Theory of fiber optic Raman polarizers

Abstract

We propose and apply a theoretical description of a Raman amplifier based on the vector model of randomly birefringent fibers to the characterization of Raman polarizers. The Raman polarizer is a special type of Raman amplifier with the property of producing a highly repolarized beam when fed by relatively weak and unpolarized light. [2,3]. The analytic theory of [2] is limited by the condition that the beat length L B is smaller than the birefringence correlation length L c , and therefore its validity is questionable when applied to Raman polarizers, which, as we shall see, require the opposite inequality L B ≫ L c . The full-scale numerical approach in [3] accurately models a randomly birefringent fiber consisting of fiber spans with randomly distributed values and orientations of the birefringence. Typically, thousands of such realizations are required for getting accurate statistics. Hence, the required computer time is 3 to 4 orders of magnitude longer than for the numerical modeling involved in the theory presented below. In addition to the much faster performance, our theory is formulated in terms of a set of deterministic differential equations and, as such, allows for a simple physical interpretation.Starting The components of the Stokes vector are written in terms of the two polarization components V s1 and V s2 of the slowly varying signal field in the appropriate reference frame as S

Suppression of Relative Intensity Noise in Fiber-Optic Raman Polarizers

“…Input temporal variations of the polarization of the signal beam disappear at the device output, where the signal SOP is strictly aligned along the (0, 1, 0) direction. As expected, the output signal SOP coincides with the SOP of the pump beam, see [4].…”

“…From the practical point of view, fiber optic Raman polarizers differ from typical Raman amplifiers in that a peculiar type of gain fiber should be selected. Namely, a Raman polarizer requires a low polarization mode dispersion (PMD) fiber, i.e., with PMD coefficients lower than 0.02 ps , see [1], [4], [5]. The lower the PMD coefficient, the greater the PDG, the better the performance of the Raman polarizer, the worse the RIN induced over the signal.…”

“…Although computer-friendly theories of the -interaction in telecommunication (i.e., randomly birefringent) fibers for both geometries were recently developed in [4], [5], here we shall limit ourselves to the copropagating case. Detailed derivations can be found in [5]: here we only provide the final equation for the evolution of the Stokes vector of the signal beam:…”

“…Details for the computation of the diagonal self-and cross-polarization modulation matrices and , diagonal Raman matrix , and coefficient can be found in [4], [5]. We self-consistently numerically solved the equations for the pump and signal Stokes vectors with the following parameters of the telecommunication fiber: the beat length m, the PMD coefficient ps , the attenuation coefficient dB/km, the Raman gain coefficient W km , the fiber length 1.5 km.…”

Two-Section Fiber Optic Raman Polarizer

“…The new approach developed by Sergeyev et al [7,8] have shown that it is possible to simultaneously mitigate both PDG and PMD by using a fiber with a particular spin profile or, more specifically, a two-section fiber ('two-section approach') in which the first section has no spin and the second one is periodically spun. A new approach based on using anisotropy of Raman amplification (Raman polarizer) to repolarize and amplify signal rather than to depolarize it has been recently developed by Martinelli et al [9] , Kozlov et al [10] and Ursinini et al [11]. In this approach, arbitrary at the input, signal SOP with low input power is attracting to the pump SOP with high input power (polarization pulling) and so is stabilizing [9][10][11].…”