Compact frequency standard at 1556 nm based on the two-photon transition in rubidium at 778 nm

Abstract

This is a progress report on the realization of a compact and transportable frequency standard at 1556 nm based on a two-photon transition in rubidium at 778 nm. These hyperfine transitions present great metrological interest. They have a narrow theoretical linewidth of 150 kHz when observed with a 1556 nm laser, and their absolute frequency is known with an uncertainty of 5.2 X 10<SUP>-12</SUP>. In this experiment, we use a high power 1556 nm DFB laser and reduce its linewidth to the 10 kHz level using optical feedback from a confocal cavity. We generate its second harmonic in a periodically poled LiNbO<SUB>3</SUB> crystal and use this signal to injection-lock a Fabry-Perot laser emitting 42 mW at 778 nm. The slave laser is used to observe the Doppler-free two- photon transitions: two counter-propagating beams excite rubidium atoms which emit a blue fluorescence on resonance. This 420 nm light is detected on the side of the Rb cell with a photomultiplier. Such an optical frequency standard at 1556 nm, standing in the multiwavelength telecommunications systems window, becomes an attractive source for absolute frequency calibration of WDM components, optical spectrum analyzers and wavemeters.