Vladimir Smotlacha

Two-way optical time and frequency transfer between IPE and BEV

This paper describes our experience with long term two-way optical time transfer between Czech and Austrian national time and frequency laboratories maintaining their national time and frequency standards generating the national time scales UTC(TP) and UTC(BEV), respectively. The optical link utilizes either dedicated optical fibers or Dense Wavelength Division Multiplex (DWDM) channels in all-optical production telecommunication networks. Results from the optical transfer, i.e. clock comparison and link delay including diurnal and seasonal effects are shown and also compared with GPS Common-View and Precise Point Positioning.

Time transfer using fiber links

This paper describes two-way time transfer over an optical network. The method is based on newly developed adapters utilizing channels in a DWDM (Dense Wavelength-Division Multiplexing) all-optical network. Results of several tests performed in real production network including the time transfer between atomic clocks in Prague and Vienna over more than 500 km long optical link are presented. In addition, the comparison of Common View GPS and optical time transfers is given.

All optical two-way time transfer in strongly heterogeneous networks

We experimentally compared different options for interconnection of remote places in precise time transmission infrastructures. We aimed at lines with bidirectional single path amplification, as they provide the best results in term of time uncertainty. For links with high reflections and without accessible midpoints, the distributed Raman amplification has been tested. In case of dark channels with fibre based “last miles”, the Raman amplification in the last miles only has been verified. According to author’s knowledge, this is the first time when the distributed amplification is used for modulated precise time transmission over fiber. For reference, the traditional lumped EDFAs suitable for transmission links or dark channels with accessible mid-point have been compared.

Photonic services, their enablers and applications

The paper introduces photonic service as a new type of multi-domain, end-to-end network service, that besides traditional data transmission enables also non-data or real-time communication, e.g. the remote control and sharing of different, typically unique, devices. We proved the concept of photonic services on a specialized metrology application - comparison of time scales, where time offset between atomic clock is measured. The experiment has been conducted over distance of 550 km (342 miles).

Optical infrastructure for time and frequency transfer

The paper describes optical infrastructure for time and frequency transfer in the Czech republic. The infrastructure is heterogeneous and utilises resources of the Czech academic optical network. It allows to interconnect Cesium standards in distant sites with the national time and frequency laboratory and to distribute accurate time and stable frequency. We also present results and compare them with other methods of time transfer.

Precise time transfer on the IPE - VUGKT line - a detailed analysis

We present implementation of a pilot bidirectional line IPE-VUGKT for precise time transmission. Directional non-reciprocities are evaluated together with uncertainties. Emphasis is given to the evaluation of the Sagnac effect. Our approach improves significantly the uncertainty.

Transfer of stable optical frequency for sensory networks via 306 km optical fiber link

Optical fiber links for distribution of highly-stable optical frequencies were experimentally tested by many metrology laboratories in the past fifteen years. But recent development of new optical sensors for industrial application puts demands on a technology transfer of this high-end technology from laboratory experiments to the real industry. The remote calibration of interrogators of Fiber Bragg Grating strain sensory networks is one of important examples. We present a 306 km long optical fiber link established in the Czech Republic where a coherent transfer of stable optical frequency has been firstly demonstrated. The link between ISI CAS Brno and CESNET Prague uses an internet communication fiber where a window 1540 to 1546 nm in DWDM grid is dedicated for the coherent transfer and 1 PPS signal. The optical frequency standard at 1540.5 nm is used for the coherent transfer where compensation of the Doppler shift induced by the optical fiber is done by an acousto-optic modulator driven by a servo loop. The closed loop action is continuously recorded. This enables to compute changes in the transport delay introduced by external influences on the optical line. A comparison with a different measuring method based on analyzing the transport delay of a 1 PPS signal transmitted via the same DWDM window is done. This comparison is a subject of results of the paper.

On calibration of network time services

This paper proposes methods for calibration of two network time services—NTP servers and time-stamp authorities. The calibration is described in conformity with metrological principles like other time distribution systems. The authors have built up the calibration sets and tested them in cooperation with five European time and frequency laboratories. The paper also presents and discusses experimental results collected by measurement of time servers between involved laboratories.

Atomic clock comparison over optical network

This paper deals with an accurate time transfer and atomic clocks comparison. It presents an adapter for clock comparison over an optical network, especially utilizing dense wavelength division multiplexing (DWDM). The adapter is a field-programmable gate array (FPGA) based device with DWDM transceivers and frequency synthesis functionality. For the second generation of adapters an embedded interpolating time interval counter in FPGA was developed. We also present results of interpolating counter functionality evaluation in respect to SR620 universal counter.

The hardware architecture and device for accurate time signal processing

The paper describes architecture, design and implementation of accurate time interval counter into the FPGA (field-programmable gate array) structure. We embedded it into the specialized adaptor for time transfer over optical links. We also present achieved results of comparison with precise commercial counter SR-620.