A diode-laser optical frequency standard based on laser-cooled Ca atoms: Sub-kilohertz spectroscopy by optical shelving detection

We report an optical frequency standard at 657 nm based on laser-cooled/trapped Ca atoms. The system consists of a novel, compact magneto-optic trap which uses 50 mW of frequency-doubled diode laser light at 423 nm and can trap > 10 Ca atoms in 20 ms. High resolution spectroscopy on this atomic sample using the narrow 657 nm intercombination line resolves linewidths (FWHM) as narrow as 400 Hz, the natural linewidth of the transition. The spectroscopic signal-to-noise ratio is enhanced by an order of magnitude with the implementation of a “shelving” detection scheme on the 423 nm transition. Our present apparatus achieves a fractional frequency instability of 5 × 10−14 in 1 s with a potential atom shot-noise-limited performance of 10−16 τ−1/2 and excellent prospects for high accuracy. PACS. 32.80.Pj Optical cooling of atoms; trapping – 42.55.Px Semiconductor lasers; laser diodes – 42.62.Fi Laser spectroscopy