The face of optical communications has changed drastically in the last few years. New – seemingly contradictory – challenges have emerged, relating to the imminent capacity crunch looming upon optical networks on one hand, and the necessity to operate communication networks in a more energy efficient manner on the other. In response to the first of these challenges, modulation formats that have been commonplace in radio frequency communications for a long time have finally been considered for use in optical systems. These formats include quaternary phase shift keying and quadrature amplitude modulation, and are capable of accommodating more information over a certain bandwidth by encoding more than one bit of information on each transmitted symbol. However, and following from what is known to communications engineers as the Shannon theorem, the more efficiently these formats use the available bandwidth, the higher the optical power that needs to be transmitted. Therefore, a long fiber link can no longer be viewed as a linear channel of information and optical nonlinearities during transmission need to be taken into account, especially when several data channels are occupying different wavelengths within the same optical fiber.

To tackle these new engineering challenges, optics has found an unexpected ally: Fast electronic circuits have always been central in optical communications but their role is becoming ever more prominent today. Powerful digital signal processing systems are crucial for the detection of advanced modulation formats. At the same time, they can correct for the effects of chromatic, as well as polarization mode dispersion in the optical link. Several works have studied how digital signal processing can also be used to mitigate the effects of optical nonlinearities, however compensating for nonlinear effects that originate from the presence of independent optical signals is clearly not a trivial problem. In addition, these techniques rely on computationally intensive numerical algorithms that inevitably burden the energy consumption of the receiving system.

The paper by Turitsyn et al. aims at simplifying the algorithms designed to compensate for nonlinear optical effects in fibers. To achieve this, the paper suggests the inclusion of an additional purely-dispersive element at the start of the transmission line. Then the nonlinear interaction between neighboring wavelength channels becomes less prominent and the problem of reconstructing the original data from the received signal can be solved using analytical expressions. The proposed technique is significant in that it offers the potential of reducing the impact of nonlinearities in transmission. This may have profound effects in the ultimate capacity of communication systems by allowing the theoretical capacity limits to be approached closely in practical systems. However, while simplifying the calculation of the nonlinear backward propagation of the signals, the technique introduces the trade-off of an increased amount of dispersion that needs to be compensated for. Undoubtedly, this first theoretical study will eventually be complemented with further works, both numerical and experimental, which will test and verify its relative strengths in practice.

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