The key component that revived coherent fiber communications in the mid-2000 was high-speed digital signal processing. In the 1990s, coherent receivers used optical phase-locked loops (OPLL) to align the phases and dynamic polarization controllers to match the polarization of the received signal with that of the LO. However, dynamic polarization controllers are bulky and expensive , and each channel of a WDM system needs a separate polarization controller. Phase locking in the optical domain using OPLL is difficult as well. With the advances in high-speed DSP, phase alignment and polarization management can be done in the electrical domain, as discussed in Sections 11.5 and 11.7, respectively. Linear impairments such as chromatic dispersion (CD) and polarization mode dispersion can be compensated using equalizers, as discussed in Sections 11.6 and 11.7, respectively. It is also possible to compensate for the interplay between dispersion and nonlinearity by using digital back propagation (DBP), in which the nonlinear Schrödinger equation is solved for a virtual fiber whose signs of dispersion, loss, and nonlinear coefficients are opposite to those of the transmission fiber. DBP is discussed in Section 11.8.
11.2 Coherent Receiver
Fig. 11.1 shows a schematic of the coherent IQ receiver with digital signal processing. The in-phase and quadrature components of the received signal can be written as (see Chapter 5, Eqs. ( ...