No abstract

Controlled mode-locked laser cooling of sodium atoms has been demonstrated for the first time. The broadband laser spectrum covers most of the Dopplerbroadened atomic resonance spectrum. Atoms of a broad velocity range are thus decelerated simultaneously. In order to stop this process at a defined atomic velocity a two-mode laser beam tuned to resonance with both sodium groundstate levels is copropagating with the atomic beam and counterpropagating to the mode-locked laser beam. In this optical molasses sodium atoms of v=20m/s have been piled up to densities of about 2. 106cm -3. The density dependence on variable laser parameters has been investigated systematically.

A new scheme for locking an acoustic resonance of an acousto-optic modulator to the exciting rf field applied for laser mode locking is presented. The resonance of the mode-locking quartz prism is temperature-tuned to match the synthesizer frequency. Deviation from perfect matching is sensed by observing the relative phase of the rf wave reflected at the device, and the error signal is fed back by a servo loop adjusting the heat delivered by a heating transistor to maintain the temperature required for exact resonance. The performance of the resonance-tracking loop was checked on several different resonances of the modulator (used in an argon-ion laser), because switching to another resonance is a simple and attractive way for changing the laser pulse repetition rate (or mode spacing) without replacing the mode-locker unit. Excellent loop response was found in all cases. The new tracking principle provides a widely extended tracking range, improved tracking accuracy and laser stability, and simplifies the laser adjustment.