# Nanobeacon: A time calibration device for the KM3NeT neutrino telescope

S. Aiello  INFN, Sezione di Catania, Via Santa Sofia 64, Catania, 95123 Italy
A. Albert  Université de Haute Alsace, rue des Frères Lumière, 68093 Mulhouse
Cedex, France  Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000
Strasbourg, France  M. Alshamsi  Université de Paris, CNRS, Astroparticule et
Cosmologie, F-75013 Paris, France  S. Alves Garre  IFIC - Instituto de Física
Corpuscular (CSIC - Universitat de València), c/Catedrático José Beltrán, 2,
46980 Paterna, Valencia, Spain  Z. Aly  Aix Marseille Univ, CNRS/IN2P3, CPPM,
Marseille, France  A. Ambrosone  INFN, Sezione di Napoli, Complesso
Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli, 80126 Italy
Università di Napoli “Federico II”, Dip. Scienze Fisiche “E. Pancini”,
Complesso Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli, 80126
Italy  F. Ameli  INFN, Sezione di Roma, Piazzale Aldo Moro 2, Roma, 00185
Italy  M. Andre  Universitat Politècnica de Catalunya, Laboratori
d’Aplicacions Bioacústiques, Centre Tecnològic de Vilanova i la Geltrú, Avda.
Rambla Exposició, s/n, Vilanova i la Geltrú, 08800 Spain  G. Androulakis†222†
Deceased NCSR Demokritos, Institute of Nuclear and Particle Physics, Ag.
Paraskevi Attikis, Athens, 15310 Greece  M. Anghinolfi  INFN, Sezione di
Genova, Via Dodecaneso 33, Genova, 16146 Italy  M. Anguita  University of
Granada, Dept. of Computer Architecture and Technology/CITIC, 18071 Granada,
Spain  M. Ardid  Universitat Politècnica de València, Instituto de
Investigación para la Gestión Integrada de las Zonas Costeras, C/ Paranimf, 1,
Gandia, 46730 Spain  S. Ardid  Universitat Politècnica de València, Instituto
de Investigación para la Gestión Integrada de las Zonas Costeras, C/ Paranimf,
1, Gandia, 46730 Spain  J. Aublin  Université de Paris, CNRS, Astroparticule
et Cosmologie, F-75013 Paris, France  C. Bagatelas  NCSR Demokritos, Institute
of Nuclear and Particle Physics, Ag. Paraskevi Attikis, Athens, 15310 Greece
B. Baret  Université de Paris, CNRS, Astroparticule et Cosmologie, F-75013
Paris, France  S. Basegmez du Pree  Nikhef, National Institute for Subatomic
Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  M. Bendahman
Université de Paris, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
University Mohammed V in Rabat, Faculty of Sciences, 4 av. Ibn Battouta, B.P.
1014, R.P. 10000 Rabat, Morocco  F. Benfenati  INFN, Sezione di Bologna, v.le
C. Berti-Pichat, 6/2, Bologna, 40127 Italy  Università di Bologna,
Dipartimento di Fisica e Astronomia, v.le C. Berti-Pichat, 6/2, Bologna, 40127
Italy  E. Berbee  Nikhef, National Institute for Subatomic Physics, PO Box
41882, Amsterdam, 1009 DB Netherlands  A. M. van den Berg  KVI-CART University
of Groningen, Groningen, the Netherlands  V. Bertin  Aix Marseille Univ,
CNRS/IN2P3, CPPM, Marseille, France  S. Biagi  INFN, Laboratori Nazionali del
Sud, Via S. Sofia 62, Catania, 95123 Italy  M. Boettcher  North-West
University, Centre for Space Research, Private Bag X6001, Potchefstroom, 2520
South Africa  M. Bou Cabo  Instituto Español de Oceanografía, Unidad Mixta
IEO-UPV, C/ Paranimf, 1, Gandia, 46730 Spain  J. Boumaaza  University Mohammed
V in Rabat, Faculty of Sciences, 4 av. Ibn Battouta, B.P. 1014, R.P. 10000
Rabat, Morocco  M. Bouta  University Mohammed I, Faculty of Sciences, BV
Mohammed VI, B.P. 717, R.P. 60000 Oujda, Morocco  M. Bouwhuis  Nikhef,
National Institute for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB
Netherlands  C. Bozza  Università di Salerno e INFN Gruppo Collegato di
Salerno, Dipartimento di Matematica, Via Giovanni Paolo II 132, Fisciano,
84084 Italy  H.Brânzaş  ISS, Atomistilor 409, Măgurele, RO-077125 Romania  R.
Bruijn  Nikhef, National Institute for Subatomic Physics, PO Box 41882,
Amsterdam, 1009 DB Netherlands  University of Amsterdam, Institute of
Physics/IHEF, PO Box 94216, Amsterdam, 1090 GE Netherlands  J. Brunner  Aix
Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  R. Bruno  INFN, Sezione
di Catania, Via Santa Sofia 64, Catania, 95123 Italy  E. Buis  TNO, Technical
Sciences, PO Box 155, Delft, 2600 AD Netherlands  R. Buompane  INFN, Sezione
di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia ed. G,
Napoli, 80126 Italy  Università degli Studi della Campania ”Luigi Vanvitelli”,
Dipartimento di Matematica e Fisica, viale Lincoln 5, Caserta, 81100 Italy  J.
Busto  Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  B. Caiffi
INFN, Sezione di Genova, Via Dodecaneso 33, Genova, 16146 Italy  D. Calvo
IFIC - Instituto de Física Corpuscular (CSIC - Universitat de València),
c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  S. Campion
Università La Sapienza, Dipartimento di Fisica, Piazzale Aldo Moro 2, Roma,
00185 Italy  INFN, Sezione di Roma, Piazzale Aldo Moro 2, Roma, 00185 Italy
A. Capone  Università La Sapienza, Dipartimento di Fisica, Piazzale Aldo Moro
2, Roma, 00185 Italy  INFN, Sezione di Roma, Piazzale Aldo Moro 2, Roma, 00185
Italy  V. Carretero  IFIC - Instituto de Física Corpuscular (CSIC -
Universitat de València), c/Catedrático José Beltrán, 2, 46980 Paterna,
Valencia, Spain  P. Castaldi  INFN, Sezione di Bologna, v.le C. Berti-Pichat,
6/2, Bologna, 40127 Italy  Università di Bologna, Dipartimento di Ingegneria
dell’Energia Elettrica e dell’Informazione ”Guglielmo Marconi”, Via
dell’Università 50, Cesena, 47521 Italia  S. Celli  Università La Sapienza,
Dipartimento di Fisica, Piazzale Aldo Moro 2, Roma, 00185 Italy  INFN, Sezione
di Roma, Piazzale Aldo Moro 2, Roma, 00185 Italy  M. Chabab  Cadi Ayyad
University, Physics Department, Faculty of Science Semlalia, Av. My Abdellah,
P.O.B. 2390, Marrakech, 40000 Morocco  N. Chau  Université de Paris, CNRS,
Astroparticule et Cosmologie, F-75013 Paris, France  A. Chen  University of
the Witwatersrand, School of Physics, Private Bag 3, Johannesburg, Wits 2050
South Africa  S. Cherubini  INFN, Laboratori Nazionali del Sud, Via S. Sofia
62, Catania, 95123 Italy  Università di Catania, Dipartimento di Fisica e
Astronomia ”Ettore Majorana”, Via Santa Sofia 64, Catania, 95123 Italy  V.
Chiarella  INFN, LNF, Via Enrico Fermi, 40, Frascati, 00044 Italy  T. Chiarusi
INFN, Sezione di Bologna, v.le C. Berti-Pichat, 6/2, Bologna, 40127 Italy  M.
Circella  INFN, Sezione di Bari, via Orabona, 4, Bari, 70125 Italy  R.
Cocimano  INFN, Laboratori Nazionali del Sud, Via S. Sofia 62, Catania, 95123
Italy  J. A. B. Coelho  Université de Paris, CNRS, Astroparticule et
Cosmologie, F-75013 Paris, France  A. Coleiro  Université de Paris, CNRS,
Astroparticule et Cosmologie, F-75013 Paris, France  M. Colomer Molla
Université de Paris, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
IFIC - Instituto de Física Corpuscular (CSIC - Universitat de València),
c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  R. Coniglione
INFN, Laboratori Nazionali del Sud, Via S. Sofia 62, Catania, 95123 Italy  P.
Coyle  Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  A. Creusot
Université de Paris, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
A. Cruz  International Centre for Radio Astronomy Research, Curtin University,
Bentley, WA 6102, Australia  G. Cuttone  INFN, Laboratori Nazionali del Sud,
Via S. Sofia 62, Catania, 95123 Italy  R. Dallier  Subatech, IMT Atlantique,
IN2P3-CNRS, Université de Nantes, 4 rue Alfred Kastler - La Chantrerie,
Nantes, BP 20722 44307 France  B. De Martino  Aix Marseille Univ, CNRS/IN2P3,
CPPM, Marseille, France  I. Di Palma  Università La Sapienza, Dipartimento di
Fisica, Piazzale Aldo Moro 2, Roma, 00185 Italy  INFN, Sezione di Roma,
Piazzale Aldo Moro 2, Roma, 00185 Italy  A. F. Díaz  University of Granada,
Dept. of Computer Architecture and Technology/CITIC, 18071 Granada, Spain  D.
Diego-Tortosa  Universitat Politècnica de València, Instituto de Investigación
para la Gestión Integrada de las Zonas Costeras, C/ Paranimf, 1, Gandia, 46730
Spain  C. Distefano  INFN, Laboratori Nazionali del Sud, Via S. Sofia 62,
Catania, 95123 Italy  A. Domi  Nikhef, National Institute for Subatomic
Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  University of
Amsterdam, Institute of Physics/IHEF, PO Box 94216, Amsterdam, 1090 GE
Netherlands  C. Donzaud  Université de Paris, CNRS, Astroparticule et
Cosmologie, F-75013 Paris, France  D. Dornic  Aix Marseille Univ, CNRS/IN2P3,
CPPM, Marseille, France  M. Dörr  University Würzburg, Emil-Fischer-Straße 31,
Würzburg, 97074 Germany  D. Drouhin  Université de Haute Alsace, rue des
Frères Lumière, 68093 Mulhouse Cedex, France  Université de Strasbourg, CNRS,
IPHC UMR 7178, F-67000 Strasbourg, France  T. Eberl  Friedrich-Alexander-
Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics,
Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  A. Eddyamoui  University
Mohammed V in Rabat, Faculty of Sciences, 4 av. Ibn Battouta, B.P. 1014, R.P.
10000 Rabat, Morocco  T. van Eeden  Nikhef, National Institute for Subatomic
Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  D. van Eijk  Nikhef,
National Institute for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB
Netherlands  I. El Bojaddaini  University Mohammed I, Faculty of Sciences, BV
Mohammed VI, B.P. 717, R.P. 60000 Oujda, Morocco  S. El Hedri  Université de
Paris, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France  A.
Enzenhöfer  Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  V.
Espinosa  Universitat Politècnica de València, Instituto de Investigación para
la Gestión Integrada de las Zonas Costeras, C/ Paranimf, 1, Gandia, 46730
Spain  P. Fermani  Università La Sapienza, Dipartimento di Fisica, Piazzale
Aldo Moro 2, Roma, 00185 Italy  INFN, Sezione di Roma, Piazzale Aldo Moro 2,
Roma, 00185 Italy  G. Ferrara  INFN, Laboratori Nazionali del Sud, Via S.
Sofia 62, Catania, 95123 Italy  Università di Catania, Dipartimento di Fisica
e Astronomia ”Ettore Majorana”, Via Santa Sofia 64, Catania, 95123 Italy  M.
D. Filipović  Western Sydney University, School of Computing, Engineering and
Mathematics, Locked Bag 1797, Penrith, NSW 2751 Australia  F. Filippini  INFN,
Sezione di Bologna, v.le C. Berti-Pichat, 6/2, Bologna, 40127 Italy
Università di Bologna, Dipartimento di Fisica e Astronomia, v.le C. Berti-
Pichat, 6/2, Bologna, 40127 Italy  L. A. Fusco  Aix Marseille Univ,
CNRS/IN2P3, CPPM, Marseille, France  T. Gal  Friedrich-Alexander-Universität
Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-
Straße 1, 91058 Erlangen, Germany  J. García Méndez  Universitat Politècnica
de València, Instituto de Investigación para la Gestión Integrada de las Zonas
Costeras, C/ Paranimf, 1, Gandia, 46730 Spain  F. Garufi  INFN, Sezione di
Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli,
80126 Italy  Università di Napoli “Federico II”, Dip. Scienze Fisiche “E.
Pancini”, Complesso Universitario di Monte S. Angelo, Via Cintia ed. G,
Napoli, 80126 Italy  Y. Gatelet  Université de Paris, CNRS, Astroparticule et
Cosmologie, F-75013 Paris, France  C. Gatius Oliver  Nikhef, National
Institute for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands
N. Geißelbrecht  Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen
Centre for Astroparticle Physics, Erwin-Rommel-Straße 1, 91058 Erlangen,
Germany  L. Gialanella  INFN, Sezione di Napoli, Complesso Universitario di
Monte S. Angelo, Via Cintia ed. G, Napoli, 80126 Italy  Università degli Studi
della Campania ”Luigi Vanvitelli”, Dipartimento di Matematica e Fisica, viale
Lincoln 5, Caserta, 81100 Italy  E. Giorgio  INFN, Laboratori Nazionali del
Sud, Via S. Sofia 62, Catania, 95123 Italy  S. R. Gozzini  IFIC - Instituto de
Física Corpuscular (CSIC - Universitat de València), c/Catedrático José
Beltrán, 2, 46980 Paterna, Valencia, Spain  R. Gracia  Nikhef, National
Institute for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands
K. Graf  Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre
for Astroparticle Physics, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  G.
Grella  Università di Salerno e INFN Gruppo Collegato di Salerno, Dipartimento
di Fisica, Via Giovanni Paolo II 132, Fisciano, 84084 Italy  D. Guderian
University of Münster, Institut für Kernphysik, Wilhelm-Klemm-Str. 9, Münster,
48149 Germany  C. Guidi  INFN, Sezione di Genova, Via Dodecaneso 33, Genova,
16146 Italy  Università di Genova, Via Dodecaneso 33, Genova, 16146 Italy  B.
Guillon  Normandie Univ, ENSICAEN, UNICAEN, CNRS/IN2P3, LPC Caen, LPCCAEN, 6
boulevard Maréchal Juin, Caen, 14050 France  M. Gutiérrez  University of
Granada, Dpto. de Física Teórica y del Cosmos & C.A.F.P.E., 18071 Granada,
Spain  J. Haefner  Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen
Centre for Astroparticle Physics, Erwin-Rommel-Straße 1, 91058 Erlangen,
Germany  S. Hallmann  Friedrich-Alexander-Universität Erlangen-Nürnberg,
Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Straße 1, 91058
Erlangen, Germany  H. Hamdaoui  University Mohammed V in Rabat, Faculty of
Sciences, 4 av. Ibn Battouta, B.P. 1014, R.P. 10000 Rabat, Morocco  H. van
Haren  NIOZ (Royal Netherlands Institute for Sea Research), PO Box 59, Den
Burg, Texel, 1790 AB, the Netherlands  A. Heijboer  Nikhef, National Institute
for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  A. Hekalo
University Würzburg, Emil-Fischer-Straße 31, Würzburg, 97074 Germany  L.
Hennig  Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for
Astroparticle Physics, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  J. J.
Hernández-Rey  IFIC - Instituto de Física Corpuscular (CSIC - Universitat de
València), c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  J.
Hofestädt  Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre
for Astroparticle Physics, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  F.
Huang  Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  W. Idrissi
Ibnsalih  INFN, Sezione di Napoli, Complesso Universitario di Monte S. Angelo,
Via Cintia ed. G, Napoli, 80126 Italy  Università degli Studi della Campania
”Luigi Vanvitelli”, Dipartimento di Matematica e Fisica, viale Lincoln 5,
Caserta, 81100 Italy  G. Illuminati  INFN, Sezione di Bologna, v.le C. Berti-
Pichat, 6/2, Bologna, 40127 Italy  Université de Paris, CNRS, Astroparticule
et Cosmologie, F-75013 Paris, France  Università di Bologna, Dipartimento di
Fisica e Astronomia, v.le C. Berti-Pichat, 6/2, Bologna, 40127 Italy  C. W.
James  International Centre for Radio Astronomy Research, Curtin University,
Bentley, WA 6102, Australia  D. Janezashvili  Tbilisi State University,
Department of Physics, 3, Chavchavadze Ave., Tbilisi, 0179 Georgia  M. de Jong
Nikhef, National Institute for Subatomic Physics, PO Box 41882, Amsterdam,
1009 DB Netherlands  Leiden University, Leiden Institute of Physics, PO Box
9504, Leiden, 2300 RA Netherlands  P. de Jong  Nikhef, National Institute for
Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  University of
Amsterdam, Institute of Physics/IHEF, PO Box 94216, Amsterdam, 1090 GE
Netherlands  B. J. Jung  Nikhef, National Institute for Subatomic Physics, PO
Box 41882, Amsterdam, 1009 DB Netherlands  P. Kalaczyński  National Centre for
Nuclear Research, 02-093 Warsaw, Poland  O. Kalekin  Friedrich-Alexander-
Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics,
Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  U. F. Katz  Friedrich-
Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle
Physics, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  N. R. Khan Chowdhury
IFIC - Instituto de Física Corpuscular (CSIC - Universitat de València),
c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  G. Kistauri
Tbilisi State University, Department of Physics, 3, Chavchavadze Ave.,
Tbilisi, 0179 Georgia  F. van der Knaap  TNO, Technical Sciences, PO Box 155,
Delft, 2600 AD Netherlands  P. Kooijman  University of Amsterdam, Institute of
Physics/IHEF, PO Box 94216, Amsterdam, 1090 GE Netherlands  Utrecht
University, Department of Physics and Astronomy, PO Box 80000, Utrecht, 3508
TA Netherlands  A. Kouchner  Université de Paris, CNRS, Astroparticule et
Cosmologie, F-75013 Paris, France  Institut Universitaire de France, 1 rue
Descartes, Paris, 75005 France  V. Kulikovskiy  INFN, Sezione di Genova, Via
Dodecaneso 33, Genova, 16146 Italy  M. Labalme  Normandie Univ, ENSICAEN,
UNICAEN, CNRS/IN2P3, LPC Caen, LPCCAEN, 6 boulevard Maréchal Juin, Caen, 14050
France  R. Lahmann  Friedrich-Alexander-Universität Erlangen-Nürnberg,
Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Straße 1, 91058
Erlangen, Germany  M. Lamoureux  Université de Paris, CNRS, Astroparticule et
Cosmologie, F-75013 Paris, France  also at Dipartimento di Fisica, INFN
Sezione di Padova and Università di Padova, I-35131, Padova, Italy  G. Larosa
INFN, Laboratori Nazionali del Sud, Via S. Sofia 62, Catania, 95123 Italy  C.
Lastoria  Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  A. Lazo
IFIC - Instituto de Física Corpuscular (CSIC - Universitat de València),
c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  R. Le Breton
Université de Paris, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
S. Le Stum  Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France  G. Lehaut
Normandie Univ, ENSICAEN, UNICAEN, CNRS/IN2P3, LPC Caen, LPCCAEN, 6 boulevard
Maréchal Juin, Caen, 14050 France  O. Leonardi  INFN, Laboratori Nazionali del
Sud, Via S. Sofia 62, Catania, 95123 Italy  F. Leone  INFN, Laboratori
Nazionali del Sud, Via S. Sofia 62, Catania, 95123 Italy  Università di
Catania, Dipartimento di Fisica e Astronomia ”Ettore Majorana”, Via Santa
Sofia 64, Catania, 95123 Italy  E. Leonora  INFN, Sezione di Catania, Via
Santa Sofia 64, Catania, 95123 Italy  N. Lessing  Friedrich-Alexander-
Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics,
Erwin-Rommel-Straße 1, 91058 Erlangen, Germany  G. Levi  INFN, Sezione di
Bologna, v.le C. Berti-Pichat, 6/2, Bologna, 40127 Italy  Università di
Bologna, Dipartimento di Fisica e Astronomia, v.le C. Berti-Pichat, 6/2,
Bologna, 40127 Italy  M. Lincetto  Aix Marseille Univ, CNRS/IN2P3, CPPM,
Marseille, France  M. Lindsey Clark  Université de Paris, CNRS, Astroparticule
et Cosmologie, F-75013 Paris, France  T. Lipreau  Subatech, IMT Atlantique,
IN2P3-CNRS, Université de Nantes, 4 rue Alfred Kastler - La Chantrerie,
Nantes, BP 20722 44307 France  C. LLorens Alvarez  Universitat Politècnica de
València, Instituto de Investigación para la Gestión Integrada de las Zonas
Costeras, C/ Paranimf, 1, Gandia, 46730 Spain  F. Longhitano  INFN, Sezione di
Catania, Via Santa Sofia 64, Catania, 95123 Italy  D. Lopez-Coto  University
of Granada, Dpto. de Física Teórica y del Cosmos & C.A.F.P.E., 18071 Granada,
Spain  L. Maderer  Université de Paris, CNRS, Astroparticule et Cosmologie,
F-75013 Paris, France  J. Majumdar  Nikhef, National Institute for Subatomic
Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  J. Mańczak  IFIC -
Instituto de Física Corpuscular (CSIC - Universitat de València),
c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  A. Margiotta
INFN, Sezione di Bologna, v.le C. Berti-Pichat, 6/2, Bologna, 40127 Italy
Università di Bologna, Dipartimento di Fisica e Astronomia, v.le C. Berti-
Pichat, 6/2, Bologna, 40127 Italy  A. Marinelli  INFN, Sezione di Napoli,
Complesso Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli, 80126
Italy  C. Markou  NCSR Demokritos, Institute of Nuclear and Particle Physics,
Ag. Paraskevi Attikis, Athens, 15310 Greece  L. Martin  Subatech, IMT
Atlantique, IN2P3-CNRS, Université de Nantes, 4 rue Alfred Kastler - La
Chantrerie, Nantes, BP 20722 44307 France  J. A. Martínez-Mora  Universitat
Politècnica de València, Instituto de Investigación para la Gestión Integrada
de las Zonas Costeras, C/ Paranimf, 1, Gandia, 46730 Spain  A. Martini  INFN,
LNF, Via Enrico Fermi, 40, Frascati, 00044 Italy  F. Marzaioli  INFN, Sezione
di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia ed. G,
Napoli, 80126 Italy  Università degli Studi della Campania ”Luigi Vanvitelli”,
Dipartimento di Matematica e Fisica, viale Lincoln 5, Caserta, 81100 Italy  S.
Mastroianni  INFN, Sezione di Napoli, Complesso Universitario di Monte S.
Angelo, Via Cintia ed. G, Napoli, 80126 Italy  K. W. Melis  Nikhef, National
Institute for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands
G. Miele  INFN, Sezione di Napoli, Complesso Universitario di Monte S. Angelo,
Via Cintia ed. G, Napoli, 80126 Italy  Università di Napoli “Federico II”,
Dip. Scienze Fisiche “E. Pancini”, Complesso Universitario di Monte S. Angelo,
Via Cintia ed. G, Napoli, 80126 Italy  P. Migliozzi  INFN, Sezione di Napoli,
Complesso Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli, 80126
Italy  E. Migneco  INFN, Laboratori Nazionali del Sud, Via S. Sofia 62,
Catania, 95123 Italy  P. Mijakowski  National Centre for Nuclear Research,
02-093 Warsaw, Poland  L. S. Miranda  University of Johannesburg, Department
Physics, PO Box 524, Auckland Park, 2006 South Africa  C. M. Mollo  INFN,
Sezione di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia ed.
G, Napoli, 80126 Italy  M. Moser  Friedrich-Alexander-Universität Erlangen-
Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Straße 1,
91058 Erlangen, Germany  A. Moussa  University Mohammed I, Faculty of
Sciences, BV Mohammed VI, B.P. 717, R.P. 60000 Oujda, Morocco  R. Muller
Nikhef, National Institute for Subatomic Physics, PO Box 41882, Amsterdam,
1009 DB Netherlands  M. Musumeci  INFN, Laboratori Nazionali del Sud, Via S.
Sofia 62, Catania, 95123 Italy  L. Nauta  Nikhef, National Institute for
Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB Netherlands  S. Navas
University of Granada, Dpto. de Física Teórica y del Cosmos & C.A.F.P.E.,
18071 Granada, Spain  C. A. Nicolau  INFN, Sezione di Roma, Piazzale Aldo Moro
2, Roma, 00185 Italy  B. Nkosi  University of the Witwatersrand, School of
Physics, Private Bag 3, Johannesburg, Wits 2050 South Africa  B. Ó Fearraigh
Nikhef, National Institute for Subatomic Physics, PO Box 41882, Amsterdam,
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Box 94216, Amsterdam, 1090 GE Netherlands  M. O’Sullivan  International Centre
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Sofia 62, Catania, 95123 Italy  J. Palacios González  IFIC - Instituto de
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University, Department of Physics, 3, Chavchavadze Ave., Tbilisi, 0179 Georgia
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95123 Italy  A. M. Păun  ISS, Atomistilor 409, Măgurele, RO-077125 Romania  G.
E. Păvălaş  ISS, Atomistilor 409, Măgurele, RO-077125 Romania  C. Pellegrino
Università di Bologna, Dipartimento di Fisica e Astronomia, v.le C. Berti-
Pichat, 6/2, Bologna, 40127 Italy  INFN, CNAF, v.le C. Berti-Pichat, 6/2,
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Cosmologie, F-75013 Paris, France  Institut Universitaire de France, 1 rue
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60000 Oujda, Morocco  S. Zavatarelli  INFN, Sezione di Genova, Via Dodecaneso
33, Genova, 16146 Italy  A. Zegarelli  Università La Sapienza, Dipartimento di
Fisica, Piazzale Aldo Moro 2, Roma, 00185 Italy  INFN, Sezione di Roma,
Piazzale Aldo Moro 2, Roma, 00185 Italy  D. Zito  INFN, Laboratori Nazionali
del Sud, Via S. Sofia 62, Catania, 95123 Italy  J. D. Zornoza  IFIC -
Instituto de Física Corpuscular (CSIC - Universitat de València),
c/Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain  J. Zúñiga  IFIC
- Instituto de Física Corpuscular (CSIC - Universitat de València),
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North-West University, Centre for Space Research, Private Bag X6001,
Potchefstroom, 2520 South Africa

###### Abstract

The KM3NeT Collaboration is currently constructing a multi-site high-energy
neutrino telescope in the Mediterranean Sea consisting of matrices of
pressure-resistant glass spheres, each holding a set of 31 small-area
photomultipliers. The main goals of the telescope are the observation of
neutrino sources in the Universe and the measurement of the neutrino
oscillation parameters with atmospheric neutrinos. Both extraterrestrial and
atmospheric neutrinos are detected through the Cherenkov light induced in
seawater by charged particles produced in neutrino interactions in the
surrounding medium. A relative time synchronization between photomultipliers
of the order of 1 ns is needed to guarantee the required angular resolution of
the detector. Due to the large detector volumes to be instrumented by KM3NeT,
a cost reduction of the different systems is a priority. To this end, the
inexpensive Nanobeacon has been designed and developed by the KM3NeT
Collaboration to be used for detector time-calibration studies. At present,
more than 600 Nanobeacons have been already produced. The characterization of
the optical pulse and the wavelength emission profile of the devices are
critical for the time calibration. In this paper, the main features of the
Nanobeacon design, production and operation, together with the main properties
of the light pulse generated are described.

###### Index Terms:

time calibration; instrumentation; neutrino telescopes

##  I Introduction

The KM3NeT Collaboration [1] is building a network of underwater neutrino
telescopes at two deep locations in the Mediterranean Sea. Although the same
technology is used for the two detectors their configuration is different,
reflecting the difference in scientific goals. Astroparticle Research with
Cosmics in the Abyss (ARCA [2]) has been designed for the detection of
neutrinos of astrophysical origin with energies from $\sim$ 100 GeV to PeV,
and is located 100 km off the southern tip of Sicily (Italy) at a depth of
3500 m. Oscillation Research with Cosmics in the Abyss (ORCA [3]), which
mainly aims at studying the fundamental properties of neutrinos, is located 40
km south of the coast of Toulon (France) at a depth of about 2450 m. The
detection principle is based on the collection of Cherenkov photons produced
along the path of relativistic charged particles emerging from neutrino
interactions inside or in the vicinity of the detector.

††E-mail addresses<EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS>
The telescopes are designed as 3-dimensional matrices of light detectors,
spanning a large volume, as shown in Fig. 1. The active component is the
Digital Optical Module (DOM) [4, 5, 6], a pressure-resistant glass sphere
which houses 31 photomultipliers (PMTs). The arrival time of the Cherenkov
photons, together with the position of the DOMs allow the reconstruction of
the trajectory of the primary neutrino. The resolution of the reconstructed
neutrino trajectory in the detector depends on the accurate measurement of the
arrival time of the light on the optical sensors, with a precision better than
one nanosecond, and of their position in space, better than 20 cm. For that
reason, time and position calibration of the telescope is a critical
requirement. The main data acquisition electronic board of the DOM [7, 8] is
the Central Logic Board (CLB) [9, 10], which performs the readout of the 31
PMT channels. A power board [11] mounted on top of the CLB provides power. The
DOMs are distributed along lines, called Detection Units (DUs), each hosting
18 DOMs. In ORCA, the vertical spacing between DOMs is around 9 m, while in
ARCA it is 36 m. The DUs are anchored on the seafloor, with a spacing about 20
m in ORCA, and 90 m in ARCA and are organized in Building Blocks (BB), each
one composed of 115 DUs. The first fourteen DUs of KM3NeT, six for ORCA and
eight for ARCA, have already been deployed and are currently taking data [12].

Figure 1: Artistic view of KM3NeT. The illustration is not scaled: sunlight
does not reach the depths at which the KM3NeT detector is deployed. The total
volume of the detector, once completed, will be larger than one km3.

###  I-A The Nanobeacon system

In addition to the information encoded in the clock distribution time using a
customized version [13] of the White Rabbit protocol [14, 15], there are
several methods of in-situ time calibration. In addition to down-going muons
and photons from 40K radioactive decays in the seawater, KM3NeT has an optical
beacon system based on pulse generation devices. Two different sources of
short-pulsed light [16] are used to cross-check the time calibration of the
telescope by illuminating sets of the optical modules in a controlled
scenario: the laser beacon [17], and the Nanobeacon [18], the latter being the
subject of the present work.

Each DOM houses a Nanobeacon (Fig. 2). The Nanobeacon can produce a short
duration pulse approximately 5 ns wide (FWHM), with around 3 ns rise time
(from 10% to 90% of full amplitude) and with enough intensity to illuminate
the DOMs located on the same DU above the emitting DOM. With a good
understanding of the optical pulse shape and thanks to the control of the time
of emission, the Nanobeacon pulses can be used to monitor the relative time
offsets between DOMs of the same DU.

The intensity of the pulse and the angular aperture (with a solid angle with
50% of light emission) of the LED emission have also been proven relevant for
the calibration between DUs, as a proper selection of these values allows the
illumination of the neighboring DUs with Nanobeacon flashes. The position of
the Nanobeacon 45∘ off the axis from the top DOM is chosen to reduce the
effect of sedimentation and biofouling on the glass sphere of the DOM.

(a)

(b)

Figure 2: (a) 2D sketch of the DOM, showing the position of the Nanobeacon LED
at 45∘ from the vertical axis. The main components of the DOMs are shown: the
PMTs, the cooling system and the main electronic boards. (b) A picture of a
small part of the top hemisphere of the KM3NeT optical module with the
aluminum cooling cap visible. The DOM sphere has a diameter of 17” (43 cm).
The position of the Nanobeacon is highlighted by the circle.

The electronics and the control firmware of the Nanobeacon are presented in
Sections II and III, respectively, while studies of the LED model are
addressed in Section IV. The mass production of Nanobeacons for KM3NeT is
described in Section V , together with the test setup and the results of the
intensity tests performed just after production. The proof of concept is
illustrated in Section VI, while the characterization of the optical pulse in
terms of rise time and width are presented in Section VII. Moreover, the
results of the pulse characterization in terms of wavelength are discussed in
this Section. Finally, in Section VIII, a summary is presented together with
future plans.

##  II Nanobeacon electronics

The Nanobeacon has its origin in the LED beacon system used for the time
calibration of the Astronomy with a Neutrino Telescope and Abyss environmental
RESearch (ANTARES) experiment [19]. ANTARES, which is still operational after
more than 13 years in the Mediterranean Sea close to the ORCA site, is the
predecessor of KM3NeT and is based on the same detection principle but with
smaller instrumented volume. The ANTARES LED beacon consists of a glass and
titanium container housing 36 LEDs with their corresponding electronic
pulsers. The LED beacon has been considerably simplified in KM3NeT by
integrating the LED inside the DOM. This avoids the need for a mechanical
external container and reduces significantly the cost. The Nanobeacon is
composed of two different elements: the pulser, and the DC/DC converter. The
pulser comprises the LED and the electronics board that generates the trigger
for the optical pulse. The DC/DC converter is integrated in the power board
inside the DOM, which generates the power supply to the Nanobeacon pulser. The
power generated in the power board is fed to the Nanobeacon via the CLB. The
control of the rail and the generation of the electrical trigger are both
carried out at the CLB.

###  II-A Nanobeacon pulser

(a)

(b)

Figure 3: (a) Electronics schematics of the Nanobeacon pulser. (b) Layout of
the Nanobeacon pulser. The dimensions of the Nanobeacon pulser are 35 mm
$\times$ 20 mm.

The Nanobeacon pulser is based on the Kapustinsky circuit [20], which can
generate short pulses with simple electronics, modified for KM3NeT. The
schematics of the pulser are shown in Fig. 3(a) while the layout is presented
in Fig. 3(b). The trigger of the pulser is generated by an FPGA on the CLB,
which provides a 1.5 V negative square pulse. The pulse generated is added to
the DC voltage provided by the power supply rail. The DC voltage loads a
capacitor (C2) that is discharged to the LED during the transition of two
transistors working in opposition. The switching between transistors is
triggered by the signal provided by the FPGA, provoking the discharge of the
capacitor to the LED and the emission of a narrow optical pulse. The trigger
generated by the FPGA is provided to the Nanobeacon pulser by a three-wire
twisted cable to reduce the electromagnetic induced noise. This connection
also provides ground and power, which comes from the power board.

###  II-B Nanobeacon power supply

The Nanobeacon power supply is provided by one of the rails of the power
board, which converts the 12 V input into the voltage needed to operate the
Nanobeacon. The power rail consists of a DC/DC converter working in a buck-
boost configuration, providing a voltage configurable between 4.5 and 30 V,
via a DAC controllable by an I2C. When the output voltage of the DAC changes,
the voltage of the Nanobeacon power supply is modified accordingly. The
provided voltage determines the amount of current supplied to the LED, and
therefore, the intensity of the generated optical pulse. The power supply
includes two circuits for measuring the voltage and the current. The values
provided by these circuits are acquired with an ADC that is read by the FPGA
embedded software. As will be shown in Section VII, the voltage is correlated
with the pulse width: the lower the voltage supplied, the narrower the optical
pulse. The typical power consumption of the Nanobeacon does not exceed 0.1 W.

##  III Firmware

An Intellectual Property (IP) block, implemented in the CLB FPGA, generates
the trigger signal for the Nanobeacon. This FPGA IP core implements a Wishbone
slave [21], where 5 registers of 32 bits are accessible via the embedded
software to configure the Nanobeacon trigger signal (Table I). In particular,
the first register allows for enabling and disabling the trigger signal. For
this, one of the bits of the first register, called Enable control, is used.
Another bit, called Enable power, controls the power supply of the Nanobeacon.
There are, therefore, two different methods for disabling the Nanobeacon. In
KM3NeT, the data acquisition is organized in timeslices, which have a typical
duration of 100 ms. A second register (m1) allows for defining the delay of
the first flash after the start of the timeslices. A third register (m2)
defines the Nanobeacon pulse length. The flash period is configured in a
fourth register (m3) and the number of triggers per timeslice is defined in
the fifth one (m4). In all cases, the time is coded in ticks of 16 ns. Table I
describes the different control registers that configure the IP core to
trigger the Nanobeacon pulse. The operation of the Nanobeacon firmware is
illustrated in Fig. 4. The configuration of the last 4 registers defines the
characteristics of the flash rate, while the control of the pulse is
implemented in the first register. Before starting the LED flashing, an I2C
command is sent via embedded software to the Nanobeacon rail to set the
voltage. In addition to the configuration, the IP core sends an interrupt to
the embedded software after the end of each flashing pattern in a timeslice
has ended, giving feedback of the operation performed.

TABLE I: Control registers of the Nanobeacon IP core. The first register includes the two control enabling/disabling bits, while the remaining registers parameterize the pattern of the Nanobeacon trigger. Reg | Name | Remark
---|---|---
0x00 | Enable register | Bit 0:Enable control - Bit 1:Enable power - Bits(2-31):Not used
0x01 | Delay after TimeSlice | In 16 ns ticks (default 0 ns)
0x02 | Nanobeacon Pulse Width | In 16 ns ticks (default 64 ns)
0x03 | Nanobeacon Pulse Period | In 16 ns ticks (default 50 µs)
0x04 | Number of pulses | Number of pulses (default 100 pulses)
Figure 4: Scheme of the Nanobeacon firmware IP core working operation. The
Nanobeacon trigger is defined by the delay after the start of the timeslice
(m1, and coded in the second register), the number of pulses per timeslice
(m4, and coded in the fifth register), the duty cycle of the pulse (m2, and
coded in the third register) and its frequency (m3, and coded as period in the
fourth register). The interrupt (Nano_IRQ) generated by the firmware IP core
when the number of pulses per timeslice has ended is also shown.

##  IV Comparison of the LED model candidates

Figure 5: Results of the in-situ tests in one of the ANTARES lines of the
preselected KM3NeT LEDs. The average number of hits recorded in each storey
per flash is shown as a function of the distance from the flashing LED. Each
point represents one storey, the stories are spaced 15 m apart. The final
selected LED model for KM3NeT (HLMP-CB1A-XY0DD) is the evolution of CB26 and
CB30.

Several LED models (AVAGO HLMP-CB26, AVAGO HLMP-CB30, AGILENT HLMP-AB87, and
Nichia NSPB500S) were tested in the laboratory and in-situ in one of the
ANTARES lines. The angular aperture of the models analyzed ranges from 10º to
30º. The light intensity is one order of magnitude higher than the intensity
of the LED model used in ANTARES, where the top part of the LEDs used was
removed. The results of the tests performed using the ANTARES line are shown
in Fig. 5. Each point represents the average number of hits per flash detected
on a storey, which in the case of the ANTARES test were spaced by
approximately 15 m. The results are presented for all LED models under study
and compared to the original ANTARES LED beacon. The target storey situated at
150 m from the emitting beacon detects approximately 0.1 hit per flash emitted
by the ANTARES LED beacon, while other LED models considered for KM3NeT record
values ranging from 0.4 to 1.05 hits per flash. The angular distribution of
the analyzed LED models is wide enough to perform intraline calibration, i.e.,
to illuminate the DOMs of the same DU. As already mentioned, the first KM3NeT
tests have shown that it is also possible to illuminate the DOMs of the
nearest DUs, even in the case of ARCA where the lines are spaced 90 m away.
The final LED model chosen for KM3NeT is the HLMP-CB1A-XY0DD by Broadcom. This
model is the evolution of the two LEDs from Avago (CB26 and CB30) tested on
the ANTARES line, which were obsolete and not available at the moment of the
final decision. The main characteristics of this LED are summarized in Table
II. Its light intensity is more than twice the intensity of former models.

Figure 6: Light-tight water tank setup. A DOM is submerged in the tank at a
depth of approximately 2 m. The DOM can rotate, as shown in the figure. Three
light sensors are located on the upper part of the container, collecting the
light emitted by the Nanobeacon being flashed in the DOM.

According to the manufacturer, the effective aperture of the chosen LED model
is 15º in air. However, the angular aperture is modified by the light
transitions through materials with different refraction indices, such as the
DOM glass sphere and the seawater. In order to measure the real angular
aperture, a light-tight water tank available at the Laboratoire AstroParticule
& Cosmologie, Paris, France (APC) was used. A DOM equipped with an embedded
Nanobeacon was immersed in the water tank as shown in Fig. 6. The DOM was
mounted in a mechanical structure allowing its rotation. During the test, the
water tank was completely closed and darkened. Three light sensors were
installed on the inner walls of the water tank: two were located on one corner
of the water tank, about 10 cm below the tank cover and right above the water
surface. One of them was facing down while the other one was facing sideways.
The third sensor was installed at the top cover of the tank, over the vertical
axis of the DOM and 10 cm over the water surface. The three sensors were
recording the measured light intensity while the DOM with the flashing LED was
being rotated. Fig. 7 shows the data read out for different angle positions.
The data have been normalized to the maximum value. The angular aperture of
the selected LED model in water exceeds 40º, much larger than the nominal
value in air.

Figure 7: Light intensity as a function of the polar angle measured in the water tank setup with three light sensors (see details in the text). Data have been normalized to the maximum light intensity. TABLE II: Main characteristics of the HLMP-CB1A-XY0DD LED model. Attribute | Value
---|---
Color | blue
Wavelength (nm) | 470
Aperture (deg) | 15
Luminous intensity (mcd) | 12000
Package | T-1 3/4 (5 mm)

##  V Manufacturing and tests

The first prototypes of the Nanobeacon, eight in total, were installed on the
Neutrino Mediterranean Observatory (NEMO) Phase II prototype tower [22] and
three more were integrated on the prototype DU of KM3NeT [23]. Since then, 600
Nanobeacon boards have been produced for KM3NeT, of which at present more than
200 are in operation in the deep sea. All Nanobeacons incorporate the same LED
model, the HLMP-CB1A-XY0DD. The company in charge of the production also
performed a calibration test, measuring the light emission intensity at
different voltages. This is necessary since there is a noticeable difference
in the light intensity between LEDs. The setup used by the company included a
mechanical framework composed of two PVC pieces manufactured at the IFIC
(Instituto de Física Corpuscular, Valencia, Spain) workshop. The mechanical
framework fixes the Nanobeacon to one of the PVC pieces while it encloses the
Nanobeacon in complete darkness with the other piece. This upper piece, which
has a little hole to allow the light exit from the Nanobeacon, has a space
reserved to hold the head (818-UV) of the Newport energy meter 1835-C. The
test setup is shown in Fig. 8. The light intensity has been measured at 10 V,
16 V, 20 V, and 24 V. The measured values of the mean light intensity at each
voltage are stored in the database of KM3NeT for later use. The mean values
and the standard deviation of the 600 measurements are presented in Table III;
the standard deviation values indicate significant intrinsic variations in
intensity among individual LEDs.

###  V-A Nanobeacon reliability

Nanobeacons are expected to remain operative on a regular basis for at least
20 years, with a variable frequency depending on the calibration needs.
Currently, Nanobeacon operation is performed weekly in KM3NeT with a flashing
frequency of 10 kHz and a typical duration of 10 minutes. In order to qualify
its reliability, a FIDES aaaFIDES means ”faith” in Latin. analysis has been
applied to the Nanobeacon board as to most of the KM3NeT electronics boards
[24]. The resulting FITbbbFIT stands for Failure in Time and indicates the
number of failures per 109 operation hours. value is 10, which is a very low
value thanks to the simplicity and robustness of the Nanobeacon electronics,
corresponding to a life expectancy longer than 20 years. Moreover, the
Nanobeacon will be working only during dedicated calibration runs, which
represent a small fraction of the detector lifetime. In addition, since every
DOM includes a Nanobeacon, a high level of redundancy is assured as a single
Nanobeacon is able to illuminate several nearby DOMs.

Figure 8: Setup used for the production tests. Note the Nanobeacon between the two PVC pieces and the head of the intensity meter coupled with the PVC piece on the right. TABLE III: Results of the intensity test performed after the first batch of the Nanobeacon production. The table presents the mean and the standard deviation of the intensity measurements performed on the 600 produced Nanobeacons. Note the large values of the standard deviations. Voltage (V) | 10 | 16 | 20 | 24
---|---|---|---|---
Mean intensity (mW) | 2.3 | 22.2 | 49.7 | 85.3
Standard Deviation (mW) | 3.8 | 18.8 | 30.3 | 41.7

(a)

(b)

Figure 9: (a) Schematic view of the result of a Nanobeacon calibration run in
KM3NeT. The x-axis shows the reference storey, i.e., the position of the
light-emitting DOM in the DU (1 closest to the see storey, 18 on top); the
y-axis the target storey, i.e., the position of the light-receiving DOM in the
DU. The Nanobeacons were flashed sequentially with a delay of 3520 ns between
one storey and the following. All Nanobeacons were set to a default voltage (8
V). The plot shows that a total coverage of the detector can be achieved with
high redundancy. Therefore, the Nanobeacons have the ability of monitoring the
time offsets of the PMTs in the same DOM, and in different DOMs in the same
DU. For this particular run the DOMs at storey 11 and 17 were not operative.
(b) Time difference between the signal received in the target DOM (second
storey) and light emission in the reference DOM (first storey), corrected for
the light propagation time in seawater. Figure 10: Testbench for the
measurement of the optical pulse. Inside a dark box, a Nanobeacon controlled
by a CLB is flashed. Two PMTs are used. One (a Hamamatsu H653 PMT with a
transit time spread lower than 200 ps) with a filter in order to attenuate the
signal to a level corresponding to one Single Photo Electron (SPE). Then, the
other PMT, (a Hamamatsu H6780-03 PMT with a photo-cathode of 8 mm diameter, a
rise time of 0.8 ns and a transit time of 5.4 ns) receives the pulse directly
to be used as time reference. The output of both PMTs is conducted to one
digital oscilloscope where the skew between both signals is measured. Figure
11: Optical pulse shape obtained with the SPE (Single Photo-Electron) method
fitted with the Gaussian and Weibull functions. Note that the Weibull function
fits the experimental data better, especially at the rise time region.

##  VI Proof of concept

The time calibration potential of the Nanobeacon in terms of volume coverage
and redundancy is demonstrated in Fig. 9. Data from a calibration run taken in
May 2020 have been used, with all the Nanobeacons in the line flashed
sequentially with an operating voltage of 8 V. From the number of entries in
the plot one can deduce that all Nanobeacons provide comparable intensities.
The flashing frequency was 1kHz.

##  VII Optical pulse characteristics

The rise time and width of the optical pulse shape and the spectrum of the
emitted light must be characterized in order to evaluate their effect on the
time calibration. In this section, the results of the qualification are
presented, together with a brief description of the methods used.

###  VII-A Pulse shape

####  VII-A1 Single photoelectron method

The SPE technique [25] has been chosen to measure the rise time and the width
of the Nanobeacon emitted pulse. Fig. 10 shows the scheme of the measurement
setup. The technique allows for the reconstruction of the pulse shape
independently of the gain and rise time of the detector. A light sensor,
usually a PMT with a low Transit Time Spread (TTS) and enough sensitivity to
detect single photons, is needed. In this case, two detectors have been used
in the experimental setup: a Hamamatsu H6780-03 PMT with a photo-cathode of 8
mm diameter, a rise time of 0.8 ns and a transit time of 5.4 ns is used as
trigger and provides an electrical signal generated by the first photons
arriving from the optical pulse. A second detector, a Hamamatsu H653 PMT with
a transit time spread lower than 200 ps and high sensitivity, measures the
arrival time of the photons in the attenuated pulse. The Nanobeacon pulse is
attenuated by means of optical filters so that only one or no photoelectron
per pulse reaches the PMT. To ensure the PMT only detects single photon hits,
a level of filtering that triggers no more than one over 100 Nanobeacon pulses
is set. In this way, the probability to have no signal at all is 99%. Assuming
Poissonian statistics, it is straightforward to estimate that the average
number of single photoelectrons per flash is lower than $\mu$ = 0.01, and the
probability to have two single photoelectrons in the same flash is negligible
(0.005%).

The signals of both PMTs are measured by a digital oscilloscope with a high
bandwidth and sampling period. The time differences between the trigger and
the SPE PMT are measured and accumulated in a histogram that reproduces the
optical pulse shape and allows the measurement of the rise time and the FWHM
of the pulse. The distribution of the time differences reproduces the optical
shape because the probability that a single photon arrives at the SPE PMT is
related to the shape of the optical pulse, i.e., photons in the center of the
optical pulse have higher probability to be detected than those in the leading
or trailing region of the optical pulse. This technique has numerous
advantages with respect to the use of PMTs of high precision. First of all,
the SPE technique is based on the measurement of the temporal differences of
two signals, greatly reducing the residual errors. Moreover, the digitization
of the signals is very effective and has higher time resolution if a Digital
Oscilloscope with high bandwidth, sampling period and large memory is used.
Large memory is specially useful in order to acquire the sufficient number of
hits and increase the precision of the measurement. Finally, high precision
PMTs are considerably more expensive than the PMTs needed to perform the SPE
technique.

####  VII-A2 Results

In order to measure the rise time and the width of the LED model chosen for
KM3NeT, the pulse shape of 24 LEDs was measured using the SPE technique. The
pulse shape was measured for each LED using a voltage of 24 V and a frequency
of 1 kHz. The time distribution obtained for one of the LEDs using the SPE
technique is shown in Fig. 11. The results of the fits to Gaussian and Weibull
functions are superimposed. In Table IV the mean and the standard deviation of
the rise time and the width (FWHM) of the 24 LEDs using a Gaussian and a
Weibull function is shown. The rise time and the FWHM of the optical pulses
are about 2.69 and 4.51 ns respectively, with a statistical accuracy around
8-9%. From the figure, it can be appreciated that the Weibull function matches
the data better, in particular at the beginning of the pulse, providing a more
precise description of the rise time of the pulse.

Figure 12:  Rise time and width (FWHM), obtained from Gaussian and Weibull fits, as a function of flashing frequency and applied voltage. When varying the frequency the voltage is fixed at 24 V. When varying the voltage the frequency is fixed at 1 kHz. Note the correlation of the voltage with the pulse width. TABLE IV: Mean values of the rise time and the FWHM for a Gaussian and a Weibull fit. The results for 24 LEDs of the model HLMP-CB1A-XY0DD being powered at 24 V and triggered at 1 kHz are presented. N LEDs | Rise time (ns) | FWHM (ns)
---|---|---
|  Gaussian |  Weibull |  Gaussian |  Weibull
Mean value |  3.14 |  2.69 |  4.39 | 4.51
Standard deviation |  0.27 (8.6%) |  0.21 (7.8%) |  0.38 (8.6%) |  0.41 (9.1%)
Figure 13: Wavelength spectrum of an LED operated at 30V and 1 kHz. Both, a
Gaussian and a Weibull fit are included. Figure 14: Wavelength spectra for
different voltage values. The data are normalized to the maximum intensity.
The wavelength distribution shifts to higher wavelengths when the voltage is
reduced. Figure 15: Change of the central wavelength, calculated with the
Weibull fit, as a function of the voltage applied for a sample of 8 LEDs. The
central wavelengths vary slightly with the voltage, increasing as the voltage
decreases.

####  VII-A3 Effect of voltage and flashing frequency on the pulse shape

Rise time and FWHM of the optical pulse have been measured at different
voltages and frequencies in order to evaluate the impact of these two
parameters in the shape of the optical pulse. Each set of LED data has been
fitted using a Weibull and a Gaussian function. The voltages have been varied
between 6 V and 24 V, while maintaining a fixed frequency of 1 kHz. In another
set of tests, the frequency has been varied from 1 kHz up to 100 kHz keeping
the voltage fixed at 24 V. All the tests have been carried out using the same
test equipment (Electronics + Nanobeacon + LED). As can be seen in Fig. 12,
the impact of the frequency on the rise time and the FWHM of the optical pulse
is negligible, and the fluctuations are compatible with the measurement
uncertainties. However, the voltage has a clear impact on both rise time and
FWHM. At low voltages (6 V), both decrease to levels only half of the values
measured at higher voltages. The lower the voltage, the fainter and narrower
the optical pulse, giving lower values of rise time and FWHM. This is
especially true for voltages below 15 V. Note that the minimum rise time of
1.24 ns (Weibull fit) at 6 V is very close to the rise time of the electrical
trigger signal. The mean of the rise time of the electrical trigger signal,
after 1100 electrical pulses have been measured, has been computed to be 1.34
ns with a standard deviation of 50 ps.

###  VII-B Wavelength characteristics

The wavelength spectrum of the optical pulse is of great importance as the
water optical properties depend on the wavelength, therefore it has an impact
on the travel time of the pulse and in the time calibration. The wavelength
spectrum of the optical pulse was measured for a set of 8 LEDs with an imaging
spectrometer (Triax 190 de Horiba Scientific) at different voltages. The LEDs
were tested at 30 V, 24 V, 20 V, 18 V, 15 V, 14 V and 13 V, keeping the
frequency fixed at 1 kHz. Changes in the frequency do not affect the
wavelength distribution. The spectral wavelength of an LED is shown in Fig.
13, where a Gaussian and a Weibull fit are superimposed. The wavelength
spectra for different voltage values are shown in Fig. 14. The wavelength
spectrum distribution shape does not change, but the wavelength peak slightly
moves towards lower values when the voltage increases. This can also be seen
graphically in Fig. 15. On average, the distribution shifts 2 nm to higher
wavelength values when the voltage decreases by 5 V, this change being larger
for lower voltages. A variation of 10 nm in the light wavelength corresponds
to a change in the refraction index lower than 0.06 % [26]. This allows for
the use of the Nanobeacon as time accurate (ns resolution) source over
pathlengths higher than 100 m.

##  VIII Summary

In order to ensure the high angular resolution required for the reconstruction
of the neutrino direction in KM3NeT, a relative time synchronization of the
order of 1 ns between DOMs is needed. Different time calibration systems have
been developed to this end. One of them is the Nanobeacon device which is
presented in this article. The time properties of the basic electronics
together with the firmware used for the operation of the board are described.
The properties (rise time and width) of the optical pulse generated by the
Nanobeacon have been evaluated, taking into account the impact of voltage and
frequency on its performance. The wavelength spectrum of the optical pulses
has also been measured, as well as the effect of the input voltage. The
results obtained show that the Nanobeacon is an excellent instrument, with low
cost while fulfilling the time calibration requirements of KM3NeT. The
characterization performed, measuring the optical pulse and its wavelength
distribution, will be of great help during KM3NeT operation. The future plans
include the analysis of LED models with different wavelength (ranging from
near ultraviolet to green) and higher intensity that could be deployed in
KM3NeT for water properties characterization.

## Acknowledgement

The authors acknowledge the financial support of the funding agencies: 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;
Shota Rustaveli National Science Foundation of Georgia (SRNSFG, FR-18-1268),
Georgia; Deutsche Forschungsgemeinschaft (DFG), Germany; The General
Secretariat of Research and Technology (GSRT), Greece; Istituto Nazionale di
Fisica Nucleare (INFN), Ministero dell’Università e della Ricerca (MIUR), PRIN
2017 program (Grant NAT-NET 2017W4HA7S) Italy; Ministry of Higher Education
Scientific Research and Professional Training, ICTP through Grant AF-13,
Morocco; Nederlandse organisatie voor Wetenschappelijk Onderzoek (NWO), the
Netherlands; The National Science Centre, Poland (2015/18/E/ST2/00758);
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-C44) (MCIU/FEDER), Generalitat Valenciana: Prometeo (PROMETEO/2020/019),
Grisolía (ref. GRISOLIA/2018/119) and GenT (refs. CIDEGENT/2018/034,
/2019/043, /2020/049) programs, Junta de Andalucía (ref. A-FQM-053-UGR18), La
Caixa Foundation (ref. LCF/BQ/IN17/11620019), EU: MSC program (ref.
101025085), Spain.

## References

  * [1] S. Adrián-Martínez _et al._ [KM3NeT Collaboration], “Letter of Intent for KM3NeT 2.0”, J. Phys. G 43 373 (2016) no.8, 084001; doi:10.1088/0954-3899/43/8/084001.
  * [2] R. Coniglione. “KM3NeT-ARCA project status and plan”, EPJ Web of Conferences. 116 (2016) 11003. doi:10.1051/epjconf/201611611003.
  * [3] J. Brunner. “KM3NeT - ORCA: Measuring neutrino oscillations and the mass hierarchy in the Mediterranean Sea”, PoS ICRC2015 (2016) 1140 doi:10.22323/1.236.1140.
  * [4] S. Adrian - Martinez _et al._ “Deep sea tests of a prototype of the KM3NeT digital optical module” (KM3NeT Coll.), Eur. Phys. J. C (2014) 74:3056 doi:10.1140/epjc/s10052-014-3056-3.
  * [5] E. Leonora _et al._ [KM3NeT Collaboration], “Design and production of the digital optical module of the KM3NeT project”, EPJ Web of Conferences 136 4008 (2017); doi:10.1051/epjconf/201712604008.
  * [6] S. Adrián-Martínez _et al._ [KM3NeT Collaboration], “Deep sea tests of a prototype of the KM3NeT digital optical module”, Eur. Phys. J. C (2014) 74: 3056 doi: 10.1140/epjc/s10052-014-3056-3.
  * [7] D. Real and D. Calvo, “Digital optical module electronics of KM3NeT”, Physics of Particles and Nuclei 47 918–925 (2016); doi:10.1134/S1063779616060216.
  * [8] S. Aiello _et al._ [KM3NeT Collaboration], “KM3NeT Digital Optical Module electronics: hardware, firmware and software”, J. Astron. Telesc. Instrum. Syst. 5(4), 046001 (2019); doi:10.1117/1.JATIS.5.4.046001.
  * [9] D. Calvo and D. Real. “Status of the central logic board (CLB) of the KM3NeT neutrino telescope”, 2015 JINST10 C12027.
  * [10] D. Real _et al._ “KM3NeT acquisition: The new version of the Central Logic Board and its related Power Board, with highlights and evolution of the Control Unit”, 2020 JINST 15 C03024.
  * [11] A. Belias. “Design and development of the Power Supply Board within the Digital Optical Module in KM3NeT”, PoS TIPP2014 (2014) 188; Proceeding to the 3th TIPP, Amsterdam, 2014.
  * [12] M. Ageron _et al._ [KM3NeT Collaboration], “Dependence of atmospheric muon flux on seawater depth measured with the first KM3NeT detection units”, J. Eur. Phys. C 80, 99 (2020); doi:10.1140/epjc/s10052-020-7629-z.
  * [13] R. Coniglione _et al._ , [KM3NeT Collaboration] ”KM3NeT Time Calibration”, Proceeding to ICRC (2019).
  * [14] J. Serrano _et al._ , “The White Rabbit Project”, in _Proc. 12th Int. Conf. on Accelerator and Large Experimental Physics Control Systems (ICALEPCS’09)_ , Kobe, Japan, Oct. 2009, paper TUC004, pp. 93–95.
  * [15] M. Lipinski _et al._ , “White Rabbit: a PTP application for robust sub-nanosecond synchronization”, 2011 IEEE International Symposium on Precision Clock Synchronization for Measurement, Control and Communication, 2011, pp. 25-30, doi: 10.1109/ISPCS.2011.6070148.
  * [16] D. Real, D. Calvo [KM3NeT Collaboration], ”The Time Calibration System of KM3NeT: The Laser Beacon and the Nanobeacon”, Ad-hoc Networks and Wireless. ADHOC-NOW 2014. Lecture Notes in Computer Science, vol 8629. Springer, Berlin, Heidelberg (2015).
  * [17] D. Real [KM3NeT Collaboration], “Proposal of a new generation of Laser Beacon for time calibration in the KM3NeT neutrino telescope”, AIP Conference Proceedings 1630, 130 (2014); doi:10.1063/1.4902789.
  * [18] D. Calvo [KM3NeT Collaboration], “Nanobeacon: A low cost time calibration instrument for the KM3NeT neutrino telescope”, AIP Conference Proceedings 1630, 138 (2014); doi:10.1063/1.4902791.
  * [19] M. Ageron _et al._ [ANTARES Collaboration], “The ANTARES optical beacon system”, Nuclear Instruments and Methods in Physics Research A 578 (2007) 498-509 doi:10.1016/j.nima.2007.05.325.
  * [20] J.S. Kapustinsky _et al._ “A fast timing light pulser for scintillation detectors”, Nucl. Instr. Meth. Phys. Res A241, 612. (1985).
  * [21] R. Herveille, “WISHBONE System-on-Chip (SoC) Interconnection Architecture for Portable IP Cores”, Specifications, 2002.
  * [22] Adrián-Martínez _et al._ “Long term monitoring of the optical background in the Capo Passero deep-sea site with the NEMO tower prototype” Eur. Phys. J. C 76, 68 (2016). doi:10.1140/epjc/s10052-016-3908-0.
  * [23] Adrián-Martínez _et al._ “The prototype detection unit of the KM3NeT detector” Eur. Phys. J. C 76, 54 (2016). doi:10.1140/epjc/s10052-015-3868-9.
  * [24] D.Real _et al._ ”Reliability Studies for KM3NeT electronics: The FIDES method”, Proceeding to ICRC, Busan, 2017.
  * [25] J. E. McMillan “The single photon technique for measuring LED pulser flash width”, Sheffield Particle Astrophysics (Sheffield University – Department of Physics and Astronomy Internal Publication) (1999) SH-HEP-99-07.
  * [26] C. Parrish. “Index of Refraction of Seawater and Freshwater as a Function of Wavelength and Temperature”, http://research.engr.oregonstate.edu/parrish/index-refraction-seawater-and-freshwater-function-wavelength-and-temperature (2020). Consulted the 29th May 2021.

|  Diego Real is a PhD. in Physics and Research Engineer at Instituto de
Física Corpuscular. He received his BS in Electronics in 1997 and his MS in
Control and Electronics in 2000, both from the Polytechnic University of
Valencia. He is the author of several publications on electronics. His current
research interests include acquisition and synchronization systems for
particle physics. He is, since 2013, the Electronics project leader of the
KM3NeT telescope and member of the Technical Advisory Board of the GVD-Baikal
telescope.
---|---
|  David Calvo is a PhD. in Physics and research engineer at Instituto de
Física Corpuscular of Valencia. He received his MS in Computing in 2006 from
University Jaume I, his MS in Electronics in 2009 from University of Valencia
and his MS in electronic systems design in 2012 from the Polytechnic
University of Valencia. His research interests are focused on the digital
electronics, synchronization and readout acquisition systems. He is the author
of several publications on electronics.
---|---
|  Francisco Salesa is a Distinguished Researcher at Institut de Física
Corpuscular (IFIC) since 2019. He received his Physics Degree from University
of Valencia in 2003, his master degree in 2006, and his PhD in Physics in 2010
with the title “Time Calibration and Point Source Analysis with the ANTARES
Neutrino Telescope”, work which was awarded in 2012 with the “Premi
Extraordinari de Doctorat” for outstanding theses. He had several post-
doctoral positions at Colorado State University (US) 2011-2013, Penn State
University (US) 2013-2015, and Institute of Nuclear Physics (Poland)
2015-2019. He has worked for The Pierre Auger and The HAWC Gamma-ray
Observatories, where he was in charge of the calibration of the detector. He
is the author of several publications on detector calibrations. His research
interests include multi-messenger astronomy and the search for sources of
high-energy cosmic rays.
---|---