Time calibration of the KM3NeT neutrino telescope

Time calibration is a key element in the performance of very large volume neutrino telescopes. The angular resolution of the track reconstruction depends on the accurate measurement of the arrival time of Cherenkov photons reaching the photon sensors. The intrinsic, unavoidable limitations in time resolution (chromatic dispersion and PMT transit time spread) set the requirement for the calibration system of a water-based neutrino telescopes (sigma ≤1ns). The experience with the ANTARES deep sea neutrino telescope has shown that a system of external light sources is very useful to ensure the time calibration of the detector and to measure water optical properties. The KM3NeT neutrino telescope will consist of a three-dimensional array of optical modules arranged on vertical detection unit (DU), anchored to the sea floor and held in tension by submerged buoys. Whilst the basic timing calibration concept applied in ANTARES will be retained, the larger spacing between photo-detectors required in a cubic-kilometre-scale detector results in modified requirements for the KM3NeT system. A three-dimensional system of optical emitters has been studied: several LED models have been tested, and four models preselected as suitable for use in KM3NeT, were incorporated into ANTARES for in situ testing. Based on these results, several LED beacons will be integrated in the forthcoming deployment of a preproduction KM3NeT detection unit planned for autumn 2011. In addition, improvements and optimizations that were applied to the ANTARES laser beacon will be also tested in the preproduction deployment. In addition many improvements and optimizations were applied to the ANTARES Laser Beacon, in order to adjust a new laser setup for KM3NeT calibration purposes. The design, optimization and construction of these optical time calibration devices are described.