Evolution of Digital Optical Modulation Formats
To meet these growing bandwidth demands, optical communications solutions are gradually replacing baseband electronics and radio frequency (RF) systems, owing to the large absolute bandwidth available at optical carrier frequencies.* ™is process started on a large scale in the late 1970s and 1980s with the most demanding high-bandwidth/long-distance applications of terrestrial  and submarine  transport. With massive ˜ber-to-the-home deployments now underway worldwide, optics is currently capturing the access space , and rack-to-rack interconnects are starting to become optical . Despite the continuing improvement in electronic transmission techniques , optical solutions are expected to enter backplanes, paving the way to optical chip-to-chip and, eventually, on-chip communications once electronic transmission can no longer keep pace with the growing need for communication capacity [3-5]. At the same time, areas where optical communication techniques are already well established have to continue to support exponentially increasing capacity demands. Fiber-optic networks are therefore playing a pioneering role in pushing the boundaries of high-capacity digital data transport. Many concepts and technologies originally developed by the ˜ber-optics industry have been entering (and will most likely continue to enter) other areas of digital communications at a later point in time, adapted and augmented to meet the respective digital communication applications’ needs. For example, while optical interconnects are still largely based on single-wavelength (or at the most on few-wavelength) transmission, dense wavelength-division multiplexing (WDM) with up to ~100 wavelengths per ˜ber, well established in ˜ber-optic networking today, is expected to enter the interconnect space once the amount of parallel ˜ber needed to support the growing interconnect bandwidth demands becomes a heavier economical burden than the cost of WDM transponders. Note in this context that the local area networking standards developed for 100G Ethernet interconnects are exclusively based on multiple parallel ˜bers or few-wavelength WDM . Going to higher Ethernet rates (400 Gb/s and, eventually, 1 Tb/s) will likely require even more wavelengths for cost-eŸective interconnect solutions .