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  • 1.
    Bideberg, Glenn
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Ebenhag, Sven Christian
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Jaldehag, Kenneth
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Hedekvist, Per Olof
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Construction of a secure clock location for alternative realization of UTC(SP)2014In: 2014 European Frequency and Time Forum (EFTF), 2014, p. 498-501, article id 7331545Conference paper (Refereed)
    Abstract [en]

    Swedish standard time is regulated by law to follow UTC as maintained by the BIPM. The atomic clocks that are used to implement UTC(SP), the realization of UTC in Sweden, are located at four different sites and are reported to TAI using data from TWSTFT and GNSS links. The activities in the Time and Frequency laboratory at SP are presently undergoing an expansion with the construction of a new additional secure site and the implementation of a distributed time scale.

  • 2.
    Ebenhag, Sven-Christian
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik. Chalmers University of Technology, Sweden.
    Hedekvist, Per Olof
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik. Chalmers University of Technology, Sweden.
    Jaldehag, Kenneth
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Two-Color One-Way FrequencyTransfer in a Metropolitan OpticalFiber Data Network2013In: NCSLI Measure, ISSN 1931-5775, Vol. 8, no 2Article in journal (Refereed)
    Abstract [en]

    Two-color one-way frequency transfer through one strand of optical fiber is an alternative method to two-way frequencytransfer, and is useful if unknown asymmetries exist in the link due to different paths for each direction. The term “twocolor”refers to the ability to send signals at two different wavelengths utilizing the same fiber in one direction. The methodis suitable for implementation in existing urban Single Mode Fiber networks, for instance in networks that are utilized for dataand television communication. It is therefore able to coexist with data channels in wavelength-division multiplexing (WDM)systems. It performs as a dynamical control of transit time and simultaneously enables a real-time phase stabilized output signal.This paper presents results from a comparison of two cesium beam frequency standards separated by about 3 km over an opticalfiber network located in a metropolitan area in Sweden. The cesium standards were simultaneously compared to each other witha Global Positioning System (GPS) satellite link and over optical fibers, so that the optical fiber technique could be evaluatedwith respect to the GPS technique. The difference in frequency stability between the two methods is shown to be about 3 × 10-15over an averaging interval of 10 000 s.1. IntroductionThe need for high accuracy time and frequencytransfer has significantly increased over thelast decade. Users with the most demandingrequirements for the characterization of remotelylocated time and frequency standardsare investigating the use of optical fibers, asthe accuracy and stability of methods based onGNSS (Global Navigation Satellite Systems)such as Global Positioning System (GPS) appearto be reaching their limits. Recent workin the field of optical frequency transfer showsresults with potential accuracies below 10-17obtained in less than a day, demonstrating thatthe technique is useful for the comparison ofoptical frequency standards [1, 2]. Most of theoptical frequency transfer methods or laserstabilizations [3] focus on two-way frequencytransfer [4], whether it uses the optical phase[5-10] or intensity modulation at microwavefrequencies [11], dark fiber, or data protocolsutilized for timing in wavelength division multiplexingchannels [12-14]. When two-wayfrequency transfer methods are used, there is apresumption that the signals delays are equivalent(symmetrical) in both directions, but inmost cases the two counter wise transmissionpaths are asymmetrical and this introduces a

  • 3.
    Ebenhag, Sven-Christian
    et al.
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Hedekvist, Per Olof
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Liström, Stefan
    SUNET Swedish University Computer Network, Sweden.
    Bergroth, Magnus
    NORDUnet, Sweden.
    Time and Frequency Dissemination in an All-optical Coherent Fiber Communication Network2017Conference paper (Refereed)
    Abstract [en]

    A nationwide fiber optic communication network utilizing state-of-the-art technologies with data modulation both in the polarization and in multi-level amplitude and phase is being deployed in Sweden. The network is operated by the Swedish University computer Network, and connects all universities and several research facilities in the country through redundant connections. Since there is a limited amount of clients connected to the network, each client will be assigned a personal wavelength. With a network that is all-optical through an advanced utilization of dynamically reconfigurable optical add and drop multiplexers, each wavelength can theoretically be connected to any other client within the network, enabling broadcasting on allocated wavelengths. The coherent modulation formats also enables signal recovery through electronic digital signal processing after detection, and no optical dispersion compensation is thereby installed. This new network scheme enables a brand new implementation of frequency and time dissemination in the network. The omission of dispersion compensation fibers enhances the symmetry in duplex fiber pair transmission. These optical add-drop installations allow for future efficient frequency and time signal broadcasting from reference nodes operated by distributors such as SP Technical Research Institute of Sweden to users connected to the network.

  • 4.
    Ebenhag, Sven-Christian
    et al.
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Hedekvist, Per Olof
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Weddig, Ludvig
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Measurement and Analysis of Polarization Variations in an Optical Coherent Fiber Communication Network Utilized for Time and Frequency Distribution2018Conference paper (Refereed)
    Abstract [en]

    When fiber optic communication networks are used for ultra-stable timing or frequency transfer, there will always be an influence of polarization variation to some extent. The variations can be induced by e.g. mechanical vibration or electromagnetic effects from adjacent power lines. The output polarization from a 120 km fiber transmission of an ultra-stable optical frequency is analyzed and the requirements of a compensation device are quantified.

  • 5.
    Ebenhag, Sven-Christian
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Zelan, Martin
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Hedekvist, Per Olof
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Karlsson, Magnus
    Chalmers University of Technology, Sweden.
    Implementation of an optical fiber frequency distribution via commercial DWDM2016In: 2016 IEEE International Frequency Control Symposium (IFCS), 2016, article id 7563586Conference paper (Refereed)
    Abstract [en]

    The rapid development in communication infrastructure over the past decades entails an increasing dependence on time and frequency, as well as its redundant distribution. This places demands not only on already existing distribution methods, but also on the development of new ones to meet future needs. To meet these demands several research groups are working on high performance fiber-based frequency transfer techniques. The best achieved performance so far is the techniques using a single bi-directional fiber connection, with customized bi-directional optical amplifiers [1]. The objective of this project is to develop a method that is compatible with data communication in DWDM-systems, i.e. using the existing infrastructure, as well as to be complementary technique for time and frequency distribution. Even though it is likely to have worse performance than the bi-directional system in terms of frequency stability, it will allow for the impassable obligation to follow the deployed structure of telecom networks. The establishment and early results of the non-stabilized link has been previously presented [2, 3]. The ongoing evaluation and improvement will be aimed at finding relevant performance specifications for a connection using this technique. The work presented here is the most recent results of the frequency transfer and discusses the future plans for the fiber connection, including the added time transfer method. If proven successful, the long-term objective is to establish a distribution network for optical frequency references in Sweden.

  • 6.
    Ebenhag, Sven-Christian
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Zelan, Martin
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Hedekvist, Per Olof
    RISE, SP – Sveriges Tekniska Forskningsinstitut.
    Karlsson, Magnus
    Chalmers University of Technology, Sweden.
    Josefsson, Börje
    SUNET Swedish University Computer Network, Sweden.
    Coherent optical two-way frequency transfer in a commercial DWDM network2016In: Proceedings of the Annual Precise Time and Time Interval Systems and Applications Meeting, 2016, p. 116-120Conference paper (Refereed)
    Abstract [en]

    An experimental fiber connection for ultra-stable optical frequency transfer is established between SP Technical Research Institute of Sweden in Borås and Chalmers University of Technology in Gothenburg. The distance is approximately 60 km, and the connection is implemented in the Swedish University Computer Network. The sites are connected through an active flexible communication network where each optical channel can be configured with terminal equipment based on the user needs. The network is implemented with unidirectional optical amplifiers and duplex fibers. The signal quality and the stability when sending an optical coherent frequency utilizing a wavelength in a DWDM system fiber pair, is evaluated within this work. The aim of the system is to be ultra-stable which corresponds to a stability of 10-13 for t = 1 s (Overlapping Allan Variance), as well as providing the ability to distribute monitored ultra-stable frequency with a future traceability to UTC (SP) to multiple users within the future network. This paper describes the current status and results from the frequency transfer between SP and Chalmers.

  • 7.
    Ebenhag, Sven-Christian
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Zelán, Martin
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Hedekvist, Per Olof
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Karlsson, Magnus
    Chalmers University of Technology, Sweden.
    Josefsson, Börje
    SUNET Swedish University Computer Network, Sweden.
    Two-way coherent frequency transfer in a commercial DWDM communication network in Sweden2015In: 2015 Joint Conference of the IEEE International Frequency Control Symposium & the European Frequency and Time Forum, 2015, p. 276-279, article id 7138840Conference paper (Refereed)
    Abstract [en]

    An experimental fiber link is being established between SP Technical Research Institute of Sweden in Boras and Chalmers University of Gothenburg in Sweden. The one way fiber length is about 60 km and implemented in SUNET (Swedish University Network). The aim of the project is to evaluate the signal quality when sending a stable optical frequency utilizing a wavelength in a DWDM (Dense Wavelength Division Multiplexing) system fiber pair. The experiment uses a channel in the DWDM with the wavelength of 1542.14 nm. This wavelength is within the C band and is therefore compatible with common Erbium doped amplifiers in this network. Another aim of the system is to be ultra-stable which corresponds to a stability of 1×10-13 for τ = 1 s as well as providing the ability to distribute monitored ultra-stable frequency with a future traceability to UTC (SP) (National realization of Universal Time Coordinated within Sweden) to multiple users within the network. Measurements of an optical frequency transfer using a fiber-link based on unidirectional light signals in parallel fibers have shown promising results in a free-running setup and in a lab environment. The fractional frequency stability, analyzed as the Overlapping Allan deviation, is approximately 3×10-13 at τ = 10 s and almost 1×10-14 at 105 s.

  • 8.
    Hedekvist, Per Olof
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Jaldehag, Kenneth
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Ebenhag, Sven-Christian
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Rieck, Carsten
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Bideberg, Glenn
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Jaldehag, Kenneth
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Mätteknik.
    Time and Frequency Activities at SP2013Conference paper (Other academic)
    Abstract [en]

    The national Time and Frequency laboratory of Sweden is a part of SP Technical Research Institute of Sweden since 1995. The laboratory is responsible for maintaining the official realization of Swedish standard time and the dissemination of it within Sweden. The objectives of the laboratory include supporting and making it accessible to Swedish industry and authorities with accurate measures of Time and Frequency by instrument calibration, knowledge-transfer, time dissemination, research and development. Swedish standard time is regulated by law to follow UTC as achieved and maintained by the BIPM. The atomic clocks that are used to implement UTC(SP), the realization of UTC in Sweden, are located at five different sites and are reported to TAI using data from TWSTFT and GNSS links. The activities in the Time and Frequency laboratory at SP is presently undergoing an expansion, with the construction of a new additional secure site, the implementation of a distributed time scale and the participation in the Galileo time scale.

  • 9.
    Hedekvist, Per Olof
    et al.
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Weddig, Ludwig
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Ebenhag, Sven C
    RISE - Research Institutes of Sweden, Safety and Transport, Measurement Science and Technology.
    Analysis and compensation of polarization in an optical frequency transfer through a fiber communication network2018Conference paper (Refereed)
    Abstract [en]

    In the fiber optic link, connecting RISE research facilities in Borås with the Photonics Lab at Chalmers University of Technology in Gothenburg, the signal is substantially distorted by polarization variations. It has been verified that the variations are induced by the electrical power grid, however unknown at which segment of the link that it occurs. While this distortion is effectively handled by standard equipment for the data transmission, it deteriorates the detection of a transmitted ultra-stable frequency, using heterodyne mixing. Thus, the magnitude and severity of this distortion is quantified, and some compensation techniques are evaluated.

1 - 9 of 9
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