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11.
  • Ebenhag, Sven-Christian, 1976, et al. (creator_code:aut_t)
  • One way time transfer utilizing active detection of propagation delay variations of dual wavelenghts in an optical fiber network
  • 2011
  • record:In_t: 43rd Annual Precise Time and Time Interval Systems and Applications Meeting 2011; Long Beach, CA; United States; 14 November 2011 through 17 November 2011. - 9781622767953 ; , s. 9-16
  • swepub:Mat_conferencepaper_t (swepub:level_refereed_t)abstract
    • For time transfer on baselines exceeding 100km, GPS is the most common technical solution. The main benefit is that it is easy to install, but it is a single point of failure and it is sensitive to interruption by radio noise. For time transfer requiring high reliability a complementary backup method is therefore desirable, and time and frequency transfer utilizing optical fiber is a favorable alternative technique. The connectivity is simplified by the deployment of dense fiber optic communication networks in most countries and since it does not rely on transmission using radio waves in open air, it is robust against perturbations. The simplest and most straightforward method for high performance time transfer is the two-way technique, which is an excellent choice when the user has access to the whole system, and when both transmission paths are equal or with a known and predictable asymmetry. Furthermore it is most practical when the numbers of users are limited and when no security issues limit the bidirectional connectivity.A proof-of-concept for an alternative technique for fiber based time and frequency transfer, utilizing a one-way co-propagation of two light waves, has been presented previously. The technique utilizes dual wavelengths and measures the difference in group velocity to estimate the delay variation of the timing signal in one of the wavelength channels. This paper presents the recent improvements on this method, including new equipment, new algorithm and a demonstration of real-time compensation of delay time variations.
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12.
  • Ebenhag, Sven-Christian, 1976, et al. (creator_code:aut_t)
  • Single way fiber based time transfer with active detection of time transfer variations
  • 2010
  • record:In_t: 42nd Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Reston Virginia, November 15-18.
  • swepub:Mat_conferencepaper_t (swepub:level_refereed_t)abstract
    • With an increasing demand for distributing accurate time, preferably accessible in real time, better techniques for time transfer is needed. The inevitable variations in the time transfer delay must be detected and compensated for, if an absolute precision better than µs-level is needed, for transmission distance exceeding a few km. This is usually solved through two-way transfer, where the transmitter can estimate and forward the information on the actual transfer time at every instant, and this can be performed both over aerial transmission and fiber. If the number of users is high, there will be a complex and large network of two-way time signal transmissions. In comparison a single, one-way broadcasting is more desirable, and would be enabled if the variations in transmission time could be estimated from the received data at the far end. Some preliminary results of this time transfer technique based on transmission of a repetitive signal, modulated on two lasers at different wavelengths and transmitted through an optical fiber, has been presented previously. These data showed a strong correlation between a change in transfer time at one wavelength, and the transfer time difference for the signals at the two wavelengths. In this paper, the setup and the measurement results have been improved and new data is gathered, which shows improvement in the reliability and quality of this technique. There is to our knowledge no other technique for time transfer over wire-line solutions, where the variations in transfer time is measured and compensated for, that only needs transmission in one direction.
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13.
  • Ebenhag, Sven-Christian, 1976, et al. (creator_code:aut_t)
  • Time Transfer over a 560 km Fiber Link
  • 2008
  • record:In_t: EFTF 2008 - 22nd European Frequency and Time Forum. ; , s. Paper E5A04 -
  • swepub:Mat_conferencepaper_t (swepub:level_refereed_t)
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14.
  • Ebenhag, Sven-Christian, 1976, et al. (creator_code:aut_t)
  • Two-Color One-Way Frequency Transfer in a Metropolitan Optical Fiber Data Network
  • 2013
  • record:In_t: NCSLI International Measure. - 1931-5775. ; 8:2, s. 10-
  • swepub:Mat_article_t (swepub:level_refereed_t)abstract
    • Two-color one-way frequency transfer through one strand of optical fiber is an alternative method to two-way frequency transfer, and is useful if unknown asymmetries exist in the link due to different paths for each direction. The term “two-color” refers to the ability to send signals at two different wavelengths utilizing the same fiber in one direction. The method is suitable for implementation in existing urban Single Mode Fiber networks, for instance in networks that are utilized for data and 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 optical fiber network located in a metropolitan area in Sweden. The cesium standards were simultaneously compared to each other with a Global Positioning System (GPS) satellite link and over optical fibers, so that the optical fiber technique could be evaluated with respect to the GPS technique. The difference in frequency stability between the two methods is shown to be about 3 × 10-15 over an averaging interval of 10 000 s
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15.
  • Hedekvist, Per Olof E, 1967, et al. (creator_code:aut_t)
  • Active optical pre-compensation in short range frequency transfer in optical single-mode fiber
  • 2011
  • record:In_t: 2011 Joint Conference of the IEEE International Frequency Control Symposium/European Frequency and Time Forum Proceedings. - 1075-6787. - 9781612841106 ; , s. 315-316
  • swepub:Mat_conferencepaper_t (swepub:level_refereed_t)abstract
    • Short distance fiber based optic distribution of time and frequency is often considered sufficiently stable and therefore, uncompensated one way transfer is used. However, when fiber distances increase to a few km, and a substantial amount of the fiber is installed outdoors or in aisles and attics without temperature control, the delay through the fiber may vary substantially. Since the variations are slow, a microwave frequency modulated on an optical carrier may be assumed to remain sufficiently stable. The time delay through the fiber will nevertheless depend on the extremes of the temperature variation which must be taken into account or compensated for. This study presents the design issues of a real time compensation and some results of an optical compensation technique.
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16.
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17.
  • Jaldehag, R. T. Kenneth, 1962, et al. (creator_code:aut_t)
  • Time and frequency activities at SP in Sweden
  • 2009
  • record:In_t: 41st Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Santa Ana Pueblo. NM, November 16-19. - 9781617386541 ; , s. 231-251
  • swepub:Mat_conferencepaper_t (swepub:level_scientificother_t)abstract
    • The national time and frequency laboratory of Sweden has since 1995 been a part of the measurement technology department at the SP Technical Research Institute of Sweden. The laboratory is responsible for maintaining a realization of Swedish standard time and the dissemination of this time scale in Sweden. The objective of the laboratory is to support and supply Swedish industry and authorities with accurate measures of time and frequency by instrument calibration, knowledge transfer, time dissemination, and research and development. Swedish standard time is connected by law to UTC as maintained by the BIPM. UTC (SP) is the realization of UTC in Sweden and is traceable to UTC via BIPM and time transfer using the GPS and TWSTFT techniques. This paper describes the generation and maintenance of UTC (SP) and the equipment, including clocks and time transfer equipment, needed for this task as well as the concept of a "Distributed Time Scale" using alternate versions of UTC (SP) maintained at sister laboratories in Sweden. The paper presents also activities related to the dissemination of Swedish standard time including GPS time transfer, Network Time Protocol (NTP), telephone time code, and a speaking clock. Finally, research activities including time transfer in optical fiber networks, continuous GNSS carrier-phase processing, and Kalman-filter-based ensemble clock generation are briefly presented.
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18.
  • Jaldehag, R. T. Kenneth, 1962, et al. (creator_code:aut_t)
  • Time and frequency transfer using asynchronous fiber optical networks: progress report
  • 2009
  • record:In_t: 41st Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Santa Ana Pueblo. NM, November 16-19.
  • swepub:Mat_conferencepaper_t (swepub:level_scientificother_t)abstract
    • SP Technical Research Institute of Sweden has since 2004 been running a project withthe aim of performing time and frequency transfer using commercial asynchronous fiberopticalnetworks. The project is motivated by the need for an alternative and complementarytime transfer method on a national basis with the goal of reaching accuracy and stabilitycomparable to satellite-based methods. Previous results using an OC-192/STM-64 10-Gb/spacket over SONET/SDH network, show that time transfer accuracy of the order of a fewnanoseconds is possible on baselines exceeding 500 km [1]. The method is based on passivelistening on existing data traffic and the detection of certain bit sequences in the SDH frameheaders continuously transmitted by the network routers. By using two-way time transfer, itis possible to estimate and compensate for symmetric delays in the optical fibers. The methodrelies on that time dependent residual delays are small or can be can be compensated forand constant residual delays can be calibrated.This paper briefly revises the method and presents new results in comparison with theGPS carrier-phase technique, with focus on residual effects due to temperature variationswhich have shown to have significant impact on the stability and accuracy. It also discusseshardware miniaturizations as well as new ideas for active time transfer using bit-sequencegenerators/transmitters in dedicated wavelength slots of the optical network. Finally, the useof a subset of the IEEE standard 1588-2008 (Precise Time Protocol, PTP) for data transportis briefly discussed.
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19.
  • Jaldehag, R. T. Kenneth, 1962, et al. (creator_code:aut_t)
  • Time Transfer Using Frame Detection in Fiber-Optical Communication Networks: New Hardware
  • 2011
  • record:In_t: Frequency Control and the European Frequency and Time Forum (FCS), Joint Conference of the IEEE International.
  • swepub:Mat_conferencepaper_t (swepub:level_refereed_t)abstract
    • In this paper, a new, recently developed hardware for time transfer using passive listening and detection of SDH frame headers in fiber-optical networks is described. The method has been presented earlier, and results, using prototype equipment and an experimental fiber-link, have shown that time transfer with a precision of the order of a few nanoseconds is possible over links with network distances exceeding 500 km. In order to further develop the method and make it available to regular users of time keeping equipment, it has been essential to minimize the space requirements of needed hardware and to make the implementation and installation more easily and straight forward. The new hardware is in this paper described in detail.
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