Introduction With data traffic volume increasing by around 40% each year, the prevailing 10 Gb/s optical networks are quickly becoming saturated. As the long-haul technology research tries to avert the ‘capacity crunch’ with novel fibers, advanced optical components and sophisticated digital signal processing, the change in technology for metro networks, enterprise and datacenters has to be smooth yet swift. For the roadmap to 100G metro networks, economic viability is of paramount importance together with greater space, power and bandwidth efficiency. It is essential that the upgrade takes advantage of the current infrastructure with minimal disruption to existing services and is inherently flexible to further accommodate newer equipment as per demand. Overlaying flourishing 10 Gb/s services with additional co-propagating 10 Gb/s channels in different colors, or wavelengths, is already common practice. Network operators eager to increase capacity have begun overlaying 40 Gb/s channels onto available fiber without leasing more dark fibers as the best pragmatic approach. With the advent of 100G technology, the interest is now shifting from 40G to 100G installations. This paper provides a primer in 100G technology developments and examines important parameters and prerequisites supporting practicable 100G metro network solutions. An example of complementing the existing 10 Gb/s services with new 100 Gb/s WDM channels is also discussed. Figure 1 Preconditions and requirements for 100G metro evolution Direct or Coherent Detection: Which one is for Metro? The transmit signal strength is limited by laser heat dissipation and power consumption. Consequently, significant research and development efforts have been undertaken to improve the sensitivity of the receiver. Two technology alternatives exist: direct detection or coherent detection. Predominantly suited to trans-oceanic submarine or terrestrial long haul applications, the performance of the coherent detection is undeniably superior to that of direct detection. In the case of coherent detection, the opto-electric conversion process is linear. Thus the phase information embedded in the optical signal is preserved permitting the straight forward electrical compensation of fiber linear effects including chromatic dispersion (CD) and polarization mode dispersion (PMD). However, the hardware required to perform coherent detection is somewhat more elaborate and comprises a local oscillator, a 90° Hybrid module necessary to discriminate the phase quadrature’s of the received optical signal, and four balanced photodiodes to detect the signal from a single polarization as seen in Fig. 2.
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