PURPOSE: To facilitate the conversion of a light source, by performing the branching, synthesis and modulation of the light from the light source by two optical fibers attached by a connector and a receptacle before transmitting said light to an optical system part.

CONSTITUTION: Light from a light source 1 is coupled with a fiber 2 through a condensing lens 12 and the connector 3 at the other terminal of said fiber 2 is coupled with the receptacle 4 attached to a separate fiber 5. Emitted light of the fiber 5 is guided to an optical system part 7 by a lens 13. The optical system part 7 is constituted of a beam splitter 8, condensing lenses 9, 10 and a light receiving element 11.