Passively biased fiber-optic Sagnac interferometric sensor architecture, for gyroscope and current sensor in particular, is disclosed. One embodiment uses a 3.times.3 coupler entirely made of circular polarization maintaining fiber that serves as a beam splitter and meanwhile a passive bias. An alternative is to use a 3.times.3 hybrid coupler consisting of two linear polarization maintaining fibers and one conventional single-mode fiber, with the former two connected in a common interferometric sensor circuitry, and with the latter one cut short at both ends to form matched terminations. Still another alternative is an integral unit of Faraday rotator, whose central part is a fiber-optic magneto-optic 45.degree. rotator, with a "zero to fast" fiber-optic quarter wave plate attached to one side, and a "fast to zero" fiber-optic quarter wave plate to the other. Advantages of passive bias are simplicity in construction, no need of manual adjustment and operational stability.