An all-diode-laser optical frequency reference using laser-trapped calcium

The 657 nm calcium intercombination line is an attractive optical frequency/length reference due to its high line Q (10¹²) and insensitivity to external perturbations. In this paper we describe progress towards an all-diode-laser-based reference using laser-trapped Ca. To generate 35 mW of 423 nm trap light, we double the frequency of a 150 mW, 846 nm diode laser with a nonlinear crystal (KNbO₃) inside a resonant build-up cavity. We use this light to trap 2×10 ⁷ atoms in a compact, magneto-optic trap. Spectroscopy of the narrow 657 nm transition is performed with a frequency-stabilized diode laser system. With the trapped atoms we have measured optical Ramsey fringes with a linewidth of 6 kHz. We discuss future improvements as well as potential performance which could achieve a fractional frequency instability of <3×10^-15