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 fundamental and higher-order mode shapes agreed with theoretical models. Quantitative measurements of modal discrimination substantiate the predictions of large discrimination between the fundamental and second-order modes.
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