Ultralow long-term drift of cryogenic optical reference resonators

We have recently demonstrated that cryogenic sapphire reference cavities can be used to stabilize lasers to <3/spl middot/10/sup -15/ relative instability for integration times over minutes. Here, we report the direct proof of an ultralow long-term drift of this system by comparison of a cryogenic cavity with a completely independent molecular reference. As a first application, a CW optical parametric oscillator (OPO) was stabilized to this reference resonator, one of the important steps toward high-resolution spectroscopy with OPOs.