FIGURE 3.2 Recorded experimental data constellation of optical 1024-quadrature-amplitude-modulation (QAM) signal (60 Gb/s) using coherent optical transmission over 150 km. The two axes are for in-phase and quadrature directions, with the total constellation representing data points encoded in amplitude and phase without (a) and with (b) digital nonlinear compensation. SOURCE: Reprinted, with permission, from Koizumi Y., K. Toyoda, M. Yoshida, and M. Nakazawa. 2012. 1024 QAM (60 Gb/s) single-carrier coherent optical transmission over 150 km. Optics Express 20(11):12508-12514.
Space Division Multiplexing (SDM). After time, wavelength, and polarization multiplexing, space multiplexing is now being investigated to further increase the transmission capacity. In a space-division-multiplexing system, each independent data channel is carried by an orthogonal spatial dimension. Two emerging approaches include using the following: (1) a special multi-core fiber, each individual core of which transmits an independent data stream;51 and (2) a few-mode fiber, with each independent data channel on one of the orthogonal spatial modes (see Figure 3.3). In both approaches, the key challenge is the crosstalk. Unique challenges for multi-core fiber systems include the following: (1) further increasing the number of cores, (2) decreasing the inter-core nonlinear effects, and (3) developing multi-mode/multi-core network elements, such as an erbium-doped fiber that has matched multi-cores, so that all cores can be amplified simultaneously in a single fiber element. For multi-mode systems, unwanted mode conversions among different spatial modes are a natural occurrence which leads to several-channel crosstalk.
51 The Climate Group. 2008. SMART 2020: Enabling the Low Carbon Economy in the Information Age. A report by the Climate Group on behalf of the Global eSustainability Initiative. Available at http://www.smart2020.org/_assets/files/02_Smart2020Report.pdf. Accessed June 26, 2012.