Theoretical Study Of Upconversion Ultraviolet Laser Based On 4f5d State In Pr～(3+): ZBLAN Fiber

It has been over forty years since the first laser system was successfullydemonstrated in the world. In this period, laser techniques and category has beenlargely developed. However, the development in the modern scientific field needsmore improvement on the laser system itself, which make lasers develop towardall-solid-state, miniaturization, tenability, ultraviolet and far infrared,easy-operation. For the trivalent RE ions doped in host materials, the energy levels belongingto the 4fN-15d configuration present continuous due to the strong couplingbetween the 5d electron of the active ion and its crystalline field environment andhence radiative transitions from these levels produce broad-band UV spectralemissions. Obviously, the 4fN-15d levels are possible upper laser levels forbroad-band tunable UV laser. Tunable UV laser based on 5d upper levels havebeen realized in LiCaAlF6/LiLuF4 crystals doped with Ce3+. However, since4fN-15d direct excitation occurs in or near the vacuum UV spectrum region, thisexcitation is not easily accessible by conventional laser sources and brings forthcomplexity of UV laser system. Therefore, realizing 4fN-15d upconversionexcitation of RE dopant is a significant approach. To realize upconversion UV laser based on 4fN-15d state in laser medium,theoretical studies of upconversion excitation kinetics and upconversion UV laserkinetics based on 4f N-15d state are necessary. In this thesis, we theoretically studythe feasibility of realizing upconversion UV laser based on 4f N-15d state inPr3+:ZBLANfiber. 第 110 页 共 114 页吉林大学硕士学位论文 Firstly, by utilizing steady population rate equations and propagationequations, we investigate the kinetics of 4f5d upconversion excitation of Pr3+ inPr3+:Yb3+:ZBLAN and Pr3+:ZBLAN fiber. In Pr3+:Yb3+:ZBLAN fiber, 4f5dupconversion excitation can be realized through Yb3+ sensitized excitation andPr3+ excitated state absorption. And there exist the population inversions between4f5d state and I6 or S0 level. In Pr3+:ZBLAN fiber, 4f5d upconversion excitation 1 1can be realized through two-step excitation. And there exist the populationinversions between 4f5d state and I6, S0 level or the highest Stark level of H4, 1 1 33F4 level. It is relatively easier to realize population inversions between 4f5d stateand the level of 4f2 and even to realize UV laser in Pr3+:ZBLAN fiber than inPr3+:Yb3+:ZBLAN fiber. In addition, the distributions of powers and absorptioncoefficients along the fiber, population density of all levels, the effective fiberlength, fluorescence power and pump efficiency are calculated and discussed. Secondly, we investigate the laser kinetics for upconversion UV oscillatorbased on transitions from the 4f5d state to the H4 state and I6 state in 3 1Pr3+:ZBLAN fiber, respectively. The threshold pump powers, output power, slopeefficiency and optimum cavity length are calculated and discussed. There existslots of difference between these two laser system. In upconversion UV oscillatorbased on transitions from the 4f5d state to the H4 state, threshold pump powers 3and slope efficiency become higher as the fiber length is increased. When thefiber length is long enough, slope efficiency tends to a constant (10%). Given thecharacteristics of the laser, there exists an optimum cavity length to obtainoptimum output power. And the optimum cavity length is 14 m when pumppowers are large enough. In upconversion UV oscillator based on transitions fromthe 4f5d state to the I6 state, high reflectivity R2 and short fiber cavity length are 1better for low threshold pump powers. Great background loss and low pump...