Application of the three-mode operation of a broad-band laser in intracavity dispersion frequency-domain spectroscopy

The beats of a three-mode dye laser, containing a component with a frequency close to the difference between neighbouring mode spacings, the technical fluctuations of which cancel out, are considered. The aim is to develop a laser spectroscopic method with a detection limit of the order of 10^-10 — 10^-11 cm^-1 Hz^-1/2, determined by the natural frequency fluctuations and by the spectral resolution limited by the homogeneous width of the resonance line. Use was made of the unequal changes in two mode spacings of an equidistant three-mode spectrum, caused by the dispersion of the absorption line of an intracavity absorber, and of the detection of the resulting difference frequency.