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. 2020 May 22;11:2568. doi: 10.1038/s41467-020-16265-x

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — a–c Spectra of the soliton crystal frequency comb after flattening a, modulation and transmission through either 75 km spooled in-lab fibre b or through the field-trial link c. The spectrum a is measured with 12.5 GHz resolution to resolve the individual comb lines, whereas b and c are plotted at 50 GHz resolution to illustrate average channel powers. Flattening equalised the comb line power to within 1 dB. After modulation and amplification, the channels were shaped by the EDFA gain spectrum. The inset in b depicts the test channel spectra captured with a 150 MHz resolution optical spectrum analyser (Finisar WaveAnalyzer), highlighting the odd and even sub-bands modulated onto each comb line in the test band. d Constellation diagrams for a comb line at 193.4 THz (1550.1 nm) for both X- and Y-polarisation channels. ‘Back-to back’ denotes the transmitter directly connected to the receiver, ’75 km in-lab fibre’ indicates reception after transmission through 75 km of spooled fibre inside the lab (as per b), whereas ’76.6 km field fibre’ denotes reception after transmission through the field-trial link (as per c). BER and Q² related to the constellations are noted on each.