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A 223 km unrepeatered link transmission is experimentally demonstrated, transmitting 490 polarisation-division multiplexed channels with adaptively optimised geometrically-shaped constellation quadrature amplitude modulation signals. The transmission band covered nearly the entire S-, C-, and L-bands, spanning 121 nm (15.6 THz) of optical bandwidth. Lumped Thulium- and Erbium-doped fibre amplifiers were used for amplification, and bidirectional distributed Raman amplification, together with pre-emphasis of signal launch power spectrum, were used to mitigate the interchannel stimulated Raman scattering (ISRS) effect. The signal power pre-emphasis and the powers of the Raman pumps were experimentally optimised with a differential evolution algorithm to improve the received signal-to-noise ratio and the throughput. The closed-form ISRS Gaussian noise model was used to support and explain the experimental results: it accurately reproduces the evolution of the signal spectral power and estimates the contributions of nonlinear interference noise and amplified spontaneous emission noise in the unrepeatered link. The combined use of the hybrid amplification scheme, adaptive constellation shaping, and system optimisation techniques resulted in a total throughput of 122.62 Tb/s, from the generalised mutual information (113.95 Tb/s after decoding), achieving the highest throughput to date for unrepeatered links over 200 km.
Yang et al. (Mon,) studied this question.