Phase Locking of Modes in Lasers

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Abstract
An investigation is made of the phase‐locking effects in lasers. It is shown that the phases of the simultaneously oscillating modes of lasers are not independent variables as sometimes thought. The calculations have been limited to three modes and homogeneously broadened lines. It is found that the sum of the phases of the low‐ and high‐frequency modes as measured relative to the phase of the center mode assumes a definite value, which depends in turn upon the location of the active medium within the cavity. This phase relationship has a profound effect on experiments conducted with multimode lasers. For example, it is found that there should be essentially no fundamental‐frequency beat note as observed by a square‐law detector when a small laser crystal is placed in the center of an optical cavity.