Excitation of two-level atoms in mode-locked laser fields

Lin-sheng He

S. Tudorache

E. Krüger

The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.

Timothy Carty, M. Sargent

We consider a single-mode unidirectional ring laser containing a homogeneously broadened two-level medium whose atoms are injected in a coherent superposition of states. Provided that appropriate phase-matching conditions are met, the injected coherence contributes directly to the polarization of the medium and hence to the laser gain. We assume that the field is classical and perfectly phase matched with the injected coherence. We find that laser oscillation can occur below the usual laser threshold, and even in the absence of an inversion, with a frequency locked to the atomic line center. Mode locking is unstable for sufficiently strong relative excitation.

Constantine Mavroyannis

Man Mohan, Vinod Prasad