An optical fiber coupler, a process for fabricating the same and an optical amplifier using the sam

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The optical fiber coupler comprises : a rare earth-doped fiber (4) as an optical amplifying element; a quasi rare earth-doped fiber (2) for entrance of input signal light; a fused-stretch fiber portion (3) as a multiplexing element which connects a part of the rare earth-doped fiber and a parts the quasi rare earth-doped fiber, wherein the fused-stretch fiber portion formed by stretched a fused mixture of parts of the rare earth-doped fiber and the quasi rare earth-doped fiber; and a single mode (1) fiber which is jointed at a fused joint portion to another shortened rare earth-doped fiber connected to the fused-stretch fiber portion on a side connected by the quasi rare earth-doped fiber. The quasi rare earth-doped fiber has substantially equal propagation constant to the rare earth-doped fiber without substantially containing rare earth elements. The optical fiber coupler is fabricated by the steps of jointing a single mode fiber with a rare earth-doped fiber by fusing abutted end faces of both fibers to form a combined fiber; and fusing parallel contact parts of a quasi rare earth-doped fiber and of the rare earth-doped fiber of the combined fiber and then elongating the fused parts in a desired diameter to form a fused-stretch fiber portion.