Technical Insight
Scientists calculate limits of fiber-optic communications (Fiber News)
The ability to send multiple wavelengths down a single fiber in DWDM systems has greatly increased the capacity of fiber-optic networks. However, the adoption of DWDM has imposed another set of limitations to capacity. In a letter to Nature, Partha Mitra and Jason Stark of Lucent Technologies Bell Labs (Murray Hill, NJ) estimated the fundamental information capacity of typical optical fibers in DWDM systems (Nature 2001 411 p1027). The capacity of fibers is ultimately limited by the noise introduced by optical amplifiers and the nonlinear response of the fibers. Current optical systems can transmit less than 2 Tbit/s, with laboratory experiments now demonstrating 10 Tbit/s. Mitra and Stark calculate that it is theoretically possible to transmit 100 Tbit/s down a single fiber. Fiber cores exhibit a nonlinear response to the range of wavelengths used in DWDM systems. Mitra and Stark claim that the information capacity of a fiber in a DWDM system is mostly limited by an effect called cross-phase modulation (CPM), in which each signal in the fiber disturbs neighboring signals. The optical waves in the fiber are intensity-modulated to encode the information in the signal. CPM arises because the modulated intensity perturbs the refractive index at the fiber core, thereby distorting the other signals present. As the power of an optical signal is increased, the information capacity also increases. This stops at a point where CPM effects begin to rise rapidly, causing a steep drop in capacity. The maximum capacity of the fiber increases with the bandwidth of each signal and the spacing between individual signals. Fiber capacity decreases with the total number of signals and also with the number of amplifiers in the system.
