Imperfectly mode-locked laser emission and its effects on nonlinear optics

The output of an ideally mode-locked laser, namely, one having equal phase angles of the modes, consists of a train of bandwidth-limited short pulses. The influence of deviations from equal phase angles on the laser output is investigated. Random distributions of the phases of the modes around equal phase angles introduce only limited fluctuating background, whereas the duration of the pulses remains bandwidth-limited. A systematic deviation of the phase angles at least quadratic with mode number is necessary for lengthening of the pulses. The results obtained by the superposition of these two effects are consistent with the experiments published so far. The efficiency of second- and higher-harmonic generation is discussed by help of the moments of the intensity probability distributions. It is shown how these distributions change as a result of harmonic generation and optical mixing. From intensity-correlation measurements, the moments of the intensity probability distribution can be obtained directly, as well as information on the time behavior of the light field. Experimental arrangements used to measure intensity correlations are described. © 1969 The American Physical Society.