| In nano-sized quantum dots (QDs), energy bands and optical emission spectra are discretized due to the confined motion of electrons and holes in three dimensions. Thus, the QDs show unique properties, such as the significantly enhancement of the fluorescence intensities, broaden Stokes shift, and disarable biocompatibility etc. Because of these properties, in recent years, QDs have attracted much attention, showing potential applications in quantum dot lasers, quantum dot solar energy cells, high-speed all optical logical devices and quantum dot LED. Among Ⅳ-Ⅵ materials, a kind of reasonable QDs is PbSe. With a large Bohr radius of excitons (-46nm), PbSe QD characterizes a narrow emission peak, high quantum yield, and emitting fluorescence wavelengths coving almost near infrared band (1000-2300nm).In this dissertation, a PbSe QD doped fiber laser (QDFL) is studied in detailes. To obtain suitable corss-sections of the PbSe QD, first, we estimated the absorptance of PbSe quantum dots in the range of1240-2230nm by the neural network of Matlab back propagation (BP) and Mie’s scattering theory. Then, we establish a computational kinetics model of the PbSe QD doped fiber laser with single mode, optimizing the pumping wavelength, the fiber length, the doping concentration and the reflectance of output mirror by using a genetic algorithm. It was reported in experiments that the absorption-and emission peak wavelengths of the PbSe QD doped in the sodium-boron-aluminosilicate glass treated with550℃can be at1566nm and1676nm, respectively. For1566nm, the calculated absorption cross section is2.98×10"20m2using the σa(λ)=α(λ)/Np equation. Further, the refractive index and absorptivity ranging1240-2230nm available in the quantum dots doped fiber laser (QDFL) can be predicted by using Matlab back propagation (BP) neural network according to the given data within200-1240nm. Using these evaluated refractive index and absorptivity, we further caculate the absorption cross sectionat at1566nm, which is6.71×10-20m2, in accord with the experimental values in the order of magnitude.Further, the PbSe QDs doped fiber laser (PbSe QDFL) is proposed based on high temperature melting experiment and genetic algorithm theory. For the PbSe QDFL, the rate equations and lasing generation equation in a linear resonator are solved numerically. The available pumping wavelength λv, fiber length L, doping concentration N and reflectance R2of the output mirror are determined by using the genetic algorithm. It is indicated that the pumping power (Pp) shows an obvious threshold (2W) on the condition of definition of the pumping effeciency. Upon the pumping power of2W, the maximum output at1676nm comes to1.36W, with the pumping efficiency of68%. The optimized doping concentration can be expressed by Pm(N)=Po exp(-Nth/(N+C))(where Nth is doping concentration threshold, Pm is maximum output power, and Po is initial value of output power) using above λp, L and R2available. It is indicated that the lasing power can be increased slightly when N <0.1×1021m-3. When N increases over0.1×1021m-3to5×1021m-3, the lasing power shows an exponential increases, then, reaching a saturation when N>5×10m-3. However, the available fiber length shows a decrease with the N increase. Namely, the higher the doping concentration is, the shorter the saturation fiber length.On these grounds, a conclusion can be gotten as follows:PbSe QDs have been doped into the sodium-boron-aluminosilicate glass with550℃heat treatment and then we stretched it into an optical fiber. Such a QDFL has high pumping efficiency, low exciting threshold, tunable doping density, and short saturated fiber length, compared with the conventional fiber lasers that are doped with rare earth ions (e.g. Yb3+, Er3+). The proposed QDFL can be developed into a novel multi-wavelength and/or tunable wavelength due to wavelength depending on the size of QDs. |
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