PURPOSE:To improve remarkably the measurement accuracy by inserting a double refractive substance into one of polarization plane maintaining optical fibers which have been made to branch to two, and making it have a sensor function. CONSTITUTION:The linearly polarized light which has been emitted from a light source 1 is made to branch to two by a polarization plane maintaining optical fiber directional coupler 3 through an optical fiber 2, and made incident on optical fibers 4, 5. In a sensor part 6, a double refractive substance whose refractive index is varied when a substance quantity is impressed is inserted and connected to the fiber 4. Subsequently, the fibers 4, 5 which have been fetched from the sensor part 6 are led to a coupler 8 and two beams of light are synthesized and made to interfere, its interference light P0 is fetched from an optical fiber 9, led to a receiver, and an intensity of the interference light P0 is measured. In this case, since the optical fiber itself is not made to function as the sensor used, two pieces of fibers 4, 5 can be set to the same condition, and especially, an intensity variation of the interference light caused by a temperature variation can be made small, the measurement accuracy is improved remarkably.
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