Theoretical Study Of Cavity Fiber Raman Laser

Stimulated Raman scattering (SRS) is one of the most efficient ways of frequency conversion. Due to the broad spectrum involved in the SRS process, this technique has expanded the scope of the available laser wavelengths. SRS is advantageous in many aspects, such as good beam quality, short duration, no need for phase matching and high conversion efficiency. Recently, SRS in optical fibers has become a focus of research which draws a lot of attention. With potential long distance, small cross section and thus large energy density, optical fibers can be used as continuous wave (CW) Raman lasers, which enable CW pumping. Furthermore, with the development of double clad rare-earth doped fiber lasers, fiber Bragg gratings and high-gain Raman optical fibers, fiber Raman lasers as a novel laser source have attracted intensive research interest.Theories concerning SRS in gases and crystals have been fully developed. However, since utilization of SRS in fibers relatively lagged behind, the theories, especially those on fiber Raman lasers are far from mature. One example is the intra-cavity fiber Raman lasers which have been reported experimentally not long ago.In their paper, Yucheng Zhao et al reported nanosecond fundamental laser, first-order Stokes and second-order Stokes generation in intracavity fiber Raman laser without employing traditional Q-switching or mode-locking techniques. This phenomenon seems amazing and arouses great interest.Due to the complexity of the interactions involved in this system, the physical mechanism underlying this novel pulsed laser source, and the stability of its output remain unclear. In our thesis, with the nonlinear propagation equations to account for the SRS process in the Raman fiber, and the rate equation for the common laser fiber, we build up a physical model to study the evolution dynamics in this novel source. Numerical simulation results qualitatively agree with the experimental results. Besides, we find that strictly speaking, the output is not stable. Furthermore, according to the results, we think the pulse mainly originates from the complex nonlinear interactions among the laser media, the fundamental wave, the first and the second Sotkes waves, rather than the saturated Raman gain proposed by the authors.