Stimulated Raman scattering (SRS) laid in the basis of the photonic devices development such as fiber Raman lasers and fiber Raman amplifiers. In contrast with phenomenological definition of SRS threshold in the early experiments, in present work we will analyze the Raman laser threshold in the single mode fibers from the point of view the strong gain description in the active laser materials. Our analysis show that the Raman laser threshold for monochrome signal wave can be directly obtained from standard coupled equations using only fundamental fiber parameters. The quantitative data of laser threshold as function of wavelength for several widespread Raman fibers are presented.

The spectroscopic analysis of the frequency distribution of the amplification of optical radiation due to the Raman effect (Raman gain profile) in single-mode fibers based on silica glass has been carried out in the region of Stokes frequency shifts from 0 to 1400 cm −1 . The Raman gain profiles are determined from the experimental spectra of spontaneous scattering for widespread fibers, namely for pure SiO2, GeO2, P2O5, and TiO2 doped fibers. The analytic expressions of the Raman gain profiles are given. They are obtained, by using the Gaussian decomposition by means of 11-12 modes, and the experimental profile is approximated with an accuracy of not less than 0.3%. The decomposition results are analyzed in terms of the fundamental oscillatory dynamics of molecular nanocomplexes in amorphous glass, as well as in the application aspect of the modeling of photonics devices. Examples of the proposed method applications are presented for the analysis of noise parameters of the fiber Raman amplifiers and for the generation bandwidth in fiber Raman lasers. K e y w o r d s: optical amplification, Raman gain, fiber Raman lasers, fiber Raman amplifiers.

Nonlinearity of the stimulated Raman scattering (SRS) process in 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 viewpoint of the strong gain description of active laser materials have been made in this work. It has been shown that the absolute transparency regime for optical transmission in fibers and Raman laser threshold for a monochrome signal wave can be directly obtained from the standard coupled equations using only fundamental fiber parameters. Limiting condition when material of fiber core starts transformation from the natural state with attenuation of the Stokes wave to the state in which the Stokes wave is amplified due to the pumping power may be expressed in a simple analytical form. The numerical data of laser threshold as a function of the wavelength and the examples of gain bandwidth determination for several widespread Raman fibers have been presented.

The properties of TiO2-doped fiber are considered as optoelectronic material in our work. The advantages of such fiber have been studied with the aim of its application to active medium in Raman fiber lasers. The comparison of spontaneous Raman spectra and corresponding gain profiles in TiO2-doped and GeO2-doped fiber is presented. Raman gain profiles were obtained over a broad spectral range of Stokes shifted frequencies up to 1400 cm-1(42 THz). The spectral decomposition using multimode Gaussian components has been performed for both Raman gain profiles. High accuracy analytic form of Raman gain profile of TiO2-doped fiber is obtained using 12 components. The pump power of Raman gain threshold is introduced as the function of wavelength within the telecommunication windows for both fiber types. Our spectroscopic analysis allows presenting the numerical results on lasing bandwidth and Raman gain threshold inTiO2-doped single-mode fiber. It has been shown that the lasing bandwidth in TiO2-doped fiber may be almost twice wider than the lasing bandwidth in standard GeO2-doped fiber.

The new decomposition approach for the Raman gain spectrum approximation in an arbitrary optical fiber is presented in this paper. A nonlinear fitting procedure of the Raman gain profile based on the LevenbergMarquardt method was applied, which provides a nearly perfect match for the observed Raman gain spectra in silicabased fibers. The optimal set of Raman gain parameters is reduced to simple analytical expressions that can be integrated and differentiated in symbolic form, and can be easily evaluated numerically. The results of simulations of stimulated Raman gain spectra in several widely used fibers are presented.