Study On The Polarization Dependent Gain In Fiber Raman Amplifiers

The fiber Raman amplifier which is based on the stimulated Raman scattering is one of the most important devices in optical communication systems because the fiber Raman amplifier has fast response time, big saturated output power, easily coupling, low noise, large bandwidth and so on. And the progress of the optical fiber manufacturing technology in recent years, as well as the emergence of high power semiconductor laser such as the pump light source has created favorable conditions and broad space for the rapid development of FRA. However, due to the influence of outside pressure and temperature the fibers show residual asymmetry and makes the light polarization change randomly, which results in the gain polarization correlation, namely the polarization dependent gain. Because the transmission performance of the optical fiber communication system is degraded by the the polarization dependent gain in fiber Raman amplifier, thus the study on the characteristic of the PDG in FRA can be useful and important for reducing the PDG in a FRA. The main aspects of this paper are as follows:(1) it summaries the domestic and overseas research status, background and senses of the PDG in FRA. (2) it shows the theory of FRA, the polarization theory of the signal in fiber, the structure forms of FRA and the nonlinear coupled equations of FRA. Moreover, the polarization factor which is relative to the PDG of FRA is deduced in detail. (3) the PDG of FRA with forward single pump and multiple signals as well as the PDG of FRA with backward single pump and multiple signals is investigated systematically, by numerical method based on fourth rank Runge-Kutta. And the results show that the PDG in FRA with single pump and multiple signals induced mainly by the pump-signal Raman interaction. The results also show that the PMD in fiber has a big influence on the PDG. (4) the characteristics of PDG in FRA are studied. The PDGs induced by all waves Raman interaction, by pump-signal Raman interaction, by pump-pump Raman interaction and by signal-signal Raman interaction are studied respectively in FRAs with forward pumps, backward pumps and bidirectional pumps. The influences of the pump powers, pump wavelengths and the number of the pump on the PDG in FRA with three different pump patterns are compared.