Theory of fiber optic co- and counter-pumped Raman polarizers
Authors: Wabnitz S., Kozlov V.V., Nuño J., Ania-Castañón J.D.
Autors Affiliation: University of Brescia, Department of Information Engineering, Via Branze 38, Brescia 25123, Italy; St.-Petersburg University, Department of Physics, St.-Petersburg, Petrodvoretz, 198504, Russian Federation; Instituto de Optica, Consejo Superior de Investigaciones Cientificas (CSIC), 28006 Madrid, Spain
Abstract: Fiber optic Raman amplifiers are polarization-dependent by their nature: the state of polarization (SOP) of the outcoming signal beam strongly depends on the SOP of the pump beam. Thus, in isotropic fibers the SOP of the signal acquires the SOP of the pump beam towards the output, because the Raman gain for a signal SOP that is aligned along the pump SOP is two orders of magnitude larger than the Raman gain for the orthogonal signal polarization. However, telecommunication fibers are not isotropic but exhibit a birefringence which stochastically changes along the propagation distance. Moreover, the pump and the signal beams experience a different birefringence. This mismatch has the consequence that SOPs of the two beams do not change in unison when they propagate through the fiber. On average, the signal beam sees the two orthogonal polarizations of the pump beam with equal frequence. This situation is characteristic to high-PMD fibers (here, PMD stands for the polarization mode dispersion), namely, with PMD coefficients 0.05 ps/km1/2. In this case the signal beam experiences an average Raman gain which is half of the maximal Raman gain. Thus a simple scalar model of Raman amplification is appropriate for describing high-PMD fibers, since all polarization-dependent properties of the Raman gain are washed out by fiber birefringence.
KeyWords: Fiber birefringence; Isotropic fibers; Orders of magnitude; Orthogonal polarizations; Orthogonal signals; Propagation distances; Pump beams; Raman amplification; Raman amplifier; Raman gain; Scalar model; Signal beams; State of polarization; Telecommunication fibers; Two beams, Amplifiers (electronic); Birefringence; Electron optics; Fiber amplifiers; Laser theory; Light amplifiers; Optics; Polarization; Pumping (laser); Quantum electronics, FibersDOI: 10.1109/CLEOE.2011.5943131