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Ultra-Long Fiber Raman Lasers – Design ConsiderationsI. Koltchanov, D. Kroushkov, A. Richter
Abstract: In frame of the European Marie Currie project GRIFFON
the usage of a green approach in terms of reduced power consumption and maintenance costs
is envisioned for long-span fiber networks. This shall be accomplished by coherent transmission in unrepeatered links
(100km – 350km) utilizing ultra-long fiber Raman laser (URFL)-based distributed amplification, multi-level modulation
formats, and adapted Digital Signal Processing (DSP) algorithms.
The URFL uses a cascaded 2-order pumping scheme where two ~1365nm pumps illuminate the fiber. The URFL oscillates at ~1450nm whereas amplification is provided by stimulated Raman scattering (SRS) of the 1365nm pumps and the optical feedback – realized by two FBGs at the fiber ends reflecting at 1450nm. The light field at 1450nm provides amplification for signal waves in the 1550nm range due to SRS.
In this work we present URFL design studies intended to characterize and optimize the power and noise characteristics of the fiber links. We use a bidirectional fiber model describing propagation of the signal, pump and noise powers along the fiber length. From the numerical solution we evaluate the on/off Raman gain and its bandwidth, the signal excursion over the fiber length, OSNR spectra, and the accumulated nonlinearities. To achieve best performance for these characteristics the laser design is optimized with respect to the forward/backward pump powers and wavelengths, input/output signal powers, reflectivity profile of the FBGs and other parameters.
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