We review our recent experiment on the Terabit-class coherent optical communication using a photonics integrated circuit-based optical amplifier. The 25.6-Tb/s 16-channel wavelength-division multiplexed (WDM) transmission (over 81-km fiber) proved the potential of such on-chip amplification for future coherent applications.
KEYWORDS: Signal to noise ratio, Forward error correction, Modulation, Interference (communication), Digital signal processing, Optical networks, Signal processing, Optical fibers, Optical communications
Optical fiber channels present flat channel response per wavelength in general, owing to the ultra-wide available bandwidth of optical fiber and optical amplifiers. However, the recent transport capacity upgrade per wavelength from 10 to >100 Gbaud has given rise to severe power fading at the high frequency range. Especially, the modern optical networks may rely on a massive number of 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 channel bandwidth, and leads to an optical channel with colored signal-to-noise ratio (SNR). To address this issue, we utilize the water-filling, an optimum power allocation that determines the capacity of colored-SNR Gaussian channels, and proposes multicarrier entropy loading to offer a theoretically optimum strategy to approach the Shannon capacity. Within each subcarrier, probabilistic constellation shaping is exploited to design Gaussian sources. Compared with the conventional uniform-entropy modulation, entropy loading possesses fundamental advantages on channel coding: it maximizes the channel mutual information under fixed channel coding rate when the system operates below the channel capacity, and approaches the capacity with less coding overhead. Entropy loading can be generalized to any applications under colored-SNR Gaussian channels beyond the optical communication.
KEYWORDS: Optical communications, Modulation, Digital signal processing, Polarization, Receivers, Signal detection, Channel projecting optics, Palladium, Single sideband modulation, Internet
To cope with the exponential growth of the Internet traffic, optical communications has advanced by leaps and bounds. For several decades, Intensity modulation with direct detection (IM-DD) dominates the commercial short-reach optical communications. However, when upgrading the data-rate distance product to 1000 Gb/s·km per wavelength and beyond, IM-DD faces severe performance barrier. Aiming to improve the electrical SE and extend the transmission distance, advanced DD modulation formats have been proposed through a so-called self-coherent (SCOH) approach, where a carrier is transmitted together with the signal to achieve a linear mapping between the electrical baseband signal and the optical field. In that way, the impact of the CD can be removed from the received signal, greatly extending the transmission distance of the DD system. Particularly, Stokes-vector direct detection (SV-DD) has been proposed to realize linear complex optical channels as well as enhance the electrical spectral efficiency and transmission reach. In this talk, we present the principle and discuss the performance of SV-DD systems.
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