On the frequency-dependent optical length of a laser resonator with an internal Fabry-Perot etalon

Recently, it was shown that the frequency-dependent optical length of a laser resonator containing a Fabry-Perot etalon can be obtained by a linear average over the paths of the partial waves inside the etalon. The author shows that the resonance frequencies obtained from the optical length so derived are not consistent with those expected from the phase-shift considerations.<
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