Beat spectrum of a dye laser generating three nonequidistant modes

The optical beat spectrum of a three-mode jet dye laser was investigated. When the modes became nonequidistant the combination tones shifted from their calculated frequencies, the intensity became modulated at the frequency of the deviation from the equidistant distribution, and the generated modes acquired side frequencies. The beat spectrum showed not only splitting of the fundamental frequencies, but was also enriched with components of frequencies which were single and double multiples of the difference between the intermode frequency intervals. Technical fluctuations of these differences, associated with an instability of the optical length of the empty cavity, were correlated and were compensated in the low-frequency difference components.