Large Normal Dispersion Fiber Design Near2μm Wavelength And Its Application In Mode-locked Lasers

Thulium doped mode-locked fiber laser has many features, such as high poweroutput, small size, high efficiency, broad-band tunable, so it has importantapplications in medical, spectroscopy, military and some other fields.However, limited by the soliton area theorem and sideband spectrum, solitonpulse energy produced by passive mode-locked fiber laser is generally is generallyless than0.1nJ. In order to improve the mode-locked laser pulse energy, researchersput forward some laser schemes and structures. Among them, normal dispersionmode-locked fiber laser which is able to output high energy pulse has become theresearch hotspot recently. However, for the2μm wavelength, the group velocitydispersion is negative for conventional optical fiber. So it is a big challenge to designa fiber cavity with a large normal dispersion.Based on the requirements for the development of high power mid-infraredmode-locked fiber laser, we have numerically studied how to design the fiber withlarge normal dispersion near2μm wavelength and how to use it in normal dispersionmode-locked fiber lasers. In this thesis, the main results can be summarized asfollows.First, we have numerically studied the group velocity dispersion (GVD)characteristics of fused silica fibers near2μm wavelength and its control method.Based on the theory of optical waveguide, the transcendental equation wasderived for the micro-nano optical fiber with different structures, and thecharacteristics of GVD for the fibers have been discussed. The factors influencing thevalue of GVD have been clarified. By appropriate design, the fiber with GVD greaterthan3000ps2/km can be obtained by tapering the conventional fiber. It also can beseen that the GVD can be adjusted by introducing a thin dielectric layer. The GVDvaries continuously with the increasing thickness and refractive index of the coat,indicating that the possibility for fine modification of the GVD.Second, the thulium-doped passively mode-locked fiber laser with the designedlarge normal dispersion fiber has been studied numerically.The non-linear Schr dinger equation for the pulse propagating in the fiber hasbeen deduced, and the codes and programs to simulate the dissipative soliton in thethulium-doped fiber laser have been improved. The stable output and time-domain characteristics have been studied numerically for the thulium doped mode-lockedfiber lasers based on semiconductor saturable absorption mirror and large normaldispersion fiber component in2μm wavelength. When the GVD is positive of thewhole laser cavity, under the condition of no additional filter, we can get thedissipative soliton pulse. The formation of the dissipative soliton laser is theself-consistent results under the interaction of group velocity dispersion, self-phasemodulation, the gain saturation and gain narrowing filter. The GVD and self-phasemodulation can result in widened pulse time domain and frequency domain. However,the gain saturation and gain narrowing filter can narrow the pulse.