Contents I. Introduction
A standard single-mode fiber can theoretically carry up to 30~50-Tb/s capacity. However, today most advanced commercial transmission systems can only support up to 10-Tb/s capacity (with 100 optical channels and 100-Gb/s per-channel data rate). It is clear that there is still a big gap between the theoretical capacity limit and today most advanced transmission data rates. To fill the gap, effort has been taken to develop various more spectrum-efficient transmission techniques. In the time domain, the techniques are evolved to transmit from 2.5-Gb/s, to 10-Gb/s, to 40-Gb/s, and to 100-Gb/s data rates. In the frequency domain, we see the evolution from coarse wavelength division multiplexing (CWDM) to dense WDM (DWDM). Moreover, the channel frequency spacing also tends to be smaller from 200 GHz, to 100 GHz, to 50 GHz, and to 25 GHz. In addition, new multiplexing techniques are being considered as an important dimension to increase optical transmission capacity. Mature multiplexing techniques include time division multiplexing (TDM), WDM, and recent polarization mode division multiplexing (PDM). With the advent of multi-mode and multi-core fiber transmissions, mode division multiplexing (MDM) and fiber division multiplexing (FDM) also join the multiplexing camp. In addition, advanced modulation formats, such as DQPSK, QAM, and so on, and coherent transmission technique are another two key enablers for optical transmission systems to further increase the capacity.
