  We report an experimental study of the 1064nm transition dipoles in neodymium
doped yttrium aluminum garnet (Nd-YAG) by measuring the coupling constant
between two orthogonal modes of a laser cavity for different cuts of the YAG
gain crystal. We propose a theoretical model in which the transition dipoles,
slightly elliptic, are oriented along the crystallographic axes. Our
experimental measurements show a very good quantitative agreement with this
model, and predict a dipole ellipticity between 2% and 3%. This work provides
an experimental evidence for the simple description in which transition dipoles
and crystallographic axes are collinear in Nd-YAG (with an accuracy better than
1 deg), a point that has been discussed for years.
