WDM: From Interference Filters to Integrated Optics

Abstract

If the wavelength-division multiplexing (WDM) technique is used for optical communications with glass fibers, then wavelength-selective devices are in general required. Their function is to spatially combine or separate, at the route ends, the wavelength channels simultaneously transmitted unidirectionally or bidirectionally along a single glass fiber. Micro-optical WDM devices with interference filters have proved effective in initial experimental WDM fiber-optic links. Increasing use of WDM, which is of great interest for subscriber lines, in particular in its bidirectional form, would therefore lead us to expect an additional boost for the interference filter market. However, it should be noted that a series of other micro-optical WDM devices /1,2/ have been tested, to some extent as rivals to interference filter devices, and integrated-optical WDM devices /3/ will also be available in the future. Due to the good properties of standard single-mode fibers in the wavelength regions around 1.3 µm and 1.55 µm, the development of WDM devices is being concentrated on these wavelength regions. The parameters important for the application of these devices are insertion loss a, far-end crosstalk attenuation af (for unidirectional operation) and near-end crosstalk attenuation an (for bidirectional operation). Typical values are a = 1 dB, af = 20 dB and an > 40 dB.