We measured the higher-order modes of a diffractive-optic graded-phase
resonator by suppressing the lower-order modes of a flash-lamp-pumped
Nd:YAG laser with spatial filters. A diffractive-optic end mirror was
used to establish a flattop fundamental mode. Measurements of the fun
damental and higher-order mode shapes agreed with theoretical models.
Quantitative measurements of modal discrimination substantiate the pre
dictions of large discrimination between the fundamental and second-or
der modes.