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Optical fiber directional couplers fabricated by fused biconical-taper technology have found widespread use in optical communications transmission equipment, in optoelectronic instruments and sensors and in the laboratory proper. Such a broad range of applications is a direct result of the versatility of the biconical-taper approach to the implementation of various types of spitter devices, manifest by the impressive range of high-performance devices that can be made with comparative simplicity using any type of silica fiber, specifically multimode step-or graded-index fiber and monomode polarization-perserving or telecommunications fiber. Devices that have been fabricated from multimode fiber include four-port directional couplers, star couplers with 400 and even more ports and power combiners - the latter using special techniques to ensure the required asymmetric coupling. Notably, combinations of such devices have been successfully incorporated into star-, linear-and hybrid-topology fiber-optic data buses. Four and six-port directional couplers have been made from both monomode polarization-maintaining and standard fiber designed for telecommunications applications. As well, a biconical-taper monomode optical fiber splitter having a continuously variable coupling ratio which can also be operated as a mechanically activated optical by-pass switch for use in fiber optic local area networks has been demonstrated. Recent innovations include a tuned-port low-mode number device that shows very strong asymmetric coupling. In this paper the devices that can be made using fused biconical taper technology will be classified and their operating principles reviewed. Device properties will be discussed, where possible with reference to fundamental optical characteristics of optical bicones, and techniques for tailoring these properties to specific applications, such as wavelength division multiplexing will be explained. Also potential sources of noise arising from the use of monomode and multimode splitters in combination with lasers will be identified. Finally applications of these devices that are either unique or of general interest will be surveyed.
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