Optical fiber coupler and optical fiber amplifier

An optical fiber coupler comprising:

• a first optical fiber having a core, an inner clad coating the core, and an outer clad coating the inner clad and having a smaller refractive index than the inner clad, signal light being capable of propagating through the core and pumping light being capable of propagating through the inner clad and the outer clad;
• a second optical fiber having a core, the pumping light being capable of propagating through the core as multimode light;
• and
• a fusion-drawn portion formed by arranging the first optical fiber and the second optical fiber so that their longitudinal directions are in the same direction and fusing the outer clad of the first optical fiber and the core of the second optical fiber,
• wherein, in a predetermined length along a longitudinal direction of the fusion-drawn portion, width of the first optical fiber and width of the second optical fiber respectively become gradually smaller from one longitudinal end of the fusion-drawn portion toward a center of the fusion-drawn portion, and
• wherein the outer clad of the first optical fiber has a refractive index not smaller than that of the core of the second optical fiber.