OPTICAL COUPLING SYSTEM

PROBLEM TO BE SOLVED: To attain a high output and high reliability by a method wherein lights coupled as clad mode lights are efficiently converted into a propagation mode in a semiconductor laser module which couples the lights emitted from a semiconductor laser to a signal mode optical fiber. SOLUTION: A signal mode optical fiber is constituted by a core; a first clad having a smaller refractive index than the core and formed in the periphery of the core; and an outer peripheral layer having a smaller refractive index than the first clad and formed in the periphery of the first clad. Also, in an optical path of the signal mode optical fiber, there is provided a diffraction lattice which converts laser beams coupled as clad mode lights into propagation mode lights propagating in the same direction.