High-resolution spectral hole-burning spectroscopy at 580 nm with an ultrastable cw OPO

Summary form only given. In order to test the equivalence principle of general relativity, which encompasses the time invariance of fundamental constants and the universality of the gravitational red-shift, ultra-stable frequency sources are required with a fractional frequency instability of <10/sup -17/. For comparison, the best current frequency standards typically have frequency instabilities of /spl sim/10/sup -15/. An important aspect in the push forward towards lower levels of instability is to use optical transitions in atoms with ultra-narrow linewidths as frequency references. The narrowest known optical transition in a solid is in Eu/sup 3+/: Y/sub 2/SiO/sub 5/ at 4K. We have carried out the spectroscopy of this transition and burnt persistent spectral holes into the homogeneously broadened absorption with a hole linewidth <1 MHz. This is the first demonstration of high-resolution spectroscopy with a cw OPO in the visible spectral region.