  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 perform 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.
