We describe theoretical and experimental demonstration for optical detection of ultrasound using a spectral hole engraved in cryogenically cooled rare-earth ion doped solids. Our method utilizes the dispersion effects due to the spectral hole to perf
orm phase to amplitude modulation conversion. Like previous approaches using spectral holes it has the advantage of detection with large etendue. The method also has the benefit that high sensitivity can be obtained with moderate absorption contrast for the spectral holes.
