| Potassium titanyl phosphate (KTiOPO4, or KTP) has large second-order susceptibility, large angular (temperature) bandwidths, large thermal conductivity, high damage threshold, resistance to the deliquescence and stable chemical and mechanical characteristic, which make it extensively used as the nonlinear crystal for frequency doubling of medium-low power Nd3+ laser. However, laser-induced damage in KTP, termed gray tracking, is always observed in 1064nm second harmonic generation (SHG) and optical parametric oscillation (OPO) pumped at 532nm. This damage will dramatically increase the absorption of KTP in the visible and infrared band, resulting in sharp decline in frequency conversion efficiency. What is more serious is that the permanent damage will be produced when KTP is overheated. Optimizing the crystal growth conditions and adopting special fluxes as well as heat treatment techniques have been proven effective in improving the resistance of KTP to gray tracking. The gray-tracking resistance KTP (GTR-KTP) has been introduced accordingly. It has been demonstrated that GTR-KTP has improved absorption loss and damage threshold in comparison with the common KTP (CKTP).In this dissertation, by using the all-solid-state lasers, we have theoretically and experimentally studied the intracavity nonlinear frequency conversions based on the GTR-KTP, including high power SHG, OPO, stimulated Raman scattering (SRS) and mixed frequency conversion processes. Compared with CKTP, GTR-KTP has greatly decreased absorption at the visible and infrared wavelengths. This is favorable to improve the performance of nonlinear frequency conversions, such as average output power, conversion efficiency, temperature characteristics and output beam quality. Meanwhile, the corresponding rate equations have been established to simulate the experimental processes. In addition, the mixed frequency conversion processes incorporating with GTR-KTP have also been investigated. The main content of this dissertation includes:1. By introducing the nonlinear frequency conversion losses, the rate equations of intracavity OPO and Raman laser have been given, respectively. To evaluate the established theoretical models, the intracavity OPO (IOPO) and SRS experiments based on KTA crystals have been performed. It was found that the corresponding experimental results were in good agreement with theoretical results, which confirms the applicability of the theoretical model. (Chapter 2)2. The gray-tracking test has been carried out. The results indicated that GTR-KTP had good resistance to the gray tracking in comparison with CKTP. A comparative study of a frequency-doubling 532nm laser based on GTR-KTP and CKTP was also presented. Under the laser diode (LD) pump power of 180W and repetition frequency of 10 kHz, the maximum average output power at 532nm was 40.6W for GTR-KTP, which was increased by 50% compared with that obtained in CKTP. With the intracavity SHG configuration, GTR-KTP was proved to have larger temperature bandwidth than that of CKTP. Moreover, the intracavity SHG temperature tuning curve were found to be different from that obtained with extracavity SHG configuration for the two crystals. Considering the great difference between the two kinds of KTP in absorption at 1064 and 532nm, a qualitative evaluation method for the KTP's resistance to gray tracking was presented. In addition, a 20W all-solid-state GTR-KTP green laser with long-time stability was made. (Chapter 3)3. By theoretically calculating the spectral transmissions induced by the etalon effect, it has been found that the shared cavity OPO had much wider transmission bandwidth than that of coupled OPO. The corresponding experimental results indicated that the line-width for the shared OPO was apparently wider than that of the coupled OPO. Therefore, the mentioned theoretical analysis can offer an explanation for the improved power stability of shared cavity OPO. (Chapter 4)4. Due to the fact that GTR-KTP has a lower absorption coefficient at infrared wavelengths than that of CKTP, the IOPO performance incorporating the two crystals has been studied. The GTR-KTP IOPO excitated by a diode-end pumped acousto-optic (AO) Q-switched Nd:YAG laser was investigated. Under the incident LD power of 11.4W and repetition frequency of 15 kHz, the maximum signal average output power was 1.2W. This corresponded to the optical conversion efficiency of 10.5%, increased by 25% compared with that obtained in CKTP IOPO. In addition, an efficient GTR-KTP IOPO with the shared cavity configuration and excited by a diode-end pumped composite Nd:YAG/Cr4+:YAG laser was also demonstrated. Under the incident LD power of 8.4W, the maximum average output power of 900mW at 1572 nm was obtained. A theoretical model for this compact GTR-KTP IOPO was also presented. Theoretical analysis on the pulse characteristics of the signal was performed, which showed a good agreement with that obtained experimentally. (Chapter 4)5. The X(ZZ)X spontaneous Raman spectrum of GTR-KTP has been measured, with the Raman gain coefficients relative to KTA given accordingly. A GTR-KTP second Stokes Raman laser intracavity driven by a diode-pumped AO Q-switched Nd:YVO4 laser was demonstrated. With an incident pump power of 9.5W, the GTR-KTP intracavity Raman laser, operating at the repetition rate of 20 kHz, produced the maximum average output power of 860mW at 1129 nm, corresponding to the optical conversion and slope efficiency of 9.1% and 11.6%, respectively. When the GTR-KTP was substituted with CKTP, a lower average output power of 720mW was obtained under the same pump condition and cavity setup as the GTR-KTP Raman laser. A theoretical model for this GTR-KTP SRS laser was also presented. In addition, the GTR-KTP Raman laser intracavity excited by a diode-end pumped composite Nd:YAG/Cr4+:YAG laser was also demonstrated. Under the incident LD power of 8.1 W, the maximum average output power of 420mW at 1129nm was obtained, with the optical conversion and slope efficiency being 5.2% and 11.4%, respectively. The corresponding Stokes pulse width and repetition rate were respectively 2.2 ns and 5.9 kHz. (Chapter 5)6. The mixed frequency conversion processes based on GTR-KTP have been studied experimentally. The synchronized dual-wavelength emissions at 1534 and 1572nm was realized by the mixed OPO conversion in GTR-KTP and KTA crystals. Both the two crystals were inserted into the diode-pumped Nd:YAG/Cr4+:YAG fundamental resonator. At an incident LD pump power of 7 W, the maximum output powers of the two wavelengths were all 230mW, with the corresponding pulse width and repetition rate measured to be 3.9 ns and 5.5 kHz, respectively. When the AO and passively Q-switched Nd:YAG lasers were respectively used as the excitation source, the simultaneous SRS and OPO conversions could be realized in one GTR-KTP crystal. For the AO Q-switching, under the incident LD power of 10W and repetition frequency of 15 kHz, the maximum average output powers of 1129 and 1572nm were150 and 180mW, respectively. The corresponding pulse widths were 22 and 3ns, respectively. For the passively Q-switching, under an incident diode laser power of 8.6 W, the maximum average output powers at 1096 nm and 1572 nm were 1.1 W and 0.36 W, respectively. The corresponding minimum pulse widths at 1096 nm and 1572 nm were 2.8 and 1.1 ns, respectively. (Chapter 6)The main innovations of this dissertation are as follows:1. A comprehensive study of high power SHG conversion based on GTR-KTP and CKTP was presented. It was found that GTR-KTP had advantage over CKTP in the output power, temperature characteristic and output beam quality.2. A qualitative evaluation method for the KTP's resistance to gray tracking is presented.3. The theoretical analysis on the improved power stability of shared cavity OPO configuration was first demonstrated.4. With the shared cavity OPO configuration, an efficient eye-safe GTR-KTP IOPO excited by a diode-end pumped composite Nd:YAG/Cr4+:YAG laser was demonstrated. Under the incident LD power of 8.4 W, the maximum average output power of 900mW at 1572 nm was obtained, corresponding to a diode-to-signal conversion efficiency of 10.7%. This was the highest conversion efficiency obtained with the shared cavity configuration.5. The X(ZZ)X spontaneous Raman spectrum of GTR-KTP has been measured, with the Raman gain coefficients relative to KTA given accordingly. The intracavity SRS conversion based on the GTR-KTP was studied. With an incident pump power of 9.5W, the intracavity GTR-KTP Raman laser produced the maximum average output power of 860mW at 1129 nm, corresponding to the optical conversion and slope efficiency of 9.1% and 11.6%, respectively.6. Some useful exploring in the mixed frequency conversions has been made. The synchronized dual-wavelength emissions at 1534 and 1572nm was realized by the mixed OPO conversion in GTR-KTP and KTA crystals. In addition, the simultaneous SRS and OPO conversions have been successfully realized in one GTR-KTP crystal. At an incident diode laser power of 8.6W, the maximum average output powers at 1096nm and 1572nm were 1.1 W and 0.36 W, respectively. |
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