Reproducible optical second-harmonic generation using a mode-locked laser

Experiments have shown a one-to-one correspondence between the intensity of a fundamental pulse of light and its second harmonic. This correspondence is observed when the fundamental is each pulse of light in one train of pulses generated by a mode-locked laser. Such fundamental light shows no variation in the spatial distribution of intensity from pulse to pulse in one train of pulses and no randomness in the phases and amplitudes of the locked modes. As a result of these conditions, the ratio of harmonic intensity to the square of the fundamental shows little or no deviation from the mean. To contrast this, the wide fluctuations in this ratio observed when the laser is not mode-locked are also presented. In addition, the deviation from a square law in the relatianship between harmonic and fundamental intensities in one burst laser pulse is demonstrated. These deviations are found less often when the laser cavity is short (40 cm) than when it is long (3 meters). They may be attributed to the appearance or disappearance of different modes of oscillation during the pulse and possible variations in the mode amplitudes.