| Nanocrystal quantum dots (QDs) develop rapidly for its superior advantages over their bulk materials in recent years. QDs have narrow emission spectra, high quantum fields and high photoluminous intensities upon pumping excitation. People can obtain tunable wavelengths of absorption-emission peak and their full width at half maximum (FWHM) through artificial regulation of the size. What’s more, it’s expected to achieve white laser based on three primary colors and wavelength tunable laser by doping different types and sizes of QDs together. With the development of preparation techniques of QDs and optical fibers, people are more and more concerning on QD fiber lasers (QDFLs).In previous works, we achieved a PbSe QD doped fiber laser with a ring resonator in an experiment for the first time. In order to improve the performance of the PbSe QDFL, this dissertation conducts a theoretical study on the PbSe QDFL based of the experiment. The main work is as follows. (1) A theoretical model is established according to the two-level approximate model of PbSe QDs and the ring resonator used in the previous esperiment, including the rate equations, laser propagation equations and laser circulating conditions for the ring resonator. In addition, the calculation method of relevant parameters is presented in details.(2) The single/multi-mode laser pumping threshold Pth observed in the experiment is simulated numerically by solving the rate equations and using the Matlab programming. The calculated thresholds (Pth=22mW for single mode, Pth=16mW for multimode) are close to the experimental results (Pth=25mW for single mode, Pth=17mW for multimode). The inversion condition of population density, i. e., N2/N1=0.45, is given by investigating the laser power distributing along the longitudinal fiber, which can be explained by the cross section ratio of absorption to emission at the1550nm.(3) The laser output power in single mode varying with the length of QDF, density of QD, coupling ratio and level lifetime is simulated numerically. The results show that the doping density threshold and "quenching" density situation changes with the QDF length. A desirable coupling ratio is (80～90)%, which is consistent with the experiment results of (85～90)%and independent on the pumping power. The saturation phenomenon of pumping efficiency is also investigated. As a result, the saturation pumping power increase with length of QDF, while the pumping efficiency decrease with length. According to the simulation results, the upper level lifetime of PbSe QD suggested be as115ns, which locates in the range of100-300ns given by literatures.(4) Parameters affecting the output power are optimized by using a genetic algorithm, including the length of QDF, density of QDs and coupling ratio. For the single mode laser, the optimized parameters can increase the output power by2-3times. In addition, optimization on the length of QDF and coupling ratio can even increase the available range of doping density (1×1020～4×1023m-3). The favorable doping density N is related with QDF length. The optimized coupling ratio is90%for the coupler, which is (80～90)%used in the experiment.Finally, an optical circulator in the resonator is introduced to optimizing and improving the laser performance, which can ensure the lasing oscillating is maintained steadily at1550-nm wavelength. |
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