PURPOSE: To reduce the effect of an unnecessary reflected light by a method wherein a reflected light to be an interference light is separated from a transmission channel of an incident light by providing a coupler for a sensor in the middle of the channel of the reflected light. ;CONSTITUTION: A light source 11 emits a laser light FM-modulated by a sine wave signal and an optical gate 12 makes the light a pulse periodically in a prescribed time interval. One branch of the pulse obtained by a coupler 13-1 for branching is divided and supplied to a sensing fiber 15-1 and a reference fiber 16-1 by a coupler 14-1 for a sensor which is a bidirectional optical coupler, and the divided pulse is reflected after it reaches reflectors 17-1 and 18-1 with Faraday rotators, and returns again to the coupler 14-1. This reflected light is made to branch to an incidence port (connected to the coupler 13-1) and a port (connected to a coupler 19-1 for coupling) being opposite to the incidence port. The reflected light sent to the coupler 19-1 in this way is sent to an O/E converter 20 through a reflected light transmission channel for exclusive use.;COPYRIGHT: (C)1996,JPO
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