| Laser operating in the2～3μm wavelength region has attracted considerable interest due to its potential applications in eye-safe laser radar, remote chemical sensing, topographic surveying, atmospheric monitoring and laser medicine surgery. Since1960s, considerable research efforts have been made to develop various solid state lasers activated by Tm3+and Ho3+. The host involves various laser crystals and glasses. Benefited from the great success in laser diodes and optical fibers, fiber laser has recently become the main direction of solid state lasers due to its outstanding merits such as excellent beam quality, high energy conversion efficiency, lower thermal effects, low cost, compactness and easy to integrate. At present, most of2.0μm fiber laser hosts are limited to silica and fluoride glass. However, there still exist some issues in the practical application. On one hand, higher phonon energy (1100cm-1) of silica glass results in low energy conversion efficiency, and suppresses laser output beyond2.0μm region. On the other hand, poor chemical stability of fluoride glass has limited its application. Besides, the manufacture process is complex and expensive. Therefore, the choice of suitable host is the key factor for developing economic and efficient2～3μm fiber laser. Nowadays, transparent oxyfluoride GC is considered as a novel excellent optical material due to the combined properties of oxide glass and precipitated fluoride nanocrystals. In this case, it exhibits both high luminescent efficiency and environmental suitability. In the present work, a series of crystallization processes and compositions were studied to fabricate transparent GC. The particular attention was given to the2.0μm emission of Ho3+in GC This work would lay a basis for the development of economic and efficient2.0μm fiber laser.This dissertation is composed of five chapters. In chapter1, the research progress in2～3μm fiber lasers and GC was reviewed. Additionally, we briefly introduced the structure and operating principles of RE ions doped fiber laser as well as the spectroscopic properties and transition characteristics of RE ions.In chapter2, the experiment methods and theory background are introduced, including sample preparation procedures, physical and spectroscopic properties measurements, and the spectroscopic parameters calculation using J-O theory and McCumber theory.To circumvent the drawback of Ho3+, i. e., lack of pumping bands matched with commercial laser diode, we developed Tm3+/Ho3+and Yb3+/Ho3+-codoped systems and systematically investigated their luminescent properties upon excitation of808nm and980nm LD. Intense2.0μm fluorescence originating from Ho3+:5I7â†'5I8transition was achieved through ET processes from Tm3+and/or Yb3+ions to Ho3+ions. Emphasis is given to (1) The first investigation is conducted on the crystallization properties of Tm3+/Ho3+-codoped60SiO2-20ZnO-20BaF2and50SiO2-26.4Al2O3-10.6Na2O-13LaF3glass. Based on DSC results, transparent GCs containing BaF2and LaF3nanocrystals were developed by controlled crystallization. The local environment of RE ions in PG and GC was investigated via optical spectra of Eu3+, which acts as the fluorescence probe and the calculation of J-O parameters for Tm3+and Ho3+, respectively. It is found that crystallization gives rise to the remarkable decrease in Ω2, the blue shift and Stark splits of absorption bands. These results indicate the structural change of the sites of Tm3+and Ho3+resulted from the partition of RE ions into BaF2nanocrystals. We investigated the properties of2.0μm emission along with ET process in Tm3+/Ho3+-codoped glass and GCs under808nm excitation. After excitation of808nm LD, Tm3+/Ho3+-codoped GCs give rise to intense2.0μm emission band featured by obvious Stark splits. The integrated intensity of2.0μm emission increases by a factor of about6as compared with that of PG. We discussed the influence of heat-treatment time and Tm3+content on emission intensity. It is found that the prolongation of heat treatment facilitates2.0μm emission while suppresses the upconversion luminescence. This could be attributed to more efficient CR (3H4,3H6â†'3F4,3F4) process of Tm3+-Tm3+pairs and ET process from Tm3+(3F4) to Ho3+(5I7), benefited from the reduced distance between RE ions. The microscopic parameters for Tm3+â†'Ho3+ET and backward process were evaluated using the Dexter’s model. The ET constant for Tm3+â†'Ho3+process was determined to be26times as large as that of backward process, which indicates that Tm3+â†'Ho3+ET is dominant and quite efficient.(2) We investigated the spectroscopic properties and ET process in Yb3+/Ho3+-codoped glass and GC under980nm excitation. The spectroscopic parameters of Ho3+in PG and GC were evaluated using J-O theory. In the case of GC, the peak emission cross section for Ho3+5I7â†'5I8transition was determined to be6.43×10-21cm2. The large absorption cross section of Yb3+(1.2×10-20cm2) would provide better potential for designing compact and efficient laser systems pumped by commercially available high-power InGaAs LD. Efficient1.2and2.0μm emissions have been achieved in Yb3+/Ho3+-codoped GCs under980nm excitation, demonstrating the occurrence of ET from Yb3+to Ho3+. Analysis on the upconversion and ET mechanism reveals that the low-phonon-energy environment in GCs facilitates the upconversion and1.2μm emissions. However, the expected enhancement of2.0μm emission is hardly observed. The microscopic parameters of ET processes between Yb3+and Ho3+ions were evaluated using phonon sideband theory. It is found that the Yb3+â†'Ho3+forward ET is efficient and dominant, and mainly assisted by one-phonon emission with the contribution rate of89.314%.In chapter5, a series of RE-doped oxyfluoride germanate GCs were developed from50GeO2-22Al2O3-13LaF3-15LiF glass. It is demonstrated that Ho3+ions can generate two NIR photons at1013and1190nm via two-step sequential transitions after the absorption of one incident photon within the300-560nm region. The quantum efficiency is estimated to be about110%。The1.37μm and1.45μm luminescent properties of Ho3+-doped PG and GC were compared. Calculated excited states absorption and stimulated emission cross sections as well as estimated spectral gain coefficients have demonstrated the great potential of GC as a host for E-band optical amplification. |
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