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A noise-free signal detection technique, based on the retardation method, is described. In this method, a phase-modulated light, after being transmitted through an optical transmission line, is divided into two beams in an interferometer, and temporal correlation is made on optical phase for these light beams. The original signal is regenerated by detecting light intensity variation caused by the interference phenomenon. The light detection part structure and its optimum parameters are described. The basic retardation detection system behavior was successfully confirmed experimentally. Characteristics of the retardation method are clarified, compared to those for the heterodyne detection method.