This paper describes new scheme for obtaining a differential signal to lock an optical cavity at a resonance peak. This scheme utilizes a unique property of non-planar cavities; due to an additional phase factor originated from the geometric configuration of the optical path, the two circular polarizations produce two resonance peaks. An ideal signal that crosses zero at the resonance peaks can be obtained using a simple setup consisting of a polarizing beam splitter and photo-diodes. The principle and the results of an experiment are also presented.
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