Abstract

In a passively mode-locked femtosecond ring dye laser the two counterpropagating pulses have a relative frequency difference Av that can be adjusted from a few hertz to megahertz by rotation of the cavity. These opposing pairs of pulses can thus be used to establish coherence between sublevels of a A transition in a resonant vapor. Sagnac spectroscopy is the distortion of the (previously linear) Sagnac response of the laser when such an atomic vapor (Sm) is inserted in the cavity. Even though the bandwidth of this unstabilized laser is of the order of 10¹³ Hz, spectroscopic measurements can be performed with 10 Hz accuracy. Applications include magnetic field measurement, lasing without inversion, and giant index of refraction.

© 1992 Optical Society of America