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Physical Review A

Stochastic Toda-oscillator model of the bad-cavity laser

Tetsuo Ogawa

Published Oct 1, 1990• 13 citations
Publisher

Abstract

The statistical noise properties of the laser radiation in a low-Q (bad) cavity are theoretically investigated. The bad-cavity laser system is shown to be exactly equivalent to the stochastic Toda oscillator (STO) in the case of negligible polarization noise. Transforming the STO Langevin equation to the Fokker-Planck equation with a position-dependent diffusion coefficient, analytical expressions of the probability distribution are obtained as particular solutions in a stationary state with the aid of the expansion into a complete orthogonal set. We predict novel statistical features of the laser light, e.g., a power tail of the intensity distribution function, non-Gaussian nature of the field fluctuation, and super-Poissonian photoelectron statistics. General solutions are also given in a closed form in terms of the matrix continued fraction to compare with the particular solutions. The good-cavity case is reanalyzed in our formalism to root out differences between them.

DOI
10.1103/physreva.42.4210

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The statistical noise properties of the laser radiation in a low-Q (bad) cavity are theoretically investigated. The bad-cavity laser system is shown to be exactly equivalent to the stochastic Toda oscillator (STO) in the case of negligible polarization noise. Transforming the STO Langevin equation to the Fokker-Planck equation with a position-dependent diffusion coefficient, analytical expressions of the probability distribution are obtained as particular solutions in a stationary state with the aid of the expansion into a complete orthogonal set. We predict novel statistical features of the laser light, e.g., a power tail of the intensity distribution function, non-Gaussian nature of the field fluctuation, and super-Poissonian photoelectron statistics. General solutions are also given in a closed form in terms of the matrix continued fraction to compare with the particular solutions. The good-cavity case is reanalyzed in our formalism to root out differences between them.
We investigate theoretically the statistical properties of the bad-cavity laser with field and population fluctuations. This system is equivalent to the stochastic Toda oscillator, whose probability distribution obeys the Kramers equation with a position-dependent diffusion coefficient. An approximate probability distribution function is calculated in the stationary state by the method of the orthogonal polynomial expansion. We predict novel statistical features of the laser intensity resulting from the power tail of the intensity distribution. Diverging intensity moments and the deviation of the photon-counting statistics from the Poisson type are also given.
The noise characteristics of a single-mode laser with a low-Q cavity are investigated theoretically. After the electric field is adiabatically eliminated from the Maxwell-Bloch equations, coupled Langevin equations with both additive and multiplicative white noises are examined. The equations are solved using Rice's method in a framework of quasilinear Fourier analysis. Noise spectral densities are calculated analytically to study the dependence of the relative intensity noise (RIN) on the pumping. Through an investigation of the auto- and cross-correlations of the light intensity and population noises, their variances are obtained in order to compare their properties to the good-cavity case. The stationary intensity cumulants, the photon counting coefficient, and the photon counting probability are explicitly derived and compared with the results of a Fokker-Planck analysis previously carried out for both the good- and bad-cavity cases.>
We formulate a running-wave, single-mode model for purely dispersive optical bistability that incorporates amplitude and frequency fluctuations in the incident-field, cavity-length fluctuations, and thermal noise in the radiation field and in the material. This model is given by a set of three Langevin-type equations for the real and the imaginary parts of the slowly varying electric field and for the material variable. In the white-noise limit, it is equivalent to a Fokker–Planck equation in three variables. Even if our model can also describe a Kerr medium or a two-level system in a suitable range of parameters, we are interested mainly in the case of miniaturized all-optical bistable devices that utilize semiconductor dispersive media. In this situation, we can adiabatically eliminate the field variables and therefore reduce the problem to a single stochastic differential equation in one variable, which contains several terms of additive and multiplicative noise. We find that noise in the imaginary part of the slowly varying electric field does not contribute to this equation. In the white-noise case, our stochastic equation is equivalent to a Fokker–Planck equation in one variable, whereas in the case of colored noise we obtain a one-dimensional Fokker–Planck equation only in the limit of short correlation times. Finally, we calculate the stationary solution of the resulting Fokker–Planck equation. We show that the presence of colored noise narrows the peaks of the probability distribution at steady state with respect to the white-noise case.
In the general framework of the system size expansion of Van Kampen and Kubo, we consider the Fokker-Planck equation for a model of absorptive bistability in the bad-cavity limit. The physical system is described by the reduced atomic density operators after adiabatic elimination of the cavity field variables. Mapping of the master equation into ccc-number form according to the normal-ordering mapping scheme yields known results for the atomic fluctuations and correlation functions; however, it also leads to a Fokker-Planck equation with a non-positive-definite diffusion matrix. The symmetrical-order-mapping scheme eliminates this difficulty. The leading contribution to the system size expansion yields a Fokker-Planck equation for the symmetrical-ordered density function having a positive-definite diffusion matrix. The atomic expectation values and fluctuations previously derived from the quantum Langevin equations emerge naturally from this Fokker-Planck equation.
In the limit of large field-relaxation rates as compared to the rates of collective and atomic relaxation, we model the stochastic evolution of a bistable system with the aid of the Langevin equations for atomic variables. This semiclassical approach leads to a Fokker-Planck equation for the joint probability distribution of the atomic polarization and inversion. The Monte Carlo integration scheme, consisting in construction of approximate random trajectories, enables us to evaluate numerically the adiabatic values of the output field, intensity, and intensity fluctuations as functions of time. The final model used in numerical work is equivalent to ignoring microscopic fluctuations and considering delta-correlated external fluctuations impressed on the driving field.