Doppler-free two-photon absorption spectroscopy with a frequency-modulated dye laser

A new method of Doppler-free two-photon spectroscopy has been developed in which a frequency-modulated (FM) dye laser is used to perform high-resolution spectroscopy. This provides an ideal method of locking a broadband FM laser to a narrow atomic or molecular reference line, a necessary step in using the FM laser for optical-frequency metrology. The method may also be used to test how closely the FM dye laser approaches a pure FM oscillation.