Adaptive Sagnac interferometer with dynamic population grating in saturable rare-earth-doped fiber.

Sagnac fiber interferometer with the dynamic population grating formed in the rare-earth doped fiber is proposed for homodyne adaptive detection of optical phase modulation. The configuration is shown to be a simple all-optical fiber sensor suitable for linear high sensitivity detection of mechanical vibrations, acoustic signals, thermo-optic effect etc. Theoretical consideration shows that the quadratic response of this interferometric configuration associated with the amplitude dynamic grating is observed in the reflected wave mainly, while the recorded phase grating results in the linear energy exchange between the transmitted and reflected from the Sagnac loop light waves.