Reflection type magnetooptic sensor

PROBLEM TO BE SOLVED: To provide sufficient intensity of signal even using a light with strong absorption by a bithmus substituted rare earth group iron garnet single crystal film (BIG), by arranging two of a light inputting and a light outputting passages near a focal point of a lens and forming a polarizer and a board into a taper shape having a specified angle of inclination. SOLUTION: Optical fibers 6, 7 of a light inputting and a light outputting passages fixed in capillaries 8, 9, a cylindrical lens 5, a polarizer 3, and a BIG 1 provided with a board 2 formed with a mirror 4 are housed in a case 10, and sealed with the resin 11 so as to form a reflection type optical magnetic field sensor. At this stage, the optical fiber 6, 7 are arranged near a focal point of the lens 5, and the polarizer 3 and the board 2 are formed into a taper shape having an angle of inclination at 3-20 degree. With this structure, intensity of the reflected light at a coupling part of the lens 5 and the polarizer 3 is remarkably smaller than the reflection at a coupling part of the inputting side optical fiber and the lens 5, and the reflected light, which enters the outputting side optical fiber, is remarkably reduced. Input of the reflected light to be a cause of the generation of noise is reduced, and the sufficient intensity of signal is obtained.