions describing fluctuations in the numbers of photons in the modes and fluctuations in the total intensity. Although the fluctuations in individual modes are large, the integrated intensity fluctuates negligibly. An analysis is made of the influence of quantum intensity fluctuations on the attainment of maximum sensitivity in intracavity laser spectroscopy.
A short-cavity dye laser has been constructed to study fluctuations arising from quantum noise and mode coupling in multimode systems. Typical dye lasers (i.e., long-cavity) have been found to have cw dynamics that are dominated by coherent wave mixing effects.1 The quantum noise in these lasers also manifests itself as a variance in the buildup times of the laser light intensity. Buildup times of singlemode long-cavity lasers have been studied.2 The relative importance of quantum noise is enhanced in short cavities making the turn-on transient the dominant feature in the laser light fluctuations. We have measured the turn-on transient of individual modes from a multimode short-cavity dye laser following sudden turn-on of the pump power. The shortness of the cavity has the effect of making the decay times of the intracavity field and the molecular population similar. The observed buildup and rise times are compared with those predicted by a semiclassical multimode laser theory with stochastic noise added for several pump rates. Moreover, semiregular oscillations of the total intensity have been observed to occur in short bursts separating intervals of nearly constant cw laser output.
The steady-state fluctuations of an inhomogeneously broadened two-mode laser with both additive and multiplicative white noise are investigated theoretically. A Fokker-Planck equation of the intensity-distribution function is presented. Analytic expressions of the autocorrelation and cross correlation of the steady-state two-mode intensities are derived for equal pump parameters of the two modes. Large intensity fluctuations occur when the laser is operated below threshold. However, when the laser is operated far above threshold, there is almost no difference between the laser models with and without multiplicative noise.
An experimental investigation was made of fluctuations of the spectral distribution of the intensity in the wide-band spectrum of a ring cw traveling-wave dye laser. Allowance for the averaging within the limits of the instrumental function of a spectrometer was made by analyzing the noise power spectra obtained for the recorded intensity distributions. A good agreement with the theory was observed. The threshold of detection of an intracavity spectrometer, denned in terms of the absorption coefficient, was found to be governed by the quantum nature of the radiation used in the spectrometer and amounted to ~ 3 X 10 − 17 cm − 1 when the accumulation time was 20 s.
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.
We derive a quantum macroscopic model for a multimode semiconductor laser, starting from a complete microscopic description. Nonlinear gain terms describing saturation and mode–mode interaction are obtained by a suitable treatment of the spectral hole burning in the distribution of carriers. In the case in which there are two neighbouring active longitudinal modes above threshold and for quiet pumping, we calculate the spectrum of the total intensity fluctuations at zero frequency. For moderate pumping level the nonlinear gain terms do not reduce the degree of anticorrelation between modes, leaving the squeezing in total intensity unchanged with respect to the single-mode case. Only for very high pumping does the nonlinear gain suppression become relevant, and the total intensity noise is increased.