TRANSPARENCY OF OPTICAL NETWORKS: HOW TO 4GQKW-W MANAGE IT?

The concept of optical transparency in fibre networks and its positive and also negative impact on data transmission is reviewed and discussed. ITU-T Study Group 15 standardisation effort on optical transparency is reported. The introduction of Erbium-Doped Fibre Amplifiers (EDFA) which have replaced electronic regenerators in fibre based transmission links in early 90s resulted in optical transparency of the links. This was in contrary with electronic regenerator based links. In those a combination of electronic logic circuit along with electro-optical and opto-electrical conversions of the digital signal transmitted has been used in order to cope with signal distortion. In optical links the distortion results from physical limitations of the transmission of light signals through fibres, namely from fibre attenuation, dispersion, and nonlinear distortion. Electronic regenerators are opaque points in lightwave transmission systems, in a sense that the light cannot pass through them. EDFAs are transparent: the light signal can pass through and it is a subject of amplification. The opaque nature of electronic regenerators limits the transmission technology to use of only single data stream at the same time in so called pulsed IM/DD (Intensity Modulation / Direct Detection) systems. Obviously, they limit transmission to specific rate and format inherent to timing characteristics of the electronic logics and they do not support transmission of several signals at different wavelengths simultaneously. So the wavelength-division multiplexing (WDM) technology could not be introduced in those systems. It should be noted here that the notion of transparency has already been applied also for metallic cable based electrical links: those links are so called transparent if the output signal is proportional to the signal at the input. This suggests that the transparency is rather an analogue feature of a link, what is in contrary to digital transmission schemes and logic elements like electronic regenerators in early binary fibre communications: the output signal has mainly two levels (zero and one), dependent on the decision of the logic. Transparency in optical domain has also its common sense: the medium is transparent if the light goes through. The advent of EDFAs resulted in transparency of optical link, thus in a possibility of WDM transmission.