We present a vector theory of Raman amplification and use it to discuss the effect of polarization-mode dispersion (PMD) on the Raman gain process inside the optical fiber used for stimulated Raman scattering. We show that the PMD induces large fluctuations in the amplified signal and reduces the average value of the amplifier gain. In the case of forward pumping, fluctuations are expected to be more than 15% under typical operating conditions and can exceed 50% for fiber with a relatively low value of the PMD parameter. The PMD effects are much less severe in the case of backward pumping. Signal fluctuations reduce to less than 1% in this case when the PMD parameter exceeds .
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