Łukasz Śliwczyński

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.

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.

Feedforward compensation of propagation delay dependence in fiber-optic receivers for precise time transfer

In this note the feedforward-compensated fiber-optic receiver, designated for precise time transfer systems, is presented. The receiver features only ±2 ps dispersion of the propagation delay for input powers and temperatures in the ranges of −20 to +3 dBm and 30–42 °C respectively.

Fibre-optic delivery of time and frequency to VLBI station

The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.

Modeling of Brillouin scattering in long-distance fiber optic links with bidirectional optical amplifiers

For the dissemination of precise signals from atomic clocks (like e.g. cesium clocks/fountains, H-masers or optical clocks) an optical link operating bi-directionally over the same fiber is essential. In such a link stimulated Brillouin scattering is one of the non-linear effects that may reduce the power of forward optical signal and convert it into the noise that propagates in the backward direction. In the link that uses a number of bi-directional optical amplifiers, the conditions that trigger the Brillouin scattering process may occur relatively easily because of large effective length for the scattering process. Thus in the design phase of the link, checking of the conditions for Brillouin scattering should be a part of optimization procedure (i.e. optimizing bi-directional amplifiers gains). In the paper we consider the mathematical model of the stimulated Brillouin scattering in the long distance, fiber optic links with multiple bidirectional optical amplifiers. The model was implemented in Matlab and consists of the coupled differential equations describing the propagation of pump and scattered signals that develops due to spontaneous scattering. The presence of bi-directional optical amplifiers is modeled as point-like discontinuity of the α parameter that is used to represent the attenuation of the fiber. These discontinuities create an extra level of difficulty when numerically solving the coupled equations (the problem is stiff) so special algorithm is presented that iteratively searches for the solution. The obtained results were compared with the measurements of the real link to confirm the correctness of the solution.

Towards the distribution network of time and frequency

In the paper the genesis, current stage and perspectives of the OPTIME project are described. The main goal of the project is to demonstrate that the newdeveloped at AGH technology of fiber optic transfer of the atomic clocks reference signals is ready to be used in building the domestic Time and Frequency distribution network. In the first part we summarize the two-year continuous operation of 420 kmlong link connecting the Laboratory of Time and Frequency at Central Office of Measures GUM in Warsaw and Time Service Laboratory at Astrogeodynamic Obserwatory AOS in Borowiec near Poznan. For the first time, we are reporting the two year comparison of UTC(PL) and UTC(AOS) atomic timescales with this link, and we refer it to the results of comparisons performed by GPS-based methods. We also address some practical aspects of maintaining time and frequency dissemination over fiber optical network. In the second part of the paper the concept of the general architecture of the distribution network with two Reference Time and Frequency Laboratories and local repositories is proposed. Moreover the brief project of the second branch connecting repositories in Poznan Polish Supercomputing and Networking Center and Torun Nicolaus Copernicus University with the first end-users in Torun such as National Laboratory of Atomic, Molecular and Optical Physics and Nicolaus Copernicus Astronomical Center is described. In the final part the perspective of developing the network both in the domestic range as far as extention with the international connections possibilities are presented.

A new approach to BER estimation for ISI corrupted baseband transmission systems

This paper derives two new formulae for BER estimation when the transmission system is subject to both intersymbol interference and additive Gaussian noise. The approach undertaken here was oriented to find a simple formula giving close estimation of the true BER, which may be useful for the transmission system designer for rapid evaluation of system performance. The obtained results are compared with commonly used BER estimations. Copyright