J. Kolodziej

Bidirectional Optical Amplification in Long-Distance Two-Way Fiber-Optic Time and Frequency Transfer Systems

In this paper, the transmission of the time and/or frequency signals (e.g., 1 pulse per second and 10 MHz) coded on the optical carrier by means of an on-off intensity modulation in the two-way fiber-optic link is considered. It is assumed that the bidirectional optical amplification in the single piece of an erbium-doped fiber is exploited to compensate the attenuation of the optical path. Such configuration of the amplifiers, offering the highest possible symmetry of the propagation conditions in both directions, is well suited for the two-way transfer method exploiting the symmetry of the link. We proposed the method of estimating interfering signals and jitter, which appear at both sides of such bidirectional fiber link because of Rayleigh backscattering and amplified spontaneous emission. This method is further exploited for finding the gains of bidirectional amplifiers, allowing optimization of the performance of the link. The experiments done with 120- and 220-km-long links, incorporating one and three amplifiers, respectively, confirmed theoretical predictions and proved that the single-path bidirectional amplifiers without any components separating the directions are useful for time or RF frequency transfer. During the experiments, both field-deployed telecommunication cables and the fibers spooled in the laboratory were used. Presented methods of analysis and optimization are useful for designing and evaluating the fiber-optic links incorporating single-path bidirectional fiber-optic amplifiers and exploiting intensity modulation for time and/or frequency transfer.

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.

Mobile user tracking system with ZigBee

In the paper an implementation of mobile nodes tracking system based on ZigBee and Wi-Fi wireless networks is presented. On the base of known algorithmic as well as circuit solutions a simple yet universal system, applied in prototype application dedicated for persons localization in museum premises has been developed. Since system utilizes entirely wireless communication, it can be applied in any closed objects. The system has been preliminarily verified in real in-situ environment.

Antialiasing filter for NSDM codec

A consideration for an analog antialiasing filter performance, addressed to a nonuniform sampling coding technique was described. The filer prototype design and its implementations are presented. The Sallen-Key architecture based on CMOS inverters and the solution of achieving resistor formed by transmission gates with long MOSFETs are presented. Finally the simulation results are discussed.

OPTIME - the system grows - a new 330 km line

The OPTIME project creates an ultra-precise time and frequency signals dissemination system based on telecommunication networks. End users obtain access to these signals without incurring huge costs for the purchase of their own atomic clocks, and receive the service related to laboratories generating international atomic time scales, to which any precise time must be referred. OPTIME dissemination system is based on three main elements: reference time and frequency laboratories, local time and frequency repositories and fiber optical network with specialized transmission equipment to transfers signals between laboratories, repositories and end users. This article describes OPTIME system with particular emphasis on a new 330 km long dissemination line between Space Research Centre PAS, Astrogeodynamic Observatory (AOS) at Borowiec and National Laboratory of Atomic, Molecular and Optical Physics (KL FAMO) at Torun.

OPTIME — Time and frequency dissemination system based on fiber optical network — PIONIER

The OPTIME project creates a long range dissemination system for transfer ultraprecise time scale and the references frequency signals in telecommunication networks. The highest accuracy signal is available only on fiber optical networks, but other type of networks can be used to transfer of signals with lower accuracy to adapt it to the needs of different user groups. Article also describes experience gained during an over-a-year experiment of connection between Central Office of Measures (GUM) in Warsaw and the Astrogeodynamic Observatory (AOS) in Borowiec.

Fiber-Optic Time Transfer for UTC-Traceable Synchronization for Telecom Networks

A robust and reliable synchronization network, able to distribute signals traceable to a recognized standard, is crucial for the operation of future 5G telecommunication networks. In this article we present the results of time transfer using optical fibers. The main goal is to test the long-term capability of ELSTAB technology (developed by AGH) to deliver time and frequency signals traceable at the sub-nanosecond level to UTC in a real telecommunication environment. In an ongoing cooperation between Deutsche Telekom (DTAG), the Physikalisch- Technische Bundesanstalt (PTB) and AGH University of Science and Technology, we deliver UTC -- as realized by PTB in Braunschweig -- to a test center of DTAG located in Bremen. For this purpose, a fiber optic link has been operated since July 2015. The results obtained show that the operator of a telecom network may use such a stabilized fiber optic link as a reliable source for synchronization signals with a precision and accuracy superior to those obtained using a state-of-the-art GNSS time receiver. Moreover, a fiber optic link delivering UTC traceable signals increases the robustness and reliability of the networks synchronization chain by making it less dependent on GNSS.

Remote synchronization of atomic clocks

In the paper we present and briefly characterize the methods of remote synchronization of atomic clocks. Two groups of technical solution are presented, namely the methods employing satellites, especially GPS Common View and Two Way Satelite Time and Frequency Transfer and the group of systems, which employs the ground-base fiber optic networks. The unidirectional and bidirectional time/frequency transfer schemes are considered, and theirs main limitations are pointed out. Finally, the newly established atomic clock signals distribution network called Optime is presented. In a first step, the network was a simple link which primary objective was to connect two most important Time and Frequency laboratories in Poland - in Central Office of Measures (GUM, Warsaw) and in Astrogeodynamic Observatory (AOS) in Borowiec near Poznan. Recently, Optime has been expanded by adding three newly installed branches connecting GUM and Orange Synchronization Center and AOS with National Laboratory of Atomic, Molecular and Optical Physics in Torun. and Astronomic Center in Piwnice.

Time-frequency analysis of accelerometry data for seizure detection

Abstract This article presents an algorithm based on a short-time Fourier transform for reliable detection of epileptic seizures measured with three-dimensional (3D) accelerometry. The objective of the described work is to provide basic technical information to create useful alarm models for epileptic seizure detection using a mobile phone. The presented material is based on experimental measurements. Finally, the possibility of increasing smartphone detection capability by attaching a triaxial piezoelectric accelerometer to the patient’s wrist is suggested.