MODE STRUCTURE FLUCTUATIONS IN A PULSED DYE-LASER

We have measured and analyzed high-resolution single-pulse spectra of a typical commercial dye laser. The longitudinal mode structure was resolved by using a Fizeau interferometer as the spectrum analyzer. In mode intensities we observed strong pulse-to-pulse fluctuations that are caused mainly by the variations of the spontaneous emissions to the laser modes during the starting phase of the laser pulse. Apart from the quantum noise, frequency jitter, i.e., shifts of the whole frequency comb, was also observed. The jitter appeared to be very fast, of the order of a few megahertz and hence cannot be explained by technical noise alone. A detailed knowledge of the spectral fluctuations is needed in theoretical models of noise effects in nonlinear spectroscopy. It can also be applied to novel noise reduction techniques.