PURPOSE: To obtain coherent light even from light source light of low coherency by bringing blocks into contact with each other with inclination and disposing two fibers in such a manner that the fibers are folded back interdigitally from each other and are coupled to each other.

CONSTITUTION: The fiber 11 (extending to a-a') is embedded in the block 15 and the fiber 11 is cut by cutting off and polishing the fiber except the core shown by a black dot on a plane 13. The fiber 12 (extending to b-b') is similarly embedded in the block 16 and is similarly cut on a plane 14. The fibers 11, 12 are embedded in the blocks 15, 16 and the surfaces of the respective blocks are cut off and cut by polishing in such a manner that the cores of the respective fibers remain. The cut planes 13, 14 of the respective blocks are brought into contact with each other with the slight inclination and the fibers 11, 12 are so disposed as to be folded back interdigitally from each other and are coupled to each other. Then the light is propagated into an arrow direction in the fiber 12 by the interference when the light is propagated in the arrow direction into the fiber 11. The device acts as a light coupler.