Marcin Lipinski

Active Propagation Delay Stabilization for Fiber-Optic Frequency Distribution Using Controlled Electronic Delay Lines

In this paper, we present the novel method of stabilizing the propagation delay in the fiber-optic link using two matched electronic delay lines. For this purpose, we developed a dedicated integrated circuit using Austria Mikro Systeme 0.35-μm CMOS process. Two transmission links were tested at two independent laboratories, operating at different distances (20 and 60 km, respectively) and using slightly different setups. Both links show the Allan deviation of about 2 × 10-17 after 1-d averaging and residual fluctuations of the propagation delay of around 12 ps peak to peak. The obtained results show that such systems are suitable for distributing the signals from hydrogen masers or cesium clocks at the distances spanning tens of kilometers.

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.

Optical fibers in time and frequency transfer

In the paper we analyze the fundamental accuracy limits of the time/frequency transfer in fiber-optic transmission systems based on intensity modulation and direct detection (IM-DD) of the light signal. The unidirectional and bidirectional time/frequency transfer schemes are considered, and their main limitations are pointed out. In particular, the impact of the fiber backscattering and the temperature dependence of the chromatic dispersion are examined in the context of bidirectional transfer. Finally, experimental results are presented and related to the preceding considerations. The experiments performed with a 60 km long fiber demonstrate single-picosecond accuracy of the time transfer. Our measurements suggest that it should be possible to obtain better accuracy of time/frequency transfer than that reported in the literature for systems based on the IM-DD principle.

Frequency Transfer in Electronically Stabilized Fiber Optic Link Exploiting Bidirectional Optical Amplifiers

In this paper, we present the experimental result for the fiber optic frequency transfer link with optical regenerators exploiting the same Er-doped fiber for amplifying the signals in both directions. The measurements performed at the distance of 124 km with one amplifier and 224 km with three amplifiers showed that the presented solution is adequate for transferring the frequency signals generated by cesium clocks, hydrogen masers, and cesium fountain clocks. The Allan deviation around 9 ·10-18 and 5.5 ·10-17 was found for the averaging time of 105 s for 124- and 224-km-long links, respectively.

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.

Absolute measurement of the 1S0 − 3P0 clock transition in neutral 88Sr over the 330 km-long stabilized fibre optic link

We report a stability below 7 × 10−17 of two independent optical lattice clocks operating with bosonic 88Sr isotope. The value (429 228 066 418 008.3(1.9)syst (0.9)stat Hz) of the absolute frequency of the 1S0 – 3P0 transition was measured with an optical frequency comb referenced to the local representation of the UTC by the 330 km-long stabilized fibre optical link. The result was verified by series of measurements on two independent optical lattice clocks and agrees with recommendation of Bureau International des Poids et Mesures.

Ultrastable long-distance fibre-optic time transfer: active compensation over a wide range of delays

In this paper we describe a new solution of active delay stabilization for fibre-optic distribution of time and RF-frequency signals, which allows one to obtain both high precision and a potentially unlimited range of compensation of the fibre delay fluctuations. The solution is based on a hybrid system exploiting a pair of continuously tuned electronic variable delay lines, and a set of switched optical delays. We present a fully operational prototype of the time and frequency distribution setup based on this idea, which is capable of compensating more than 1 µs of the fiber delay fluctuations, and thus may be used in very long-haul links up to about 1000 km, without the need for any seasonal maintenance. We also report measurements of the time and frequency distribution stability, and the verification of the time transfer calibration.

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 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.