The optical data transport system of the KM3NeT neutrino telescope at the bottom of
the Mediterranean Sea will provide more than 6000 optical modules in the detector arrays with a
point-to-point optical connection to the control stations onshore. The ARCA and ORCA detectors of
KM3NeT are being installed at a depth of about 3500m and 2500 m, respectively and their distance
to the control stations is about 100 kilometers and 40 kilometers. In particular, the two detectors are
optimised for the detection of cosmic neutrinos with energies above about 1 TeV (ARCA) and for the
detection of atmospheric neutrinos with energies in the range 1 GeV–1 TeV (ORCA). The expected
maximum data rate is 200 Mbps per optical module. The implemented optical data transport system
matches the layouts of the networks of electro-optical cables and junction boxes in the deep sea. For
efficient use of the fibres in the system the technology of Dense Wavelength Division Multiplexing is
applied. The performance of the optical system in terms of measured bit error rates, optical budget
are presented. The next steps in the implementation of the system are also discussed.Agence Nationale de la
Recherche (contract ANR-15-CE31-0020)Centre National de la Recherche Scientifique (CNRS)Commission Européenne (FEDER fund and Marie Curie Program), Institut Universitaire de France
(IUF), LabEx UnivEarthS (ANR-10-LABX-0023 and ANR-18-IDEX-0001)Paris Île-de-France
Region, France; Deutsche Forschungsgemeinschaft (DFG)Germany; The General Secretariat of
Research and Innovation (GSRI)Greece Istituto Nazionale di Fisica Nucleare (INFN), Ministero
dell’Università e della Ricerca (MIUR), PRIN 2017 program (Grant NAT-NET 2017W4HA7S) ItalyMinistry of Higher EducationScientific Research and InnovationMorocco, and the Arab Fund
for Economic and Social Development, KuwaitNederlandse organisatie voor Wetenschappel¼k
Onderzoek (NWO)The National Science Centre, Poland (2021/41/N/ST2/01177)National Authority for Scientific Research (ANCS)Romania; Ministerio de Ciencia, Innovación,
Investigación y Universidades (MCIU)Programa Estatal de Generación de Conocimiento (refs.
PGC2018-096663-B-C41-A-C42-B-C43-B-C44PID2021-124591NB-C41-C42,
-C43)MCIU/FEDERGeneralitat ValencianaPrometeo (PROMETEO/2020/019)Grisolía (refs.
GRISOLIA/2018/119, /2021/192)GenT (refs. CIDEGENT/2018/034, /2019/043, /2020/049,
/2021/023) programsJunta de Andalucía (ref. A-FQM-053-UGR18)La Caixa Foundation (ref.
LCF/BQ/IN17/11620019)EU: MSC program (ref. 101025085Spain; María Zambrano program
within the framework of grants for retaining in the Spanish university system (Spanish Ministry of
UniversitiesThe European Union, NextGenerationE
