  The optical data transport system of the KM3NeT neutrino telescope at the
bottom of the Mediterranean Sea will provide each of the 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 3500 m and 2500 m, respectively; their distance
to the control stations is about 100 kilometers and 40 kilometers. 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 and the next steps in the implementation of the system are presented.
