Rare Earth Doped Tellurite Glass Fibers And Their Spectroscopic Properties For The Lasers Operating At 2.0 μm Region And The Laser Experimental Research

Lasers operating at 2.0 μm region have been a hot area due to numerous important applications such as biomedical system, material processing, eye-safe laser radar, pump sources as mid-infrared laser and mid-infrared supercontinuum generation. Currently, the research emphasis of the glass hosts used to achieve the lasers has already transferred from inchoate silica glasses and fluoride glasses to heavy metal oxide glasses. Tellurite glasses belong to the family of heavy metal oxide glasses. They possess lower maximum phonon energy compared to silica glasses and germanate glasses. On the other hand, they have better chemical stability and mechanical property than that of fluoride glasses. In addition, they exhibit high solubility for rare-earth ions and large absorption and emission cross sections. However, Te O2-ZnO-Na2 O system usually used to obtain the laser at 2.0 μm region has the weakness of lower glass transition temperature and large coefficient of thermal expansion. Thus the motivation of the present work is to search for the tellurite glass system with high glass transition temperature, good thermal stability and spectroscopic property. Based on the system, the fabrication of tellurite glass fiber by the suction method was explored and laser experiments at 2.0 μm region were also carried out.This dissertation is composed of five chapters. In chapter 1, the applications and research progress of fiber lasers operating at 2.0 μm region have been reviewed. Active ions and mechanisms to generate fluorescence or laser at 2.0 μm region were introduced. Finally, the purpose and research content of the dissertation were represented.In chapter 2, the effect of WO3 contents on structure, thermal stability and spectroscopic properties of Tm3+ doped TWZL glasses was studied firstly. It was found that the glass transition temperature was 424 °C and the crystallization peak did not appear in the measurement range with the addition of 30 mol% WO3. In addition, the glass containing 30 mol% WO3 had high emission cross section(9.54×10-21 cm2) and gain coefficient(3.8 cm-1). And then the effect of Yb3+ concentration, Tm3+ concentration and bubbling time on spectroscopic properties of Yb3+/Tm3+ codoped TWZL glasses was systematically investigated. When Yb2O3 and Tm2O3 concentration were 2 and 0.5 mol%, respectively, 1.8 μm emission intensity was the highest. The strength of interaction between Tm3+ and OHwas calculated. It was found that the value was 6.7 times as large as that of interaction between Er3+ and OH-, indicating that OH- will seriously quench 1.8 μm fluorescence and the lifetime of Tm3+:3F4 level. After bubbling dry O2, OH- contents were drastically reduced while 1.8 μm fluorescence intensity was enhanced and the lifetime of Tm3+:3F4 level was prolonged. According to fluorescence decay curves, the energy transfer efficiency from Yb3+ to Tm3+ was calculated to be up to 97%. And the energy transfer coefficient from Yb3+ to Tm3+ was determined by the phonon sideband theory, extended spectral overlap theory and hopping model, respectively. It was found that these results were very coincident. The ratio of direct energy transfer coefficient to the backward one was 223, ensuring intense 1.8 μm emission.In chapter 3, the effect of Nd3+ concentration on spectroscopic properties of Nd3+/Ho3+ codoped TWZL glasses was investigated. When Nd2O3 concentration was 0.5 mol%, 2.0 μm emission intensity was the highest. Emission cross section and gain coefficient of Ho3+ at 2.0 μm were calculated. And the energy transfer efficiency and coefficient of Nd3+â†'Ho3+ were determined. Based on these results, Yb3+ was added and the effect of Yb3+ concentration on thermal stability and spectroscopic properties was discussed in detail. With the addition of a trace of Yb3+, the thermal stability was greatly enhanced and the glass transition temperature was increased to 458 °C. In addition, 2.0 μm emission intensity was greatly enhanced. The reason that energy bridge Yb3+ improved 2.0 μm emission was explained. These results demonstrate that Nd3+/Yb3+/Ho3+ triplely doping could be a new way to achieve laser at 2.0 μm region.In chapter 4, the fabrication of tellurite glass fiber preform by the suction method was explored. Finally, no bubble fiber preform with the length of 3 cm was successfully prepared when the casting temperatures of core and cladding glass and the annealing temperature were 780, 750 and 400 °C, respectively and the casting speed was suitable. And then 125 μm multimode TWZL glass fiber with good properties was stably drawn by the optimization of drawing temperature, dropping speed of the preform and rolling speed. In addition, the extrusion technic of the fiber preform and the fabrication of the hollow glass tube were also explored. The loss of TWZL glass fiber at 1310 nm was 4.44 dB/m by the cutback method.In chapter 5, a typical F-P cavity was set up by using Tm3+ doped TWZL glass fiber as the gain medium. Under the pump of 792 nm laser diode, intense fluorescence was obtained. Compared to bulk glass, the fluorescence peak in the fiber red-shifted from 1780 to 1944 nm. 1570 nm amplifier was set up and 10 mW signal was successfully amplified to 900 mW. And then laser at 2.0 μm region appeared after Tm3+ doped double cladding silica fiber was pumped by the amplifier. The effect of silica fiber length on 2.0 μm laser threshold and slope efficiency was investigated. 20 cm was a optimal length. In this case, the maximum output power and slope efficiency were 193 mW and 31.4%, respectively. Under the pump of 1570 nm amplifier, Tm3+ doped germanate fiber laser operating at 2.0 μm region was set up. The effect of the heating temperature of FBG on 2.0 μm laser wavelength was studied. When the temperature increased from 30 to 280 °C, the laser wavelength shifted from 1937.11 to 1940.83 nm. Q-switched Tm3+ doped germanate fiber laser at 2.0 μm region was obtained by depositing the graphene on the D-type fiber. The effect of pump power on the plus repetition rate and duration was studied. When the pump power was lower than 411 mW, the plus train was very stable. With increasing the pump power from 189 to 411 mW, the repetition rate increased from 36.1 to 60.2 kHz while the plus duration decreased from 3.61 to 1.92 μs. In addition, the signal-to-noise ratio(SNR) was about 46 dB at the pump power of 411 mW, indicating this laser was very stable.