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Developments in fiber optic communications have been rejuvenated after the glut of the overcapacity at the turn
of the century. The boom of video-centric network applications finally resulted in another wave of vast build-outs of
broadband access networks such as FTTH, DOCSIS 3.0 and WI-FI systems, which in turn also drove up the bandwidth
demands in metro and regional WDM networks. These new developments have rekindled research interests on
technologies not only to meet the surging demand, but also to upgrade legacy network infrastructures in an evolutionary
manner without disrupting existing services and incurring significant capital penalties. Standard bodies such as IEEE,
ITU and OIF have formed task forces to ratify 100Gb/s interface standards.
Thanks to the seemingly unlimited bandwidth in single-mode fibers, advances in optical networks has
traditionally been fueled by more capable physical components such as more powerful laser, cleaner and wider
bandwidth optical amplifier, faster modulator and photo-detectors, etc. In the meanwhile, the mainstream modulation
technique for fiber optic communication systems has remained the most rudimentary form of on-off keying (OOK) and
direct power detection for a very long period of time because spectral efficiency had never been a concern.
This scenario, however, is no longer valid as demand for bandwidth is pushing the limit of current of current
WDM technologies. In terms of spectral use, all the 100-GHz ITU grids in the C-band have been populated with 10Gb/s
wavelengths in most of the WDM transport networks, and we are exhausting the power and bandwidth offered on
existing fiber plant EDFAs. Beyond 10Gb/s, increasing the transmission to 40Gb/s by brute force OOK approach incurs
significant penalties due to chromatic and polarization mode dispersion. With conventional modulation schemes,
transmission impairments at 40Gb/s speed and above already become such difficult challenges that the efforts to manage
these problem have severely hindered the rate of return on the investment from an economical viewpoint, let alone
100Gb/s transmission.
In addition, to enable fast turn-up of new services and reduce network operation costs, carriers are also
deploying reconfigurable optical add/drop multiplexers (ROADMs) and transparent optical networks. ROADMs impose
more impairments to transmitted signals and are important considerations in designing backbone transmission links.
Recently, advanced modulation schemes have been investigated in both the academia and industry as ways to
improve the spectral efficiency and alleviate transmission impairments. Signal processing techniques familiar to
traditional telecommunication engineers are also playing more and more important roles in optical communications
because of the fast advance in mixed signal processing and growing abundance of computational power.
In this invited talk, we review the current challenges faced in upgrading existing 10Gb/s metro and regional
WDM networks and the potential solutions to enable 40 and 100Gb/s wavelength services.
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