Wireless and Fiber Optics

Department of electromagnetic fieldlogo.jpg
Technická 2
166 27 Praha 6
Tel.: 224 352 280, Fax: 233 339 958
 

Who are we?

The optical research team engages in fiber optics, optical sensing, fiber lasers, optical beam outdoor and indoor propagation with respect to atmospheric influence on free-space optic (FSO) links and last but not least in visible light communication (VLC). Currently our scientific effort is focused on fundamental research as well as on cooperation with industry in the field of applied research.

Stanislav Zvánovec STANISLAV ZVÁNOVEC

Coordinates activities in the fields of fiber and free-space optics with extension towards the sub-millimeter region and terahertz spectroscopy.

  Matěj Komanec MATĚJ KOMANEC

Deals with nonlinear optical phenomena and their utilization in all-optical networks, with pulse lasers and analyses of optical effects in simulation software.

           
Jan Bohata JAN BOHATA

Focuses on optical fiber component development and on testing of modern fiber systems in hazardous environment.

  PETR CHVOJKA

Focuses on analyses and development of Vissible Light Communications (VLC).

           
TOMÁŠ NEMĚČEK

Focuses on advanced fiber sensors for gas and liquid detection.

  PETR PEŠEK

Deals with LTE transfer through optical infrastructures (both fiber and FSO).

           
JAN ŠÍSTEK

Engages in fiber optics and microwave techniques.

  PETR DVOŘÁK

Focuses on measurement methods in optical systems and microwave radiometry.

 

DMYTRO SUSLOV

Focuses on the supercontinuum generation in microstructured optical fibers.

  REDWAN AHMAD

Deals with microstructured optical fibers.

EZGI ERTUNÇ

Is a student of master degree program Electronics and Communications and her research includes VLC.

     

Michael Pisarik
Deals with new technological approaches and waveguide optics and with passive nonlinear optical components.

Pavel Skoda
Deals with photonic services, fiber optics, optical waveform shaping and fiber lasers.

Martin Mudroch
His work composes of system behavior simulation with employment of neural networks and FPGA programming.

Jan Spáčil
Focuses on the supercontinuum generation in microstructured fibers.

Redwan Ahmad
Focused on Microstructured optical fibers (MOF), hollow core fibers.

Norhanis Aida Mohd Nor 
Diversity techniques for free-spce optical networks (PhD student of Northumbria University, supervisor prof. Ghasemlooy, co-supervisor prof. Zvanovec).

Navid Bani Hassan
Car-to-car (C2C) communication using VLC (PhD student of Northumbria University, supervisor prof. Ghasemlooy, co-supervisor prof. Zvanovec).

 

External (or visiting) team members:

  • Joaquin Perez (Universitat Politècnica de València , Northumbria University), 2013
  • Paul Anthony Haigh (University of Bristol), 2014
  • Mojtaba Mansour Abadi (Northumbria University, Newcastle upon Tyne), 2014
  • Hatef Nouri (Ozyegin University, Istanbul), 2014
  • Amalia Nallely Castro Martínez (Universidad Nacional Autónoma de México), 2015
  • Hassan K. Bakir Al-Musawi (Northumbria University, Newcastle upon Tyne), 2015
  • Tamas Cseh, (Budapest University of Technology and Economics,Budapest), 2015
  • Svetlana Korsakova, (Saratov State University,Saratov), 2015
     

What are our research topics?

Our team research is focused on numerous areas of optical signal propagation and transmission.

First topic is represented by fiber optics, where we work on research and development of fiber sensors, fiber lasers, broadband optical signal generation and all-optical network components. We utilize state-of-art techniques and technologies. We test specialty optical fibers (microstructured, doped, special glass materials) for the purposes of liquid detection, nonlinear effects and supercontinuum generation. Furthermore we develop fiber pulsed fiber lasers, evaluate long-term effects on optical fibers, design fiber structures and much more. For these we utilize state-of-art simulation software and approaches. We work in co-operation with leading universities worldwide, Institute of Photonics and Electronics, Academy of Sciences and with a number of technological companies participating in the field of photonics.

   

 

Another research and development topic is represented by free-space optics (FSO), where we exploit several wireless optical links in the CTU campus connected into a simple network. We measure atmospheric parameters; we have a special turbulence chamber; we develop behavioral models of atmospheric turbulences, rain influence, etc. with respect to the quality of FSO links.

 

 

 

 

 

 

 

 

 

Last topic of interest is wireless optical communication inside buildings in visible light (indoor visible light communication, VLC), where transmissions over 7 Gbit/s have been achieved. Here our goal is analysis of LED technologies coverage as for room illumination and data transmission. Furthermore we test in international cooperation the employment of organic LED (OLED) for communication purposes..

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What it is good for?

The optical fiber development in the last decades have enabled extreme growth in network transmission capacity, but also employment of optical fibers in other areas, such as eg. sensing. Optical fibers are isolants, they are chemically resistant and immune to electromagnetic interference – which makes the ideal candidates for sensing in hazardous environments.  Special fiber structures then enable broadband signal generation, where application in medicine or chemical analysis is of overestimate value. Free-space optic link then enable high-speed transmission to numerous areas, where it is not possible to place optical fibers (historical town centers), or where standard high-speed transmissions are insufficient in capacity. Furthermore for VLC the LED illumination is massively applied worldwide not only indoor but also in public places. In contrast to radiofrequency region VLC provides several orders higher transmission speeds. 

What have we been working on?

Fiber optic detection of liquids

The project is focused on development of fiber optical sensors with enhanced sensitivity for detection of liquid analytes. This detection is based on enhanced overlapping of evanescent wave with the liquid, in principle we speak of refractometric measurement. In the project we develop various types of fiber optic detection units for detection of liquid (such as hydrocarbons) not only based on silica fibers, but also based on microstructure optical fibers. This project is carried out in cooperation with SQS, Fiber optics.

Broadband optical source based on soft-glass fibers

Aim of the project is the development of a broadband optical source prototype (i.e. supercontinuum source). The supercontinuum source is composed of a pulsed laser and a nonlinear media – optical fiber. The nonlinear media is based in our application on conventional and microstructured optical soft-glass fibers (fluoride, lead-silicate, chalcogenide). In parallel we are developing a pulsed fiber laser at 1550 nm and 2000 nm. This project is carried out in cooperation with SQS, Fiber optics, development of the 2000 nm pulsed laser also in cooperation with Institute of photonics and electronics, AS CR.

Optical packet switch

Project aims at a switch development, which is based on optical packet routing. Main advantage stands in keeping the data payload in optical format, while only the label – IP address – is processed electronically. Employing this approach we achieve higher data speeds than with the consideration of opto-electronic speed limits. For the switching itself we utilize special fibers and nonlinear phenomena.

Femtosecond fiber laser

We develop fiber laser with passive mode synchronization, which exploits commercially available erbium-doped optical fiber and thanks to nonlinear polarization rotation in the loop, pulse behavior is achieved. It is possible to obtain pulses having full-width-at-half-maximum (FWHM) as short as 200fs, while preserving high energy. Our goal is the loop optimization and high repetition rate.

Microwave photonics

Unlike the conventional digital baseband transmission schemes supporting only one service at a time, the radio-over-fibre (RoF) transmission network enables the coexistence of multi-service and multi-operators in a shared resources, thereby offering increased link capacity, advanced networking (i.e., dynamic resources and allocations) without the need for frequency up-down conversion. An alternative option for fiber radio networks is to transmit the RF-based information over a free space opticaů (FSO) link in place of an optical fiber or combine these networks together. This leads to the bigger flexibility of the networks, which can bring reduced costs especially in dense urban areas. However, the link availability and performance quality is mostly affected by atmospheric weather conditions such as atmospheric turbulence, fog, rain, etc.

   

Optical fibers in harsh environment

The reliability of an optical communication system in a hazardous area may be adversely affected by temperature variation, pressure, humidity, high voltage transmission, radiation, etc.. In such conditions, optical fibers undergo structural changes that may result in their transmission characteristics being temporarily or permanently degraded. Thus it desires attention when the optical networks are designing going hand in hand with optical fiber networks growth. Our experiments are focused on crucial transmission parameters monitoring in long-term horizon.

 

Research of ambient influence on novel broadband free-space optical systems

Main aim of this project stands in national cooperation with partners of Cost project IC1101 OPTICWISE, which should lead to development of a new methodology for atmospheric parameters reconnaissance based on measurement of free-space optical systems utilizing adaptive approaches. Large measurement campaign including two optical links WaveBridge 500 by Plaintree, 4-beam optical wireless link FlightStrata G od LightPointe (1.25 Gbps, VCSEL at 850 nm) and approximately 400m long link MRV Telescope 700. Parallel measurement in microwave region (5GHz link Mikrotik) is being carried out, atmospheric parameters are being measured by two metrological stations and turbulent environment close to buildings is analyzed based on temperature gradient obtained by special sensor link and also based on radiometric noise temperature measurement. In analyses, the meteoradar database is utilized; containing rain intensity behavior in the area of 250x250km (Czech Republic) with 1km step and time step of 1min for a 3-year period.

 

 

 

 

 

 

Visible Light Communication (VLC)

In recent years, visible light communications (VLC) has rapidly gained interest among research communities worldwide. It is an emerging technology for future high capacity communication links utilizing the visible range of the electromagnetic spectrum (~370 – 780 nm), which is not only license free, but free from spectral overcrowding unlike radio frequencies (RF). VLC utilizes light-emitting diodes (LEDs), modulating them at high speeds that are much faster than the human eye can detect, to simultaneously provide data transmission and room illumination. A major challenge in VLC is the LED modulation bandwidths, which are limited to a few MHz. However, gigabit speed transmission links have already been demonstrated.

Concurrently, organic LEDs (OLEDs) have been the focus of enormous attention for solid-state lighting applications due to their advantages over conventional LEDs such as ultra-low costs, mechanical flexibility and large photoactive areas. As a result, researchers are starting to investigate the performance of OLEDs in VLC systems, which is a very challenging research area, as OLED bandwidths can be approximately three orders of magnitude lower than their LED counterparts. Our group works on development of such VLC links to drive both LEDs and OLEDs in order to implement broadcasting networks featuring advanced modulation formats such as orthogonal frequency division multiplexing (OFDM) or carrier-less amplitude and phase modulation (CAP). We have many international academic and industrial collaborations with world leading research groups in the field of VLC, including Northumbria University and University College London (UCL) or University of Bristol.

 

   

    

 

Who supports our research?

  • COST project IC1101 Optical Wireless Communications - An Emerging Technology, OPTICWISE, http://www.cost.eu/domains_actions/ict/Actions/IC1101
  • COST project MP1401 Advanced Fibre Laser and Coherent Source as Tools for Society, Manufacturing and Lifescience
  • Wideband Optical Source Based on Soft-glass Fibers (in cooperation with SQS, fiber optics a.s), TACR  grant TA04010220
  • Fiber optic detection of liquids  (in cooperation with SQS, fiber optics a.s), TACR grant TA03010060
  • Optical packet switch (in cooperation with SQS, fiber optics a.s, IFE AV ČR), TAČR grant TA01011105
  • TACR – Centre of competence TE02000202 Advanced sensors and sensor data processing methods (our team - leading Working Package 6), external academic participants from University of West Bohemia, Brno University of Technology and industrial participants from AZD Praha, Honeywell International s.r.o., SQS Fiber optics, LESIKAR, a.s. and Safibra 
  • Research of Ambient Influences on Novel Broadband Optical Wireless Systems - RAINBOWS, MŠMT COST_CZ project LD12058
  • Centre for quasi-optical systems and terahertz spectroscopy, project LC06071 (with VŠCHT and VUT)
  • Quadrupole Interactions as a Powerful Tool for the Conformational and Structural Analyses of Biochemically and Astrophysically Important Molecules (with VŠCHT), GAČR project GAP206/10/2182
  • The Influence of the Atmosphere on Electromagnetic Wave Propagation for Broadband Stratospheric Links, GAČR project GP102/08/P346
  • Optical and microwave detection systems, CTU project SGS14/190/OHK3/3T/13

Who do we cooperate with?

  • Czech research teams and industrial partners

    • SQS Vláknová optika a.s.
    • Ústav fotoniky a elektroniky AV ČR, v. v. i.
    • SITEL, spol. s.r.o.
    • T-Mobile Czech Republic a.s.
    • Kabelovna Kabex a.s.
    • OFA, s.r.o.
    • PROFiber Networking s.r.o.
    • NETWORK GROUP, s.r.o.
    • Vysoké učení technické v Brně
    • Vysoká škola báňská-Technická univerzita Ostrava
    • Vysoká škola chemicko-technologická v Praze
    • Czech Metrology Institute

    International teams

    • Northumbria University, Newcastle UK
    • University of Southampton
    • University College London
    • University of Edinburgh
    • University of Oxford
    • Ecole Centrale Marseille, Institut Fresnel, Francie
    • RWTH Aachen
    • Politecnico di Milano
    • Hong Kong Polytechnic University
    • Ozyegin University Turecko
    • Cork Institute of Technology
    • University of Technology Graz

Selected publications

    • Z. Ghassemlooy, L. Nero Alves, S. Zvanovec and M. Ali Khalighi, Visible Light Communications: Theory and Applications, CRC Press, 2017.
    • P. Luo, M. Zhang, Z. Ghassemlooy, S. Zvanovec, S. Feng, P. Zhang, Undersampled-based Modulation Schemes for Optical Camera Communications, IEEE Communications Magazine, accepted… 
    • J. Bohata, S. Zvanovec, M. Komanec, J. Jaros, Z. Ghassemlooy, Adaptation of Transmitting Signals over Joint Aged Optical Fiber and Free Space Optical Network Under Harsh Environments, Optik, accepted…
    • S. Vitek, J. Libich, P. Luo, S. Zvanovec, Z. Ghassemlooy, N. B. Hassan, Influence of Camera Setting on Vehicle-to-vehicle VLC Employing Undersampled Phase Shift On-off Keying, Radioengineering, accepted...
    • E. Romanova, S. Korsakova, M. Komanec, T. Němeček, A. Velmuzhov, V. Shiryaev, Multimode Chalcogenide Fibers for Evanescent Wave Sensing in the Mid-IR, IEEE Journal of Selected Topics in Quantum Electronics, vol. 23, Issue 2, pp. 1-7, 2017.
    • H. K. Al-Musawi, T. Cseh, J. Bohata, W. P. Ng, Z. Ghassemlooy, S. Zvanovec, E. Udvary, P. Pesek, 4G-LTE Signal Transmission for the Last-mile Access Network Using a Hybrid RoMMF-FSO System under Turbulence Effects, IEEE Journal of Lightwave Technology, vol. 33, Issue 17, pp.3758-3764, 2017
    • T. Martan, T. Nemecek, M. Komanec, R. Ahmad, S. Zvanovec, Refractometric Detection of Liquids Using Tapered Optical Fiber and Suspended Core Microstructured Fiber: A Comparison of Methods, Applied Optics, vol. 56, Issue 13, pp. 1-9, 2017.
    • J. Libich, J. Perez, S. Zvanovec, Z. Ghassemlooy, R. Nebuloni, C. Capsoni, Combined Effect of Turbulence and Urban Aerosol on Free Space Optical Links, Applied Optics, vol. 56, Issue 2, pp. 336-341, 2017.
    • M. M. Abadi, Z. Ghassemlooy, S. Zvanovec, M. R. Bhatnagar, M. A. Khalighi, Y. Wu, Impact of Link Parameters and Channel Correlation on the Performance of FSO Systems with Differential Signalling Technique, Journal of Optical Communications and Networking, vol. 9, issue. 2, pp. 138-148, 2017.
    • N. A. M. Nor, Z. Ghassemlooy, J. Bohata, P. Saxena, M. Komanec, S. Zvanovec, M. R. Bhatnagar, M. A. Khalighi, Experimental Investigation of All-Optical Relay-Assisted 10 Gbps FSO Link over the Atmospheric Turbulence Channel, IEEE Journal of Lightwave Technology, vol. 35, issue 1, pp. 45-53, 2017. 
    • J. Bohata, J. Jaros, S. Pisarik, S. Zvanovec, M. Komanec, Long-term PMD Evolution and Accelerated Aging in Old Optical Cables, IEEE Photonics Technology Letters, vol. 29, no. 5, pp.519-522, 2017. 
    • R. Ahmad, M. Komanec, S. Zvanovec, Circular Lattice Photonic Crystal Fiber for Mid-IR Supercontinuum Generation, IEEE Photonics Technology Letters, vol. 28, Issue 23, pp. 2736 - 2739, 2016.
    • M. R. Bhatnagar, Z. Ghassemlooy, S. Zvanovec, M. A. Khalighi, M. M. Abadi, Quantized Feedback Based Differential Signaling for Free-Space Optical Communication System, IEEE Transactions on Communications,  vol. 64, Issue 12, pp.5176 - 5188, 2016.
    • J. Bohata, S. Zvanovec, P. Pesek, T. Korinek, M. Mansour Abadi, Z. Ghassemlooy, Experimental Verification of LTE Radio Transmissions over Dual-polarization Combined Fibre and FSO Optical Infrastructure,  Applied Optics, vol. 55, Issue 8, pp. 2109-2116, 2016.
    • A. N. Castro Martinez, M. Komanec, T. Nemecek, S. Zvanovec, S. Khotiaintsev, Fiber Optic Refractometric Sensors Using a Semi-ellipsoidal Sensing Element,  Applied Optics, vol. 55, Issue 10, pp. 2574-2579, 2016.
    • M. M. Abadi, Z. Ghassemlooy, M. Khalighi, S. Zvanovec, M. Bhatnagar, FSO Detection Using Differential Signaling in Correlated Channels Condition, Photonics Technology Letters, vol.28, no.1, pp.55-58, 2016.
    • D. Wu, Z. Ghassemlooy, W. Zhong, M.A. Khalighi, H. L. Minh, Ch. Chen, S. Zvanovec, A.C. Boucouvalas, Effect of Optimal Lambertian Order for Cellular Indoor Optical Wireless Communication and Positioning Systems, Optical Engineering, vol. 55, no. 6, pp. 066114-1-8, 2016. 
    • M. M. Abadi,  Z. Ghassemlooy, S. Zvanovec, D. Smith, M. R. Bhatnagar, Y. Wu, Dual Purpose Antenna for Hybrid Free Space Optics/RF Communication Systems, IEEE Journal of Lightwave Technology, vol. 34, no. 14, pp. 3432-3439, 2016.
    • M. Pisarik, P. Peterka, J. Aubrecht, J. Cajzl, A. Benda, D. Mareš, F. Todorov, O. Podrazky, P. Honzatko, I. Kašík, Thulium-doped fibre broadband source for spectral region near 2 micrometers. Opto-Electronics Review. 2016, 24(4), 223-231. ISSN 1230-3402.
    • T. Nemecek, M. Komanec, T. Martan,  R. Ahmad, S. Zvanovec, Suspended-core microstructured fiber for refractometric detection of liquids, Applied Optics, vol. 54, no. 30, pp. 8899-8903, 2015.
    • P. A. Haigh, A. Burton,  K. Werfli, H. L. Minh, E. Bentley, P. Chvojka, W. O. Popoola,  I. Papakonstantinou, S. Zvánovec, A Multi-CAP Visible Light Communications System with 4.85 b/s/Hz Spectral Efficiency, IEEE Journal on Selected Areas in Communications, vol. 33, no.9, p. 1771-1779, 2015.
    • J. Bohata, S. Zvanovec, T. Korinek, M. Mansour Abadi, Z. Ghassemlooy, Characterization of Dual-polarization LTE Radio over a Free-space Optical Turbulence Channel,  Applied Optics, vol. 54, no. 23, p. 7082-7087, 2015.
    • P. A. Haigh, S. T. Le, S. Zvánovec, Z. Ghassemlooy, P. Luo, T. Xu, P. Chvojka, T. Kanesan, E. Giacoumidis, P. Canyelles-Pericas, H. L. Minh,  W. O. Popoola,  S. Rajbhandari, N.J. Doran, I. Papakonstantinou, I. Darwazeh, Multi-band Carrier-less Amplitude and Phase Modulation for Bandlimited Visible Light Communications Systems, IEEE Wireless Communications, vol. 22, no. 2, p. 2-9, 2015.
    • P. Chvojka, S. Zvanovec, P.A. Haigh, Z. Ghassemlooy, Channel Characteristics of Visible Light Communications within the Dynamic Indoor Environment, IEEE Journal of Lightwave Technology, vol. 33, p. 1719 - 1725, 2015.
    • J. Libich, M. Komanec, P. Pesek, S. Zvánovec, W. O. Popoola, Z. Ghassemlooy, Experimental Verification of All-optical Dual Hop 10 Gbit/s FSO Link under Turbulence Regimes, Optics Letters, vol. 40, no. 3, p. 391-394, 2015.
    • M. Komanec, T. Martan, T. Nemecek, S. Zvanovec, Multimode Fiber Tapers for Reproducible Refractometric Liquid Detection, Optical Engineering, vol. 54, no. 4, p. 047102.1-6, 2015.
    • S. Zvanovec, P. Chvojka, P. Haigh,  Z. Ghassemlooy, Visible Light Communications towards 5G. Radioengineering, vol. 24, no. 1, p. 1-9, 2015.
    • P. Koska, Y. Baravets, P. Peterka, J. Bohata, M.  Pisarik, Mode-Field Adapter for Tapered-Fiber-Bundle Signal and Pump Combiners, Applied Optics, vol. 54, no. 4, p. 751-756, 2015.
    • S. Rajbhandari, Z. Ghassemlooy, P. A. Haigh, T. Kanesan, , Experimental Error Performance of Modulation Schemes under a Controlled Laboratory Turbulence FSO Channel, IEEE Journal of Lightwave Technology, vol. 30, art. no. 1, p. 244-250, 2015.
    • P. Dvorak, M. Mazanek, S. Zvanovec, Fire Emissivity Detection by Microwave Radiometer, IEEE Geoscience and Remote Sensing Letters, vol. 12, no. 11, p. 2306-2310, 2015.
    • V.Weinzettl, G. Shukla, J. Ghosh, R. Melich, R. Panek, M. Tomes, M. Imrisek, D. Naydenkova, J. Varju, T. Pereira,  R. Gomes, I. Abramovic,  R. Jaspers, M. Pisarik, T. Odstrcil, G. Van Oost, High-resolution Spectroscopy Diagnostics for Measuring Impurity Ion Temperature and Velocity on the COMPASS Tokamak, Fusion Engineering and Design, Vol. 96, pp 1006–1011, 2015.
    • R. Ahmad, M. Komanec, S. Zvanovec, Modified octagonal photonic crystal fiber for residual dispersion compensation over telecommunication bands, Optik, submitted...
    • J. Perez, S. Zvanovec, Z. Ghassemlooy, W. O. Popoola, Experimental characterization and mitigation of turbulence induced signal fades within an ad-hoc FSO network, Optics Express, vol. 22, no. 3, p. 3208-3218, 2014.
    • M. Pisarik, P. Peterka, S. Zvanovec, Y. Baravets,F. Todorov, I. Kasik, P. Honzatko, Fused fiber components for „eye-safe“ spectral region around 2 micrometers, Optical and Quantum Electronics, vol. 46, pp. 603-611, 2014.
    • J. Bohata, M. Pisarik, S. Zvanovec, P. Peterka, Reliability of Aircraft Multimode Optical Networks, Optical Engineering, vol. 53, no. 9, 096102, 2014.
    • M. Mudroch, S. Zvanovec, Artificial Neural Network Utilization for FSO Link Performance Estimation, Radioengineering, vol. 23, no. 1, p. 474-479, 2014.
    • S. Zvanovec, J. Perez, Z. Ghassemlooy, S. Rajbhandari, J. Libich, Route diversity analyses for free-space optical wireless links within turbulent scenarios, Optics Express,  vol. 21, Issue 6, pp. 7641-7650, 2013. 
    • P. Dvorak, M. Mazanek, S. Zvanovec, Short-term Prediction and Detection of Dynamic Atmospheric Phenomena by Microwave Radiometer. Radioengineering. 2012, vol. 21, no. 4, p. 1060-1066. ISSN 1210-2512.  
    • J. Libich, P. Dvorak, P. Piksa, S. Zvanovec, Correction of Thermal Deviations in Fabry-Perot Resonator Based Measurements of Specific Gases in MillimeterWave Bands. Radioengineering. 2012, vol. 21, no. 1, p. 459-463. ISSN 1210-2512.
    • J. Libich, S. Zvanovec, Influences of Turbulences in Near Vicinity of Buildings on Free Space Optical Links, IET Microwaves, Antennas & Propagation, 2011, vol. 5 , issue 9, p. 1039 - 1044.
    • M. Komanec, P. Honzatko, S. Zvanovec, Single-shot All-optical Sampling Oscilloscope Using a Polarization-maintaining Resonator for Pulse Replication, Microwave and Optical Technology Letters. 2010, vol. 52, no. 11, p. 2452-2456. ISSN 0895-2477.