The DWDM multiplexer or de-multiplexer is the core technology providing a platform for adding essentially unlimited co-propagating channels to existing 10G and 40G channels with the 100G upgrades and eventually enabling the ability to upgrade all the channels to 100G. In DWDM, multiple wavelengths are used to carry the optical signals through the same fiber leading to higher spectral efficiency per fiber. Pass band bandwidth, pass band flatness, stop band rejection ratio, transition slope from pass band to stop band and insertion loss are the key specifications determining selection of the multiplexers and demultiplexers. Figure 4 Composition of an AWG Traditional thin film optical filters based DWDM multiplexers provide excellent wavelength selectivity. However, since these are free-space optical devices, much care must be taken to precisely align individual optical sub-components to achieve perfect collimation of the optical beams. Additionally, the complexity increases with the increase in the channel count. Thus, arrayed waveguide gratings (AWG) based on planar light wave circuit (PLC) technology, are widely used for multiplexing large numbers of channels. The AWGs are composed of integrated multiple waveguide delay lines to render multipath optical interference. As depicted in Fig. 4, an AWG consists of input and output waveguides, two star couplers and an array of waveguides inter-connecting the two star couplers. Each waveguide within the array manifests a slightly different optical length, leading to different interference at the output when the differing paths recombine. The AWG can be built into a 19” rack type installation as shown in Fig. 5. Figure 5 DWDM unit for 19” rack type installation Depending on the transmission power budget available from the transceiver (e.g., current DWDM 100G CFP are specified to a power budget equivalent to about 20km), an optical amplifier can be introduced to overcome the additional loss encountered from the fiber attenuation, optical power splitting, and other factors. For a DWDM application, an erbium-doped fiber amplifier (EDFA) is the best choice as it provides high gain, high optical signal power, low crosstalk between wavelength channels and convenient optical coupling from and into optical fibers.
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