Abstract

It is well known that the stimulated Raman scattering (SRS) gain is maximal for equally (linearly or elliptically) polarized pump and stokes waves¹ and is a factor of 2 lower if they are depolarized. The latter case corresponds even to polarization maintaining fibers, but with relatively high birefringence. Our investigations show that in the intermediate case of low birefringence fibers excited by high power pump pulses the polarization dependence of the dispersionless SRS gain may substantially differ from the cited above. For these conditions the third-order nonlinear polarization include the additional terms² of the type 1/3 ${\text{A}}_{\text{x}}^{*}{\text{A}}_{\text{y}}^2\exp \left(-2\text{βz}\right)$ where A_x and A_y are the slowly varying amplitudes of the pump pulse components and β is the propagation constants difference. These terms are responsible for the polarization ellipse rotation effect which causes the extensively studied instability of CW waves polarized close to the fast axis of a low birefringence fiber.⁹ It is also possible to introduce the integral Raman gain connected with the slow mode component respectively where g_r is the gain coefficient and L is the fiber length. When ellipse rotation and SRS occur simultaneously the dynamics of the Raman scattering is strongly affected because the coupling of the two orthogonal components of the pump pulse, which causes periodic power exchange between them, always increases G_x and respectively lowers G_y. The relative importance of this effect depends on the angle θ between the input pump pulse polarization and the slow axis. For θ ≈ 0 the intermodal power coupling is negligible and the overall Stokes conversion efficiency is approximately equal to that for θ = 0 or θ = π/2 (determined by G= g_rP L / S_eff, where P is the total input pump power and S_eff is the fiber core effective area). If θ ≈ π/2 the power transfer is significant, the gain is distributed along the two axis and the conversion efficiency decreases.

© 1991 Optical Society of America