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| 2. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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A fiber based frequency distribution system with enchanced output phase stability
- 2009
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Ingår i: Proceedings EFTF-IFCS2009 joint conference 20-24 April 2009, IEEE catalog number:CFP09FRE-CDR. - 1075-6787. - 9781424435104 ; , s. 1061-1064
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Konferensbidrag (refereegranskat)abstract
- Experimental results on the stability of the output phase of a frequency distribution system from several days of measurement is presented, in addition to a discussion regarding the influence of control loop parameters. The setup handles the issue that the output phase stability of a system depends on perturbations along the transmission length. This is especially critical if the signal is transmitted through optical fiber, at lengths of a few 100 m. An experimental evaluation using a laser based transmitter at a wavelength of 850 nm, and 625 m of multimode fiber where 575 m where placed outdoor, a temperature dependence of 100 ps/°C was detected. Tocompensate for these slow variations in real time, a setup using two-way transmission, in conjunction with an adjustable optical delay, was constructed. This device is adjusted to induce a delay variation of equal magnitude but opposite direction, in comparison to the delay change of the fiber. Calculating the modified Allan deviation of the transmitted signal, it is apparent that without active compensation, the deviation at τ below 1000 s is comparable to the values from the measurement system without transmission. At longer integration times, however, the slow variations in the fiber transmission will deteriorate the modified ADEV substantially. When activating the dynamic adjustment of pre-delay in the system, the deviation at shorter times will increase with a few dB, however, the modified ADEV decreases continuously with τ, eventually below the values for the uncompensated system. In conclusion, activating a dynamically controlled pre-delay in a fiber based frequency transmission system will induce a small penalty on fast variations of the output phase, however giving a remarkable improvement on slower variations. The usefulness of this added functionality must therefore be determined by the application of the signal.
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| 3. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Active detection of propagation delay variations in single way time transfer utilizing dual wavelengths in an optical fiber network
- 2011
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Ingår i: Proceedings of the IEEE International Frequency Control Symposium and Exposition. - 9781612841113
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Konferensbidrag (refereegranskat)abstract
- Several communication systems of today rely on the real time accessibility of accurate time and frequency measures and there is an increasing demand for the development of new and redundant methods for the distribution of these measures. The classical two-way method is able to compensate for the inevitable variations in the time and frequency propagation delay. The two-way method is used for time transfer in free space, electrical or optical domain, but has the disadvantage of often using two different paths for transmitting back and forward. The paths may be of equal length and have equal propagation delay, but nevertheless there is often a remaining asymmetry in the propagation paths. The inevitable asymmetry between the paths in the time transfer delay must be detected and compensated for, if an accuracy better than s-level is needed for transmission distance exceeding a few km. Furthermore, if the number of users is high, there will be a complex and large network of two-way time signal transmissions. Therefore, a solution using one-way broadcasting would be more desirable, and would be possible if the variations in transmission time could be estimated from the received data at the far (user) end. The one-way method uses only one path of transmission and is possible to implement in existing Wavelength Division Multiplexing-networks. Proof of concept and results of this one-way time transfer technique based on transmission of a repetitive signal, modulated on two lasers at different wavelengths 8 nm apart 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 collected which shows improvement in the reliability and quality of this technique. The stability is improved through component analysis and minimizing error sources. The distance is improved from 38km to 160km.
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| 4. |
- Ebenhag, Sven-Christian, 1976
(författare)
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Evaluation of Output Phase Stability in a Fiber Optic Two-way Frequency Distribution System
- 2008
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Ingår i: 40th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting 2008; Reston, VA; United States; 1 December 2008 through 4 December 2008. - 9781615672394 ; , s. 117-124
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Konferensbidrag (refereegranskat)abstract
- The assembly of a real-time actively controlled fiber-optic frequency transfer link based on additions to an existing static solution is proposed and evaluated. The dynamic variations of transmission delays, caused mainly by temperature dependence of the fiber, is continuously measured and used to adjust a delay at the transmitter for net cancellation of the effect. The resulting link shows no visible temperature dependence; however, the feedback induces added noise at short integration times.
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| 5. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Fiber based frequency distribution based on long haul communication lasers
- 2009
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Ingår i: 41st Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting 2009; Santa Ana Pueblo, NM; United States; 16 November 2009 through 19 November 2009. - 9781617386541 ; , s. 57-65
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Konferensbidrag (refereegranskat)abstract
- Resent research and development of optical clocks has increased the need and requirement of better performing time and frequency transfer over baselines longer than 100 km. This need have resulted in lot of activities in transfer methods using optical fiber. Some of them in dedicated fibers others in already existing fiber networks. This study has focused on one-way transmission over fiber optic WDM-network. The results show that it is possible to perform a one-way time and frequency transfer with two wavelengths and by evaluate these two against each other create a correction signal for compensation for influences along the transmission path. This experiment has shown proof of concept, but further work is needed. Future work includes development of a physical correction component in the end of the link that incorporates the steering signal from the difference between the two wavelengths.
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| 6. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Fiber based one-way time transfer with enhanced accuracy
- 2010
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Ingår i: 24th European Frequency and Time Forum, EFTF 2010; Noordwijk; Netherlands; 13 April 2010 through 16 April 2010. - 9781467359702
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Konferensbidrag (refereegranskat)abstract
- To meet the request for access to accurate and reliable time, several time and frequency transfer methodsusing optical fibers have been developed or are under development. These fiber based techniques willovercome the issues of vulnerability in radio- and satellite solutions; however, they all rely on two-waytransmission when variations in transfer time must be compensated for. As an alternative, a one-waytransmission over fiber optic WDM-network has been proposed, with estimation of variation in transfertime based on detection of transfer time difference between two co-propagating lightwaves at differentwavelengths. The technique was presented previously when the two wavelengths were far apart. Here wepresent results from an experiment where both wavelengths are within the optical C-band, i.e. within thegain bandwidth of Erbium-doped fiber amplifiers. Thereby it is proven that the technique is usable to alarger extent than previously demonstrated.
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| 7. |
- Ebenhag, Sven-Christian, 1976
(författare)
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Frequency Transfer Techniques and Applications in Fiber Optic Communication Systems
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern society is dependent on communications and the developments of these increases constantly through a seemingly endless demand for communication services and thereby synchronization and time. This is confirmed by a vast range of research on communications, irrespective of technology and protocol. Historically, the national metrology institutes are the distributors of stable accurate time and frequency through national timescales, but that situation has changed with the arrival of Global Navigation Satellite Systems (GNSS) such as GPS (Global Positioning System). The introduction of GNSS-based solutions has resulted in improvement for system users and owners in need of time and frequency. When using a GNSS receiver, sufficient accuracy and precision is often achieved. However, a disadvantage of this development is that GNSS-solutions are based on weak radio signals that can be interfered with. The main objective of the research that forms the groundwork for this thesis is the development of new fiber based methods for time and frequency. The aim is to complement GNSS-based methods for redundancy, with the intention of strengthening the robustness of the Swedish infrastructure. The research has resulted in two unique and innovative transmission technologies, one of which has been patented (two-color, one-way).The first method is based on a non-insertion technology that utilizes passive listening to existing data frames in a fiber optical network and does not require any particular bandwidth. This technology only uses a fraction of the optical signal for time and frequency measurement from an indirect connection to the network. This method has resulted in a precision relative to the GPS carrier phase of less than 1 ns root mean square for distances exceeding 1,100 km. This precision has been achieved for all of the included experiments, conducted within the framework of the thesis, regardless of configuration.The other fiber based technology is a one-way method that uses two wavelengths (colors) for the realization of a correction algorithm and signals thereto. It was developed because the symmetry required for performing two-way time and frequency transfer is rarely precise enough.This optical fiber technique was evaluated with respect to a GPS precise point positioning technique in an urban fiber optical network. The difference in frequency stability between the two systems has been shown to be about 3 × 10-15 over an averaging interval of 10,000 s for a distance of 3 km. The method has also been evaluated in several laboratory experiments with fiber distances up to 160 km. The best performing result is presented as time resolved transit time variations compared with arrival time difference. The standard deviation of the difference between the reference measurement and the one-way, two-color technique result is 3.12 ns and the data showed temperature dependence in transit time of 6 ns / °C.
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| 8. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Measurements and Error Sources in Time Transfer Using Asynchronous Fiber Network
- 2010
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Ingår i: IEEE Transactions on Instrumentation and Measurement. - 1557-9662 .- 0018-9456. ; 59:7, s. 1918-1924
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Tidskriftsartikel (refereegranskat)abstract
- We have performed time transfer experiments basedon passive listening in fiber optical networks using Packet over synchronous optical networking (SONET)/synchronous digital hierarchy(SDH). The experiments have been performed with differentcomplexity and over different distances. For assessmentof the results, we have used a GPS link based on carrier-phase observations. On a 560-km link, precision that is relative to the GPS link of
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| 9. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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One way time transfer utilizing active detection of propagation delay variations of dual wavelenghts in an optical fiber network
- 2011
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Ingår i: 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
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Konferensbidrag (refereegranskat)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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| 10. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Single way fiber based time transfer with active detection of time transfer variations
- 2010
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Ingår i: 42nd Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Reston Virginia, November 15-18.
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Konferensbidrag (refereegranskat)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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