National Institute of Telecommunications, Poland, Research (RES)

Tomasz Kossek
ul. Szachowa 1
04-894 Warszawa
Poland
Phone: +48 22 5128159
Fax: +48 22 5128492
E-Mail: tkossek@itl.waw.pl
http://www.itl.waw.pl

Description and expertise of the organisation:
Research topics cover all sectors of modern telecommunications, particularly prospective areas such as: optical communications,
planning and design of telecommunication networks, radio terminals (e.g. for mobile networks), IP-based network management and
computer-assisted decision making in telecommunications. Interdisciplinary research on subjects like telecommunication regulations
and economics or network security is also preferred. The Institute conducts research on several issues related to the information
society. Research subjects are selected according to worldwide and European trends, especially considering participation of the
Institute in the European Union’s 6th and 7th Framework Program.
The Institute operates as an ISO-17025 certified laboratory for testing of fibre optic cables and components, SDH systems,
subscriber terminals (telephone sets, ISDN, xDSL, modems, fax machines), radio systems, cellular phones and base stations, power
equipment, EMC as well as to perform environmental and mechanical testing. NIT operates a specialized laboratory for calibration
and verification of measurement equipment (electrical and fibre optic) and maintains Polish national time/frequency standard.
R&D work frequently leads to practical implementation of its results, considered as extension or enrichment of research.
In fiber optic telecommunications domain NIT conducts research and implementation works on ultra-fast transmission in terabyte
optical transparent networks, wavelength multiplexing (DWDM and CWDM) in optical access networks, non-linear optical effects,
application of innovative photonic crystal technologies, micro-structured optical fibers and quantum communications, methods for
measurement and compensation of fiber polarization mode dispersion (PMD), reliability of optical components and networks,
implementation of optical IP networks, assurance of Quality of Service (QoS) and convergence of broadband wireless and optical
networks.

Research interests / Project ideas
1. Passive Optical Networks (PONs) seem to be the only solution for eliminating last/first mile “bottleneck”
problem. It can be observed the lack of efficient monitoring technology appropriate for passive splitters (PS)
based PON. Present measurement solutions are not suitable for PON networks with passive splitters (1:128
for the GPON standard). In order to effectively identyfy each end of fiber (ONU) and monitor the PON
network the Bragg grating with a unique wavelength can be used.
We would like to develop monitoring (measurement of physical paramters) system for PON networks.
The PON network monitoring scheme is generally based on wavelength sweeping of the monitoring source
wavelength and collecting the reflected optical signal from subscribers Bragg gratings.
The narrow spectral width of Bragg gratings are the crucial parameter for such application because of the
large number of subscibers behind the passive splitter (up to 128). Also the shape of the reflectance
characteristic of Bragg gratingd is important feature.
One of disadvantages of FBG fabricated by using a binary (uniform) phase mask is presence of the sidelobes
in its spectral characteristics. In order to minimize these side-lobes it is convenient to use apodization
for decreasing the amplitude of refractive index modulation at the ends of the grating. One of the methods for
obtaining apodized fiber Bragg gratings is an application of phase masks with variable diffraction efficiency
along its length. The main advantage of this approach is to maintain the effective index of refraction constant
along the grating, what allows to counteract the broadening of FBG spectral characteristics.
In principle two types of apodization profile can be used: Gaussian and tanh. In reality, some limitations of
electron-beam exposure system are noticed in phase mask fabrication, for that reason phase masks can be
manufactured with limited values of step height. The effect of such limitation can observed in comparisonwith the averaged intensity distributions behind ideal phase masks with variable intensity. Simulations of
intensity distribution perturbations due to phase jumps in real apodized phase masks are also very useful.
Our aim in this project is to design the phase masks for Bragg gratings of desired, unique spectral
characteristics.
Next step is to find the laboratory where the phase masks can be manufactured. There will be also the need
to find someone who can manufacture the Bragg grating using these phase masks.
Bragg gratings parameters will be measured in our laboratory.
Our final task is to build PON test network with manufactured Bragg gratings. We expect that it can be
possible to measure more accurately and reliably the parameters of PON network with passive spliters.

2. Modelling of physical constrains in a fibre access networks (WDM/TDM-PON), using self-developed Fiber Optic Communication System Simulator. Modelling, design and optimization of modern optical communication networks taking into account the quality of transmission (QoT) and physical layer impairments (PLI). Moreover, we can offer mathematical modelling approach to the evaluation of interferometric noise caused by localized Fresnel Back-Reflections (FBR) and Rayleigh Back-Scattering (RBS).
Optimization of Rayleigh-limited WDM-PONs using colorless re-modulating ONU.

Reflective SOA used for re-modulating downstream optical power in WDM/TDM-PON is a promising solution to deploy a cost effective, high bit rate (2.5 Gbit/s or above) wavelength independent (“colorless”) transmitters. However, since the optical downstream and upstream signals share the same path, this scheme may suffer from interferometric noise caused by localized Fresnel Back-Reflections (FBR) and Rayleigh Back-Scattering (RBS). The former can be removed in a properly designed network, but the latter is not fully removable.
In the PIANO+ project we wish to evaluate usefulness of a special combination of RZ codes especially tailored for efficient re-modulation, taking into account cost effectiveness and mitigation of interferometric RBS noise. Simultaneously, the influence of the gain in a remotely seeded ONUs on the upstream transmission will be evaluated.
The distortions can be strongly reduced by a line coding, i.e., using a DC-balanced data stream and high-pass filtering in the OLT receiver. To this aim, the 8B10B coding, which is widely implemented in Gigabit-Ethernet, is a good candidate. Moreover, this solution is compatible with burst-mode operation, which may be an important additional requirement in PONs. A pulse position modulation or duobinary coding can be evaluated in the downstream transmission. Another techniques for re-shaping an upstream signal in ONU (e.g. feed-forward power equalization) can be considered, as well.
Reflective modulation schemes suffer from RBS when using a single fibre. To reduce RBS impairments, we intend to evaluate different PON architectures: e.g. with a CW feeding light source located at the remote node or using a dual fibre in the trunk section.
We seek experienced partners with technical ability to experimentally evaluate proposed ideas, previously checked by means of theoretical modelling and simulations performed by our team.


Keywords
PON, monitoring, fiber Bragg gratings, FBG, WDM/TDM-PON, Rayleigh Back-Scattering, interferometric RBS noise, 8B10B coding

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