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Session P6: Present and Emerging Applications and Techniques for Time and Frequency using GNSS/RNSS/LEO and Optics

National Infrastructure for Dissemination of Precise Time and Coherent Ultra-stable Optical Frequency - CITAF
Josef Vojtecha, Vladimir Smotlachaa, Ondrej Havlisa, Martin Slapaka, Lada Altmannovaa, Jan Kundrata, Sarbojeet Bhowmicka, Rudolf Vohnouta, Radek Velca, Martin Cizekb, Jan Hrabinab, Simon Reruchab, Lenka Pravdovab, Josef Lazarb, Ondrej Cipb, Alexander Kunac, Jaroslav Roztocild aCESNET z.s.p.o., Prague, Czechia; bInstitute of Scientific Instruments of the Czech Academy of Sciences, Czechia; cInstitute of Photonics and Electronics of the Czech Academy of Sciences, Prague, Czechia; dDepartment of Measurement, Faculty of Electrical Engineering, Czech Technical University, Prague, Czechia
Location: Beacon A

Dissemination of precise time and coherent ultra-stable coherent optical services underwent significant interest and developed from first laboratory trials into operational services over large fiber plants. Very typical is, especially in Europe, the involvement of National Research and Educational Network (NREN) operators. Large scale and long term sustainability dictates to use concept where transmission fibers are shared with regular data transmissions.
Czech Infrastructure for Time and Frequency (CITAF) is a non-commercial and open activity focused on the transfer of accurate time and very stable frequency using optical networks. The national optical infrastructure for time and frequency transfer is operated on top of the CESNET network infrastructure. It is used to transmit time scales of atomic clocks in involved organizations, the Institute of Photonics and Electronics, the Institute of Instrumentation, the Faculty of Electrical Engineering of the Czech Technical University, and the Research Institute of Geodesy, Topography and Cartography. Time transfer utilizes adapters developed by CESNET and the White Rabbit system. Another application is the transfer of ultra-stable frequency over hundreds of kilometers of optical fiber with active noise cancellation.
The infrastructure already reaches two neighboring countries, Austria, Poland and Slovakia should be reached also this year.
The CITAF infrastructure is the result of more than 10 years long research activities of the CESNET association in cooperation with partners and is further developed. The achieved parameters and potential correspond to similar activities in other countries and allow us to participate in international research in time and frequency metrology.
The aim of CITAF activity is:
• to be a national platform for cooperation in research and development of methods of the maximum possible
• stability and accuracy of time and frequency transmission in the environment of optical networks;
• to strive for the establishment of a permanent national optical infrastructure for the transmission of
• time and frequency with parameters corresponding to the most stable current and developed sources of frequency
• and for its connection to the follow-up European infrastructure;
• to support joint publishing activities and cooperation in national and international projects and grants;
• to present the results of cooperation and develop an awareness of the possibilities and application of distributed very accurate time and stable frequency.
Above mentioned sharing of fibers with data channels can be achieved via using the so-called Alien Waves (AW) service in Dense Wavelength Division Multiplexing (DWDM) system. This allows fast service establishment at minimal costs, and especially it can be used for precise time transmission. For coherent ultra-stable optical frequency transmission, a bidirectional optical channel must be created to deploy active noise cancelation. A separate bidirectional channel is also required for time transmission with maximal achievable precision.
First transmissions have been established using dedicated “legacy” wavelengths: 1542.14 nm or 1540.56 nm. These wavelengths represent a drawback in modern transmission systems supporting dynamic allocation of channels with variable spectral width. Unfortunately, this has prevented some operators from fiber sharing. Thus operation in 1570nm spectral gap between C and L bands used in telecom data transmission has been proposed, laboratory verified (e.g. by CESNET)[5] and later also deployed in several countries, e.g. by Swiss NREN SWITCH [7].
In the prepared presentation, we would like to address infrastructure itself an important aspects of the operation of these time and frequency transmission services, and the possibility of direct interconnection of optical clocks using active bidirectional channel in 1458 nm region. Also use of distributed Brillouin amplification for coherent optical frequency transfer, which provides really high gain and effectively eliminates unwanted oscillations, will be discussed for proposed spectral band 1570 nm.
Keywords: precise time, coherent optical frequency, time metrology, infrastructure, optical amplification, optical fiber
REFERENCES
[1] https://clonets-ds.eu/ (31 March 2021).
[2] http://empir.npl.co.uk/tifoon/ (31 March 2021).
[3] V. Smotlacha, A. Kuna, W. Mache: Time Transfer in Optical Network. Proc. 42nd Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Reston, Virginia, USA, (U.S. Naval Observatory, Washington D.C.), (2010) 427–36.
[4] V. Smotlacha, A. Kuna, W. Mache: Optical Link Time Trans-fer between IPE and BEV. Proc. 43rd Precise Time and Time Interval (PTTI) Systems and Applications Meeting (2011) 27–34
[5] J. Vojtech et al., "White Rabbit Single Fibre Bidirectional Transmission of Precise Time Using Unconventional Wavelengths," 2020 Joint Conference of the IEEE International Frequency Control Symposium and International Symposium on Applications of Ferroelectrics (IFCS-ISAF), 2020, pp. 1-4, doi: 10.1109/IFCS-ISAF41089.2020.9234815
[6] J. Vojtech, et al, "Joint accurate time and stable frequency distribution infrastructure sharing fiber footprint with research network," Opt. Eng. 56(2) 027101 (6 February 2017) https://doi.org/10.1117/1.OE.56.2.027101
[7] Dominik Husmann, Laurent-Guy Bernier, Mathieu Bertrand, Davide Calonico, Konstantinos Chaloulos, Gloria Clausen, Cecilia Clivati, Jérôme Faist, Ernst Heiri, Urs Hollenstein, Anatoly Johnson, Fabian Mauchle, Ziv Meir, Frédéric Merkt, Alberto Mura, Giacomo Scalari, Simon Scheidegger, Hansjürg Schmutz, Mudit Sinhal, Stefan Willitsch, and Jacques Morel, "SI-traceable frequency dissemination at 1572.06 nm in a stabilized fiber network with ring topology," Opt. Express 29, 24592-24605 (2021)



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