Josef Vojtech

Large signal model of TDM-pumped Raman fiber amplifier

In this letter, we study spectral optical signal-to-noise ratio (OSNR) profile of a wide-band Raman fiber amplifier (RFA)with time-division-multiplexed (TDM) pumping. We derive a comprehensive large-signal numerical model which incorporates time variation effects and the downstream propagation of signals, upstream propagation of pumps, and downstream and upstream propagation of amplified spontaneous emission spectral components. We present results for a four-wavelength-pumped discrete RFA with TDM and continuous-wave pumping. Improvement in OSNR flatness of 0.9 dB due to TDM pumping is demonstrated.

Field-Trial of an All-Optical PSK Regenerator/Multicaster in a 40 Gbit/s, 38 Channel DWDM Transmission Experiment

The performance of future ultra-long haul communication systems exploiting phase-encoded signals is likely to be compromised by noise generated during signal transmission. One potential way to mitigate such noise is to use Phase Sensitive Amplifiers (PSAs) which have been demonstrated to help remove phase as well as amplitude noise from phase-encoded signals. Recently, we showed that a PSA-based signal regenerator based on degenerate four-wave mixing can be implemented in a network-compatible manner in which only the (noisy) signal is present at the device input (black-box operation). The developed regenerator was also able to perform simultaneous wavelength conversion and multicasting, details/analysis of which are presented herein. However, this scheme was tested only with artificial noise generated in the laboratory and with the regenerator placed in front of the receiver, rather than in-line where even greater performance benefits are to be expected. Here, we address both theoretically and experimentally the important issue of how such a regenerator, operating for convenience in a multicasting mode, performs as an in-line device in an installed transmission fiber link. We also investigate the dispersion tolerance of the approach.

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).

Surviving-Channel-Power Transients in Second-Order Pumped Lumped Raman Fiber Amplifier: Experimentation and Modeling

In this paper, the effect of addition and/or dropping of wavelength-division-multiplexed channels in an all- optical gain-clamped (AOGC) second-order (SO) pumped lumped Raman fiber amplifier (LRFA) has been investigated experimentally and analyzed by numerical simulation. Channel addition/ removal was simulated by transmitting ten signals through a counter-directionally pumped LRFA consisting of a 16-km-long dispersion-compensating fiber. The light from eight lasers was square-wave modulated at 500 Hz; power transients of the surviving channels caused by cross-gain modulation of the LRFA were monitored at the output of the amplifier. All-optical feedback loop was implemented in the form of a ring laser. Gain-clamping properties of the SO-pumped LRFA are compared with those of the first-order pumped LRFA having the same ON/OFF Raman gain. Theoretical analysis of the AOGC LRFA was based on numerical solution of coupled propagation equations for the backward propagating pump, signals, and both forward and backward propagating spectral components of amplified spontaneous emission powers.

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.

10 Gb/s and 40 Gb/s Multi-Wavelength Conversion Based on Nonlinear Effects in HNLF

We present experimental results and numerical simulations of multi-wavelength conversion of 10 Gb/s NRZ signals based on 2-pumps four-wave mixing, 10 Gb/s and 40 Gb/s RZ signals derived from supercontinuum (SC) generated in highly nonlinear fibre (HNLF). High quality SC with bandwidth exceeding 50 nm was generated when the HNLF was pumped with 25 dBm average power. Sliced SC can be used as a source for all-optical multicasting

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.

Φ-OTDR signal amplification

Phase-sensitive optical time-domain reectometry (Φ-OTDR) seems to be the most appropriate solution for acoustic vibration along standard optical fiber detection. In general the sensing system measures phase changes of the received Rayleigh back-scattered signal in the fiber. Since the back-scattered signal intensity is decreased about tens of decibels in comparison to the forward propagating pulse power level, the received signal power level is very low. That is why the main limiting parameter of the system is the power level of the back-scattered signal, which limits maximum achievable distance. For long reach sensing it is necessary to create high power optical pulses with short time-duration. Direct pulse amplification by erbium doped fiber amplifier (EDFA) is an issue because of the pulses low repetition rate. We have designed and verified a simple method using a holding beam for amplifying of pulses with low repetition rate by standard telecommunication EDFA booster instead of deployment of an expensive optical shutter. A second CW laser with a different wavelength for EDFA stabilization is used in our setup. Because a pulse losses its energy during propagation in the fiber and with longer distances by 1st order Raman amplifier (RA). In telecommunications this amplifier is used to compensate for fiber losses. The second setup uses remote amplification by remotely pumped erbium doped fiber (EDF) placed after a few tens of kilometers of sensing fiber. A pump laser is placed in the transmitter part of the system from where EDF is pumped. In this paper, we present an overview of few techniques for Φ-ODTR signals amplification and their verification by measurement.

Field-trial of an all-optical PSK regenerator in a 40 Gbit/s, 38 channel DWDM transmission experiment

We report the first field-trial of a phase and amplitude regenerator highlighting the practicality of the technology. Sensitivity improvement and mitigation of transmission-induced noise with the regenerator placed in-line or at the receiver is demonstrated.