LAUSR

Fiber optics channels provide flat channel response per wavelength in general, owing to the ultrawide available bandwidth of optical fibers and optical amplifiers. However, the recent transport capacity upgrade, which drives the signal baud-rate from 10 to >100 Gbaud, has given rise to severe power fading at high-frequency ranges, induced at levels of both optoelectronic transceivers and optical networks. Especially, the modern meshed optical networks rely on more and more reconfigurable optical add and drop multiplexers (ROADM) to enhance the network flexibility with low latency. These cascaded ROADMs bring about a well-known filter-narrowing effect that has become a severe issue in the deployed networks. This strongly limits the achievable channel bandwidth, and leads to an optical channel with colored signal-to-noise ratio (SNR). Within the linear transmission regime, the capacity-approaching strategy for an individual wavelength channel is to design a Gaussian source, which has been extensively studied recently. However, there is the lack of investigation on approaching the capacity of a channel with colored SNR. This paper addresses this issue with rigor. It reviews the optimum power allocation that determines the capacity of colored-SNR Gaussian channels, and proposes entropy loading based on multicarrier modulation that offers a theoretically optimum strategy to approach the capacity. The entropy loading advantage is verified by a 400-Gb/s coherent optical transmission through band-limited fiber channels with cascaded ROADMs. Entropy loading can be generalized to a variety of applications under colored-SNR Gaussian channels beyond the optical communication.