Metrology of nonlinear effects in fibers for optical networks

• Research unit: COST
• Project number: C99.0082

Key words (English)

Nonlinear effects; optical fibres; WDM; distributed measurements; local birefringence; local dispersion; Brillouin effect; Raman effect; Kerr effect

Research programs (English)

COST-Action 265 - Measurement techniques for active and passive fibres to support future telecommunication standardisation

Short description (English)

See abstract

Further information (English)

Full name of research-institution/enterprise: EPF Lausanne Faculté des sciences et techniques de l'ingénieur SCI-STI-LT ELE 141, Station 11

Partners and International Organizations (English)

A, B, CZ, DK, FIN, F, D, I, P, SK, E, S, CH, GB

Abstract (English)

The COST265 action is devoted to the development of measurement techniques for the characterisation of optical fibres and new components for modern optical communications. The new trend to increase the capacity of communication networks requires to simultaneously transmitting many channels at different wavelengths (WDM). The subsequent increase of optical power within the fibres makes it necessary to address the issues (and also the potential benefits) caused by optical nonlinearities. The aim of our studies along this project has been twofold: in one side, we have developed measurement techniques that should enable us to characterize (and potentially overcome) the nonlinear impairments appearing in the fibre; in the other, we have shown that the domain of nonlinear interactions opens new possibilities to develop new measuring techniques, in particular those providing distributed measurements, i.e. giving the value of a quantity at each point along a fibre. In particular, the determination of local variations of chromatic dispersion along the fibre have been a necessary concern in our work, since a successful modeling of the nonlinear effects relies on this knowledge. The results obtained in this project demonstrate that the accuracy of the nonlinear techniques is unreachable by linear techniques. These were the most outstanding results obtained from this project: · Development of an original technique to determine the local chromatic dispersion along an optical fibre. This technique provides through a direct measurement the actual value (no calculation) of dispersion at any point along a fibre. It is based on the probing of the four-wave-mixing local amplitude using the Brillouin interaction. This is the most accurate technique reported so far for this kind of measurements. · Improvement of the Brillouin local analyser by successfully testing a novel configuration using laser injection locking and development of a centimetre-resolution set-up based on a correlation technique. This is the best spatial resolution obtained to date. This measurement technique can be used to determine the core refractive index distribution along the fibre and also as a distributed sensor for strain and temperature. Some other results obtained along this project were: · Development of linear technique for the measurement of chromatic dispersion distribution along the fibre. This technique is based on the use of an optical time-domain reflectometer (OTDR) operating at different wavelengths. This is basically a commercial instrument that has been modified to include a wavelength-tuneable laser source. Such a set-up makes possible the determination of spot-size at different wavelengths, in order to get essential quantities for determining waveguide dispersion. Additionally we performed distributed measurements of Brillouin frequency over different optical fibres, in order to obtain the core refractive index distribution along the fibre. We developed a model to determine local waveguide dispersion from OTDR and Brillouin distributed measurements. We applied this model to the results of the previously developed OTDR and the Brillouin shift measurements. The use of these data in lead us to establish that variations of dopant concentrations (which are largely neglected in other models appearing in the literature) are a dominant contribution to the dispersion variations that are produced along the fibre. The main conclusion of this study is that the linear techniques proposed in the literature for distributed measurement of chromatic dispersion do not meet the requirements demanded by the next generation of optical networks. ·Measurement of Brillouin fibre properties at cryogenic temperatures and development of a new thermometric sensor for very low temperatures (down to 1K). Measurement of Brillouin fibre properties at high temperatures (up to 1000K) for the development of a distributed thermometry at high temperatures. ·Development of an experimental setup for the measurement of Raman gain in optical fibres. This setup enables a complete experimental characterization of the basic parameters governing the Raman interaction in optical fibres (namely gR(u) and Leff). Additionally, the setup can also be used to determine the pump-wavelength dependence of these parameters, by comparison of Raman gain and Raman attenuation processes. · Experimental study of the stimulated scatterings in microstructured fibres. As a result of this study such fibres turn out to offer interests only for sensing as far as stimulated scatterings are concerned. The linear loss is yet too important to leave sufficient gain through Raman scattering, while Brillouin scattering is quite identical to standard fibres and thus offers no particular feature.

References in databases (English)

Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. [phone] Swiss Project-Number: C99.0082