A diode-laser optical frequency standard based on laser-cooled Ca atoms: Sub-kilohertz spectroscopy by optical shelving detectionC. W. Oates, F. Bondu, R. W. Fox and L. HollbergEur. Phys. J. D, 7 3 (1999) 449-460DOI: https://doi.org/10.1007/s100530050589