Single-mode 2×2 optical fibre couplers have important applications in optical fibre networks, instrumentation and sensor systems. Bidirectional couplers made using the fused biconical taper technique are now provided by a large number of manufacturers. The transmission characteristics are inherently wavelength and polarization dependent, which may be of use for wavelength multiplexers and polarization splitters respectively, but is unwanted for optical power splitters [1]. The COST 217 project group decided in October 1988 to start a new Working Group (WG5) in order to develop measurement procedures for optical characterization of these and other branching devices. Up to now final agreement on such procedures is still lacking [2]. Like it was done in the other working groups [3], the 8 participants decided to start an interlaboratory comparison of measurements using different techniques. For this round robin 7 fibre couplers (named A to G) were circulated among the participants. The couplers, constructed with two identical 1300 nm single-mode fibres, were either 1300/1550 nm wavelength multiplexers or power splitters. After discussion of the first measurement results, the definitive experimental procedures were agreed on. These procedures as well as some experimental results are summarized in this paper. The definitions of the optical parameters of fibre couplers used in this paper are given in Figure 1. These properties can be measured as a function of wavelength and as a function of linear polarization angle with the degree of polarization of the light beam launched into the coupler to be 0 and 1 respectively. By rotating an analyzer in the beam, the maximum (I_(1)) and minimum (I_(2)) transmitted intensity can be determined. The degree of polarization P is then defined to be: P=(I_(1)-I_(2))/(I_(1)+I_(2)) (1)
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