Stabilization of mode-locked trains, and dark resonance of two-photon lambda-level structures

Author： Arissian Ladan Jones Jason Diels Jean-Claude

ISSN： 1362-3044

Source： Journal of Modern Optics, Vol.49, Iss.14-15, 2002-11, pp. : 2517-2533

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Abstract

Our ultimate objective is to design a combined frequency standard for optical as well as radio frequencies. A mode-locked laser provides frequency components that can be used as a ruler to measure any unknown optical source through direct beating. The frequency spacing of a pair of teeth of this comb is in itself a radio frequency reference. Fast control and correction for both the average frequency and the repetition rate of a mode-locked Ti:sapphire laser are achieved by locking the laser to a reference cavity of ultra-low expansion quartz with equal mode spacing. We measure an optical frequency with a mean square deviation of 700Hz, instability limited by the radio-frequency sources used to count the repetition rate. As a reference standard to achieve absolute accuracy, we use the Lambda transition 5S^1/2(F = 1) → 5D^5/2(F = 3) → 5S^1/2(F = 2) of rubidium. The theory for this coherent interaction shows that, with one mode resonant with the two-photon 5S^1/2(F = 1) → 5D^5/2(F = 3) transition, the fluorescence goes through a resonance for a change in repetition rate of less than 10kHz. These results suggest that, by locking to the peak of that resonant feature, optical stability and absolute accuracy better than 1kHz can easily be achieved.