Theoretical Investigations On The Characteristics Of Random Distributede Feedback Fiber Lasers

Unlike the conventional laser, the optical cavity of random laser is replaced bymultiply scattering. Its random lasing comes from multiply scattering and amplificationof light waves in disordered gain medium. Therefore, compared with the conventionallaser, random laser shows a set of specialties, such as random distribution of theradiation direction and intensity, low threshold pump power, single longitudinal modeoutput, which has broad applications in the military and technological fields.The output of random laser is unstable with irregular direction. To improve theseshortcomings, a variety of low-dimensional structures have been proposed. Wherein, therandom fiber laser which uses optical fiber as the gain medium could improve thedirectionality of its output effectively. Based on the studies all above, Turitsyn et aldeveloped a new kind of random fiber laser using standard single-mode fiber as the gainmedium based on distributed Rayleigh scattering and Raman amplification in April2010successfully. The output lasing is stable and continuous. Thus the concept ofrandom distributed feedback fiber laser has been proposed.Compared with conventional fiber laser, random distributed feedback fiber laserwith a mirrorless open cavity only consists of pump and gain medium. It has a simplestructure, showing good directionality, stable and continuous output, long-distancetransmission as well as broadband wavelength tunability. It has wide potentialapplications in nonlinear optics, optical sensing, optical communication and some otherfields, and therefore is considered to be an important novel light source.However, the theory of random distributed feedback fiber laser has not beenconsummated so far, and its operation mechanisms as well as characteristics still need tobe further investigated. In addition, the threshold pump power of random distributedfeedback fiber laser is relatively high, which limits the engineering applications in thefield of optical fiber communication and sensing to a certain extent.In this paper, we analyze three kinds of random distribution feedback fiber laserswith the open cavity, half-open cavity and ring cavity respectively. The lasing powerdistribution, spectral characteristics and coherence of these different structures have been investigated theoretically and experimentally.Firstly, we take the middle-pump regime for instance, describing the theoreticalmodel of random distributed feedback fiber laser and giving a solution of its threshold.Then, the output characteristics of random distributed feedback fiber lasers withthe open cavity, half-open cavity and ring cavity have been numerically simulatedrespectively. The evolution of pump light wave and output lasing could be expressed bythe steady-state propagation equations theoretically, which take into account the fiberlosses, Rayleigh scattering, Raman gain and the influence of frequency on other factors.We use shooting method for the numerical simulation. And the results show that theStokes light wave power distribution in different cavity structures is not the same, whichis helpful to the optimization and design for the optical fiber amplifier. Besides,according to the simulation, the open cavity has the highest pump threshold among thethree kinds of resonator structures with equal length, following by the ring cavity andhalf-open cavity successively, which could have a good agreement with theexperimental results.In addition, the output spectral characteristics of random distributed feedback fiberlaser with three different kinds of resonator structures are also observed and compared.For ring resonator, the output is unstable when its cavity length is relatively short. Withthe increase of fiber length, the bandwidth of its output spectrum would arrive to10nmunder the action of high pump power. Besides, the random distributed feedback fiberlaser with ring cavity is insensitive to external strain disturbances and temperaturechanges of environment, which make it an ideal light source for potential applications tolow-coherence interferometer system.Finally, as the application exploration of random distributed feedback fiber laser,the characteristics of middle-pump random fiber laser with variable spectrum areinvestigated experimentally. The results show that it has mixed output with the dualwavelength, wherein the first peak with shorter wavelength is completely determined byFBG, which is sensitive to the environment temperature, and the output characteristic ofthe second peak with longer wavelength is mainly due to Raman gain profile, whichexhibits a unique thermal stability. This feature may found potential applications to theoptical sensing system for random distributed feedback fiber laser.The mechanism and characteristics of random distributed feedback fiber laser based on distributed Rayleigh scattering are different from the conventional laser andtraditional random laser. The proposition of this concept provides not only a newdirection for the research of laser physics, nonlinear optics as well as other naturalsciences, but also new opportunities for the development and applications of lasers.Thus the exploration of random distributed feedback fiber laser has importanttheoretical and practical significance. The investigation of this thesis is helpful to thedesigns and applications of novel fiber lasers.