Noise and Fluctuations in Multiphoton Processes

P. Zoller

L. A. Lugiato, G. Strini

K. Wódkiewicz, Joseph H. Eberly

We consider a category of transient processes that is usually hidden from view in coherent-transient experiments. These are the very fast noise processes that give rise to relaxation effects. We present different Markov-type models of a class of these noise processes, namely, laser phase and frequency fluctuations. These models lead to different forms for the laser-field autocorrelation function, depending on which quantity, the phase or the frequency, is the primary source of noise. We show that it is possible to establish a regime such that all these kinds of noise will lead to a Lorentzian laser power spectrum. Despite this common feature, N-photon absorption exhibits observable differences depending on the source of fluctuations. For example, for diffusive noises the absorption linewidth increases as N2, but for jumplike or shotlike phase fluctuations it can be N independent.

P. Garcia-Fernandez

A model for the master equation of multiphoton processes with radiative damping of the atomic levels is proposed and a matrix method employed to solve exactly the rate equations for the matrix elements of the reduced density operator in the Fock representation. This model has been applied to the study of the low-noise properties of squeezed light in an all-optical transmission link formed from alternating sections of linear and nonlinear attenuating fibre and optical amplifiers. When the attenuators present radiative damping of the atomic levels (the lower atomic state is not the ground state) the signal-to-noise ratio is enhanced with respect to the case in which the lower atomic level is the ground state. The enhancement is improved when the input light is squeezed. The statistical properties of linear and two-photon amplification with an absorption process which exhibits radiative damping have also been analysed for coherent and squeezed inputs. The second-order fluctuations are reduced when radiative damping in the absorption process is considered except in the case of two-photon absorption for small values of the saturation parameter sa . When the value of this parameter is increased the behaviour of the fluctuations changes and the noise is reduced. These results could be of interest in all-optical transmission links and are improved when the input light is squeezed.

Dietrich Voslamber

Cao Long Van, K. Wódkiewicz

The authors present exact solutions for N=1 and 2 N-photon off-resonance ionisation in the presence of a fluctuating radiation field with pre-Gaussian statistics. These pre-Gaussian fluctuations are composed of n independent two-state jump processes (telegraphs). Such telegraph-type fluctuations have a second-order intensity coherence function g that is normalised and bounded such that 1<or=g<or=2. In the limit n to infinity the pre-Gaussian noise converges to chaotic light with the well known g=2!. The authors show that an exact discussion of the N-photon ionisation rate can be given for such pre-Gaussian fluctuations.