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Low-loss lensless 2-channel wavelength-division multiplexing (WDM) couplers may be produced on the beam-splitter principle with interference filters and three butt-coupled fiber ends not only for multi-mode Operation but also for single-mode transmission links. These devices are constructed with single-mode fibers-for bidirectional transmission only in the transmission channel, but for unidirectional operation in all channels. The interference filter, used in the experiment to separate the emissions of two LD's with wavelengths of 834 and 868 nm, may be implemented both as a longwave-pass (LWP) and a shortwave-pass (SWP) edge filter with high edge steepness and low polarization effects. WDM single-mode couplers exhibit very low insertion losses lying in the O.5-dB region. Their fat-end crosstalk attenuations attain values of between 15 and 30 dB and their near-end crosstalk attenuations are better than 65 dB.