L. Sliwczynski

Fiber-Optic Joint Time and Frequency Transfer With Active Stabilization of the Propagation Delay

In this paper, we describe the extension of our fiber-optic frequency transfer system to the time transfer capability. In contrast to standard two-way transfer schemes which offer only comparisons of two distant clocks, our system displays distribution functionality, reproducing the time and frequency signals of the reference clock in the remote location. By using active compensation of the fiber delay fluctuations, we obtained a time deviation of 0.3 ps (for time transfer) and an Allan deviation of 1.2 × 10-17 (for frequency transfer) at 105-s averaging. The experiments presented were carried out using a 60-km-long fiber loop, forming a part of the real urban network around Kraków.

Multipoint dissemination of RF frequency in fiber optic link with stabilized propagation delay

In this paper, we present the concept of accessing the signal at some midpoint of a frequency dissemination system with stabilized propagation delay, which allows building the point-to-multipoint frequency dissemination network. In the first experiments with a 160 km-long fiber link composed of a field-deployed optical cable and fibers spooled in the lab, exposed to both diurnal and seasonal temperature variations, in the access node, we obtained the Allan deviation of a 10- MHz frequency signal of about 3 × 10-17 and the time deviation not greater than 2 ps for 105 s averaging.

ELSTAB—Fiber-Optic Time and Frequency Distribution Technology: A General Characterization and Fundamental Limits

In this paper, we present an overview of the electronically stabilized (thus named ELSTAB) fiber-optic time and frequency (T&F) distribution system based on our idea of using variable electronic delay lines as compensating elements. Various extensions of the basic system, allowing building a long-haul, multiuser network are described. The fundamental limitations of the method arising from fiber chromatic dispersion and system dynamics are discussed. We briefly characterize the main hardware challenge of the system, which is the design of a pair of low-noise, precisely matched delay lines. Finally, we present experimental results with T&F distribution over up to 615 km of fiber, where we demonstrate frequency stability in the range of 1-7 × 10-17 for 105 s averaging and time calibration with accuracy well below 50 ps. Also, practical implementation of the ELSTAB in the Polish T&F distribution network is shown.

Multipoint joint time and frequency dissemination in delay-stabilized fiber optic links

This paper presents the system for dissemination of both the RF frequency (e.g., 5, 10, or 100 MHz) and time (pulse per second) signals using an actively tapped fiber-optic link with electronic stabilization of the propagation delay. In principle several nodes for accessing the time/frequency signals may be added without the degradation of the dissemination in the main link. We are discussing the algorithm of determining the propagation delay from the local end of the link to the access node that is required for calibration of the time dissemination. Performed analysis shows that the uncertainty of the time calibration at the access node may in practice be dominated by the dependence of the propagation delay of the receivers on impinging optical powers and is only weakly affected by the distance between the local and access modules. The uncertainty is, however, still low, being only about two times higher compared with the calibration uncertainty of the main link. Experimental results performed on several spooled fibers show that the accuracy of described calibration procedures, expressed as a difference from the results of direct measurement, is not worse than 35 ps.

Measurement of acoustic noise in field-deployed fiber optic cables

In the paper we are presenting the results of the measurements of the phase noise occurring in the optical fiber because of mechanical (acoustic) vibrations. The system used for these measurements, based on a dual-mixer time-difference method, is also presented. We tested two different fiber spans, one running along the motorway and the second one running along the railroad tracks. After analyzing more than 100 h of recorded acoustic noise signals we found substantial differences depending not only on the route of the fiber but even on the time of the day.

Multipoint dissemination of RF frequency in delay-stabilized fiber optic link in a side-branch configuration

In this paper we are presenting the concept of the multipoint dissemination of RF frequency signal via a fiber optic network with actively stabilized propagation delays. The idea allows to add tapping nodes and side branches to the main point-to-point frequency transfer link, constituting a tree-like dissemination network. We are demonstrating the experimental results with the 100 km-long side branch starting at the 110th km of the 170 km-long main link. At the end of the side branch we obtained the Allan deviation of a 10-MHz frequency signal of about 4×10-17, and the time deviation not greater than 1.2 ps for 105 s averaging.

Frequency distribution in delay-stabilized optical DWDM network over the distance of 3000 km

In the paper we are presenting the results of the experiments we performed with sending the frequency signals (10 MHz) to the remote location exploiting the optical dense wavelength division multiplexed telecommunication network. To stabilize the phase of the frequency signal we applied the approach with the electronic stabilization of the propagation delay. We measured the residual instability resulting from the fact that in a telecommunication network the signals in the forward and backward direction do not share the same fiber and are transmitted through different pieces of equipment when passing through reconfigurable optical add drop multiplexers or optical amplifiers. Our experiments show that results may depend substantially on the route of the link. For all tested links, however, the stability was better than the stability of the signal generated by commercial 5071A cesium standard. In case of one link even the stability better than stability of H-maser was observed for averaging times longer than 1000 s.

Optical fiber time and frequency transfer inside urban telecom network in Warsaw - Results of initial tests

In this paper, the initial results of optical fiber time and frequency transfer at the optical distance up to about 40 km inside urban telecom network have been presented. The applied optical methods have been shortly described. The results of optical measurements have been compared with GPS CV method and specifications of compared clocks. In practice, it was obtained a similar effect if compared clocks were standing very close to each other.

Delay compensation of 1 PPS timetags in fiber-optic time distribution

In various fiber-optic time and frequency distribution systems the delay of 1 PPS timetags is stabilized and precisely calibrated. However in a case of long fiber path the 1 PPS signal is seriously delayed in respect to UTC, which is cumbersome in various applications. In this work we present an idea and experimental evaluation of a circuit designed for compensating this delay. The compensation up to 1 s may be introduced in 10 ns steps, and the standard uncertainty of the compensation is app. 550 fs.

Towards sub-nanosecond synchronization of a telecom network by fiber optic distribution of UTC(k)

The operation of a mobile telecom network relies directly on the synchronization of thousands of base stations that must be robust and operate reliably 24 hours a day. Nowadays networks using advanced Long Time Evolution (LTE) standard need not only frequency syntonization, but also time synchronization that should ideally be traceable to UTC. In this work we present first results of providing UTC(PTB) (1 PPS and 10 MHz) by optical fiber to a test center of Deutsche Telekom in Bremen. The work aimed at a proof-of-concept in real telecommunication environment and the demonstration of the long-term operation capability and scalability of the approach. A potential future application is the comparison of UTC(PTB) and UTC(DTAG).