Full transparency regime for optical transmission and lasing threshold in silica fibers due to nonlinear Raman interaction

Raman nonlinearity of single mode fibers is the creation basis of the new class of modern photonic devices such as fiber Raman lasers and fiber Raman amplifiers. The quantitative analysis of the gain start conditions and the Raman laser threshold in the single mode fibers from the point of view the strong gain description of active laser materials will be made in present work. It is shown that the absolute transparency regime for optical transmission in fibers and Raman laser threshold for monochrome signal wave can be directly obtained from standard coupled equations using only fundamental fiber parameters. Limiting condition when material of fiber core starts the transformation from the natural state with attenuation of Stokes wave to the state in which the Stokes wave is amplified due to pump power may be expressed in simple analytical form. The numerical data of laser threshold as function of wavelength and the examples of gain bandwidth determination for several widespread Raman fibers are presented.