Theory of Intensity-Correlation Measurements on Imperfectly Mode-Locked Lasers

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Abstract
Intensity-correlation measurements, particularly by the two-photon fluorescence (TPF) technique, have become very popular for determining the temporal behavior of laser emission on a picosecond time scale, although they are subject to considerable difficulties of interpretation. The characteristics of the fluorescent intensity in the TPF technique are analyzed theoretically for two different physically interesting and tractable models of imperfectly mode-locked lasers: (i) a partial-locking model where some of the modes are locked and the rest unlocked, and (ii) a "domain model" where the set of oscillating modes is composed of several perfectly locked subsets randomly phased relative to one another. For each case, the dominant features of the spatial dependence of the fluorescence are calculated analytically, using approximations which are valid for realistic cases involving large numbers of oscillating modes. Computer results for the intensity of the laser field and of the fluorescence are also presented for the domain model. It is shown that the partial-locking model is incompatible with experiment, whereas the domain model is in reasonably good agreement with it. Finally, the dynamical basis of the domain model is discussed.

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