A multimode optical fibre is tapered down to a single mode fibre over 10 to 20 mm. The taper is then sandwiched against a high index prism in the manner of the prism thin-film coupler. Single guided modes can then be launched from a TEM00 mode laser beam impinging on the fibre through the prism at the correct angles of incidence and skew. Modes are selected by adjusting the position of incidence along the taper and the angles. The device can be operated in reverse as a mode selective detector. The preparation of tapers will be described and the results of experiments in which each of the guided modes of a fibre from the LP11 to the mode cut off at the LP61 were excited in turn. The technique can also be used to excite the highest order modes of non-tapered fibre. This is useful for examining propagation of modes very near to cut-off for testing purposes. We envisage that the taper launcher will be most valuable In studying mode dependent loss and coupling mechanisms in fibres and particularly in fibre cables which do not allow the study of radiative loss mechanisms directly.

© 1975 OSA

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