Stimulated Raman Scattering in Monomode Optical Fibres

A theoretical investigation of stimulated Raman scattering in step-index, monomode optical fibres is presented. The regime of validity of existing mathematical analyses has been extended to account in greater detail than previously reported for the spectral characteristics of optical fibres in which stimulated Raman scattering is occurring. The variation of fibre loss and transverse modal confinement over the range of frequencies, which are subject to Raman amplification, is considered. It is shown that the growth of total power generated by stimulated Raman scattering with the corresponding depletion of laser input power can be modelled by only two coupled differential equations. In order to do so it has been necessary to develop the concepts of an effective Stokes loss and a weighted Raman line-shape function. The coupled pair of equations has been solved analytically in the special cases where all losses are zero and where all losses are a wavelength independent constant. Numerical solutions and a quantitative discussion of the mechanism of power transfer from pump to Stokes wavelengths are offered for circumstances of inequality of pump and Stokes losses. As a result of this work is is possible to predict with greater accuracy the length of communications fibre which is free from crosstalk due to stimulated Raman scattering. A method of designing a source, fibre and detector system which is free of threshold stimulated Raman scattering has been developed. Finally, it has been shown that the mathematical model developed here agrees well with published experimental data