Abstract: The present invention is to provide a multi-diameter optical fiber link, which includes a first cable and a second cable connected in series with the first cable through an adaptor (or adaptors) and is characterized in that a first optical fiber enclosed in the first cable has a smaller diameter than a second optical fiber enclosed in the second cable. Hence, when the first and second cables are connected in series, an end surface of the first optical fiber is easily and precisely aligned within an end surface of the second optical fiber, thus allowing the second optical fiber to receive all optical signals transmitted from the first optical fiber. Consequently, the optical signals pass through the first and second optical fibers in succession, and a unidirectional signal transmission is realized in the multi-diameter optical fiber link without signal deterioration which may otherwise result from misalignment of the optical fibers.