Under Strong Pumping Fiber Random Laser Characteristics Of The Study

Random lasers are stimulated emission phenomenon of disordered opticalwaveguide medium in which light waves are both multiply scattered and amplified.Hence, the feedback is provided by light scattering in a gain medium rather than acavity, as in conventional lasers. Lasing occurs when the total gain in the cavityovercomes the total cavity loss.Since the first theoretically studied by Letokhov, random lasers have attracted agreat deal of attention because of their intrinsic interest and wide potential applicationranging from medicine to laser fusion. Compared with traditional random lasers, therandom distributed feedback fiber laser shows relative stable output, long-distanceemission and wide wavelength tunability, which are of great interest in optical fibercommunication and optical fiber sensing. Furthermore, random fiber laser is a newtype of one-dimensional laser with random distributed feedback based on Rayleighscattering (RS). Because Rayleigh scattering due to inhomogeneities within the glassstructure of the fiber is extremely weak, making the operation and properties of therandom distributed feedback fiber lasers different from those of both traditionalrandom lasers and conventional fiber lasers. The theory and characteristics of randomfiber laser have not been studied well due to short development history.In this thesis, the characteristics of random fiber laser, such as power distributionalong the fiber, pump threshold and conversion efficiency etc., are studied boththeoretically and experimentally.Firstly, we study the random laser based on the rate equation theory. Taking themiddle pump scheme as an example, the theory model was proposed and the pumpthreshold of the random distributed feedback fiber laser was calculated.Then a numerical simulation model on the random lasing characteristics of asingle mode fiber with symmetric and asymmetric end reflections is presented in thispaper. We considered three pump regimes, i.e., middle, directional and bidirectionalpumps, and also discussed the influence of pump wavelength. For numerical analysis,a steady-state propagation function of pump and random lasing waves taking into account the Raman amplification, Rayleigh scattering and fiber loss were used. Thismodel was solved numerically through the shooting method. The results show that wecan get maximum output power through optimizing the length of the fiber, because thepower distribution varies with the pump scheme. The lasing threshold varies fordifferent pump regimes. Due to the combined influence of distributed Rayleighfeedback and end reflection, a lowest lasing threshold appears (at a specific value offiber length) as fiber length increases, and the lasing threshold depends greatly on thepump regimes and end reflection. For pump at a lower wavelength, similar results canbe obtain, however, the lasing threshold (efficiency) is slightly larger (lower) due tolarger fiber loss.At last, taking directional pump scheme as an example, the output characteristicsof the random fiber laser was studies. The results show that end reflections will changethe power distribution along the fiber and reduce the pump threshold obviously.Comparing with1365pump scheme, the pump threshold power is lower while theconversion efficiency is higher for1480pump scheme. These results are in accordancewith that of the numerical simulation.As an important novel optical source, the distributed feedback fiber laser will bewidely used in nonlinear optics, optical fiber communication and sensing etc. Inaddition, it will be the ideal model for the research of nonlinear optics, laser physicsand chaos theory. The work of this thesis provides a basis reference to understand thecharacteristics of random fiber laser as well as the design and optimization of this typeof lasers.