Observation of a Sub-10-Hz Linewidth ⁸⁸Sr+ \ \ 2S1/2–²D5/2 Clock Transition at 674 nm

In this paper, a pair of diode lasers at 674 nm, locked to two independent ultralow-expansion high-finesse cavities, have been characterized by monitoring the beat frequency between them. The individual laser linewidth of 1.4 Hz for a 3-s averaging time broadened to 4 Hz at 30 s. The relative frequency stability of the beat at 1s is 2.5 10-15. One of the lasers has been used to interrogate the88Sr+2Si1/2-2D5/2transition at 674 nm, and a transition linewidth of 9 Hz has been observed