Fused taper/filter hybrid three-channel single-mode wavelength division multi/demultiplexer

The dominant choice of wavelengths in wavelength division multiplexing has been so far 1.3 and 1.55 μm, because these two bands correspond to the low-loss and wideband (electrical) regions of a single-mode fiber in the optical spectrum. Consequently, such interband wavelength division multi/demultiplexers (WDMs) have been developed by using a fused taper coupler technique or an interference filter technique,1-3 and they are widely available now. To increase the number of channels further, however, these two techniques encounter very difficult problems, if not insoluble, in terms of channel spacing in the case of a fused taper WDM or insertion loss in the case of a filter WDM.