Multi-core optical fiber, and wavelength division multiplexing coupler, and multi-core optical fiber amplifier

PURPOSE: A multi core optical fiber, a wavelength division multiplexing coupler for the multi core optical fiber, and a multi core optical fiber amplifier are provided to use few pumping light sources in which structure is simple. CONSTITUTION: A multi core optical fiber amplifier comprises double-clad multi core optical fibers (601, 608), pumping light sources (609, 610), an optical fiber, and a wavelength division multiplexing coupler (603). The double-clad multi core optical fiber comprises multiple cores, inner cladding covering the multiple cores, outer cladding covering the inner cladding. The pumping light sources output pumping light. The pumping light from the pumping light sources is inserted into the optical fiber. The wavelength division multiplexing coupler controls so than the pumping light which inserted into the optical fiber form the pumping light sources is applied to the multi core optical fiber by coupling the optical fiber with the double-clad multi core optical fibers.