Observation of a Sub-10-Hz Linewidth $^{88}\hbox{Sr}^{+ \ \ 2}\hbox{S}_{1/2}$–$^{2}\hbox{D}_{5/2}$ 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 the
⁸⁸
Sr
<sup>+

2</sup>
Si
_1/2
-
²
D
_5/2
transition at 674 nm, and a transition linewidth of 9 Hz has been observed