Rb atomic absorption line reference for single Sr+ laser cooling systems

The near coincidence of the Rb-85, 5s S-2(1/2)(F " = 2) --> 6p P-2(1/2) (F' = 2, 3) absorption resonance with the Sr-88(+), 5s S-2(1/2) --> 5p P-2(1/2) transition is exploited to provide a simple, effective frequency reference for a laser cooling/fluorescence excitation source applied to single Sr+ ions. A modulation-free frequency stabilization system has been designed which uses the differential signal from two frequency-displaced beams traversing a Rb cell and which probe the Doppler-broadened Rb S-P lineshape at microwatt power levels. The method is applied to frequency lock a 422-nm frequency-doubled diode laser system that is used for excitation of a single Sr-88(+) ion. Stable, long-term laser cooling and fluorescence are achieved using the frequency-stabilized 422-nm source resulting in observed ion confinement times without adjustment of over 8 h, together with an improvement in single-ion loading efficiency.