ABSTRACT
Laser saturation spectroscopy is widely used as a reliable and technically simple way to produce spectra with sub-Doppler resolution. Widespread application is found in the frequency stabilization of laser systems. The physical principle is based on the selective saturation of the inhomogeneously broadened transition. This appears as a “hole” in the velocity population distribution of the ground state, which can be probed by a portion of the beam of the same laser. The narrow dip can be used to resolve closely spaced absorption lines even if they would be completely washed out in the normal, Doppler-broadened spectrum. If applied to the stabilization of a laser, the narrow saturation dip can serve as a frequency standard. Transitions with small homogeneous line width in molecules without permanent dipole moment are preferred candidates for this; they can be readily utilized with this method and do not depend too much on the complexity of their spectrum. The wide application for ultra-high-resolution spectroscopy is not restricted to atoms at rest. Using lasers of very different wavelengths, saturation spectroscopy was applied even for the spectroscopy of relativistic ion beams in an accelerator.
