Frequency-dependent round-trip time in an optical cavity with a Fabry-Perot etalon

It is shown that the optical length of a laser cavity equipped with an intracavity Fabry-Perot etalon can be derived from the linear average over the paths of the partial waves. An explicit formula, the validity of which is confirmed by the usual phase-relation consideration applied in a mode-pulling theory, is obtained. An experiment to verify that this length has physical significance for the round-trip time is proposed.<
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