I. Introduction
The development of the standard single-mode fiber (SMF) in the 1970s [2] and of the erbium-doped fiber amplifiers [3] in the late 80s increased the capacity of the fiber-optical channel far beyond the required data rate in those days. This abundance of resources made it inessential to exploit the bandwidth optimally in the design of optical communication networks. Nowadays, however, with the exponential growth of the global Internet, the data demand has started meeting the limits of traditional optical systems. This ever-increasing data demand has motivated many recent efforts, including the one in the current paper, to increase the efficiency of optical transmitters and receivers.