The use of fused single-mode directional couplers as highly efficient and stable beam splitters is well known1. The degree of coupling achieved in such a device depends critically upon the guidance conditions prevailing in the constituent fibres, and thereby upon the wavelength of operation. For example, Figure 1 shows the wavelength dependence of the power emerging from the two output ports of a device which is nominally a 3 dB coupler at 1300 nm. The variation in coupling ratio with wavelength can be exploited to form the basis of a range of low loss wavelength multiplexing devices.
"Wavelength Multiplexing By Means Of Low Loss Single Mode Fused Taper Couplers", Proc. SPIE 0522, Fibre Optics '85, (22 August 1985); doi: 10.1117/12.946219; https://doi.org/10.1117/12.946219