Abstract

Efficient conversion of laser frequency noise to amplitude noise takes place when a laser propagates through a resonant medium. This is demonstrated in the case of a solitary diode laser having the usual frequency or phase noise with a bandwidth of ~30 MHz but very low amplitude noise, propagating through a Doppler broadened K vapor cell. The lineshape of the noise as a function of laser frequency near the K resonance has the characteristic “M” shape of FM spectroscopy. Measurements of the RF noise spectrum of the emergent beam in the case where the diode laser is near the 770 nm resonance of K. reveals the ground state hyperfine structure of the K atoms at 460 MHz. (See Fig. 1 for the relevant energy level diagram). In previous observations (1) for Rb and Cs, excited state hyperfine structure was observed in the RF spectra of the noise, though in a more recent report on the diode laser noise spectroscopy of Rb (2) the observation of hyperfine peaks is not reported. Our experiments reveal that the K ground state hyperfine feature, shown in the noise spectra in Fig. 2, are observed only under very specific experimental conditions of laser detuning, polarization and intensity. The feature is present only for circular polarization of the laser and becomes strong when the laser is detuned to the low frequency side of the atomic resonance and has an intensity corresponding to fairly large Rabi frequency.

© 1996 Optical Society of America