The Researches On Characteristics Of Crystal And Performance Of Diode-pumped Nd:LuVO₄ Lasers

With the development of laser technique, lasers are widely used in fields of military, industry, medical treatment, scientific research and our daily life. Diode pumped solid state lasers (DPSSL) have become the central focus of the field of lasers due to their many advantages such as compactness, high efficiency, good quality of light beam, high stability and etc. Laser crystal is an important part of laser system and its physical and spectral properties make a significant role on characteristics of laser output. Now, Nd:LuVO₄ crystal attracts our attention among new crystals. Firstly, Nd:LuVO₄ crystal can be used in low threshold microchip laser system due to its large absorption cross-section, wider absorption band and medium life time. Secondly, Nd:LuVO₄ lasers can obtain high and efficient output in high pump laser system because it has higher emission cross-section and thermal conductivity than Nd:YVO₄ crystal. At last, the output is polarized light with which high efficiency can be obtained in frequency conversion. Nd:LuVO₄ crystal was firstly reported in 2002, so only a little research work on its CW and acousto-optically Q switched properties has been done. This dissertation presents much work on theoretical and experimental researches which contains structure, spectra, thermal and laser properties of Nd:LuVO₄ crystal. The main context can be outlined as follows:(1) The history and merit of laser diode pumped all solid-state lasers were introduced and physical and optical charateristics of several laser crystals were also discussed. The work of Nd:LuVO₄ crystal at present was summarized and the expections of Nd:LuVO₄ crystal was given at the same time.(2) Raman shift and intensity were studied from classical and quantum theory. And we calculate the symmetry species of Nd:LuVO₄ crystal by group theory. The Raman-active optical phonon modes of zero wave vector were:r = 5A_1g + 7B_1g +2B_2g +10E_g. In theory, no more than 34 Raman peaks can be measured. Raman spectra were measured with scattering geometry X(ZZ)X, X(YY)X, Z(XY)Z, and Y(XZ)Y, whichcorresponds to symmetry species modes of A_1g, A_1g+B_1g, B_2g and E_g, respectively. The Raman spectra were measured by micro-Raman spectrophotometer and Raman spectra peaks were assigned one by one. Raman spectra peaks were narrow except in v₂, which implies that VO₄ and Lu/NdO₈ groups were undistorted. And intensity of 903cm^-1 inmode A_1g with scattering geometry X(ZZ)X was the strongest, whichcan be used in self-Raman lasers along a-cut Nd:LuVO₄ crystal.(3) The thermal diffusion coefficient of Nd:LuVO₄ crystal was measured by the laser flash method and then thermal conductivity could becalculated according toλ=αρC_p. Thermal conductivity decreased whenthe temperature increased, and it was larger in [001] dierection than that of [100] direction. The thermal conducitivty is 7.9Wm^-1K^-1 at 330K for a-axis while 9.7Wm^-1K^-1 for c-axis. The specific heat is 0.45Jg^-1K^-1, which is larger than that of Nd:LuVO₄ crystal and lower thermal gradient will be induced in laser crystal. So, Nd:LuVO₄ crystal have well thermal properties and is an prosperous candidate for high pump power laser system.(4) We measured the polarized absorption spectra, luminescence spectra and transmission spectra of Nd:LuVO₄ crystal. By using JO theory and Origin software, the optical parameters such as absorption cross-section,life time of metastable state ⁴F_3/2, branching ratio, oscillator strength and integrated emission cross-section were calculated. The life time of ⁴F_3/2 is 129μs. It can be seen from the calculated results that the valuesof absorption cross-section ofÏ€polarization are higher than that of a polarization. Nd:LuVO₄ crystal can provide some practical applications at 1.06μm due to the larger fluorescence branching ratio of 50.22%, oscillator strength, and integrated emission cross-section when pumped at 0.8μm. And 0.9μm laser emission is also a prosperous spectrum for its large fluorescence branching ratio and Stark split.(5) From four energy level rate equation, we obtain input and output relationship of solid-state lasers under stable conditions. The threshold and slope efficiency were also discussed for end pumped laser systems. We demonstrated a-cut and c-cut Nd:LuVO₄ laser systems with radiation at 1.06μm and 1.34μm. The transmission T of output couplers impacts the output power. At the incident pump power of 19W, output power of 7.67W was obtained, corresponding to the optical conversion efficiency of 40.3% and the slope efficiency of 50.5% with transmission T=10% at 1.06μm while for 1.34μm, the output of 2.015W was obtained for T=4% at the incident pump power of 16.18W, given optical conversion efficiency of 12.5% and slope efficiency of 14.2%. The excited absorption played an important role on radiation of 1.34um and a-cut Nd:LuVO₄ laser had lower conversion efficiency than that of c-cut Nd:LuVO₄ laser.(6) The excited absorption of saturable absorber was taken into account in passively Q-switched rate equations. By numerically solving these rate equations, a group of curves were generated. These curves clearly show the dependence of the Q-switched characteristics such as pulse width, repetition rate, peak power and pulse energy on the T₀ of saturable absorber and T of output coupler. We demonstrated an LD pumped Nd:LuVO₄ passively Q-switched laser for the first time. The higest opt-opt conversion efficiency and solpe efficiency were 10.7% and 17.6% (T₀=85%,T=40%), respectively. The narrowest pulse width was 12ns (T₀=70%,T=40%), the higest pulse repetition was 156kHz(T₀=85%,T=10%)> the maximum pulse energy and peak power were 33.9μJ and 2.83kW (T₀=70%,T=40%) , respectively. Comparison was also done between passively Q-switched characteristics of Nd:LuVO₄ and Nd:YVO₄ laser. Narrower pulse width and lower repetition rate can be obtained in Nd:LuVO₄ laser, therefore higher pulse energy and peak power can be gained.(7) A passively mode-locked Nd:LuVO₄ laser was performed for the first time. Using different output couplers and saturable absorber, we realized the stable operation of Nd:LuVO₄ mode-locked laser by using 'V-shaped' folded cavity when thermal lens effect was considered. The spot size on Nd:LuVO₄ crystal was 200μm while 34μm on the Cr⁴⁺:YAG crystal, giving the compression ratio of 6. The modulation depth of mode-locking pulse train reached 40%-50%, and also discussed the influence of small-signal transmission of saturable absorber on modulation depth.(8) The principle of OPO was studied by means of coupled wave equations. Meanwhile the researches on KTP crystal were done and when the KTP was cut with angleθ=90°andφ=0°, 1.57μm signal wave can be obtained at pump wave of 1.06um. We reported an intracavity optical parametric oscillator at 1.57μm based on non-critically nonlinear crystal KTP driven by an end-pumped acousto-optically Q-switched Nd:LuVO₄ laser. The narrowest pulse width of 1.57μm was 4ns, which was six times shorter than that of 1.06μm, indicating an efficient shorter mechanism took place in our IOPO systems. The pulse energy and peak power of 24μJ, 4.8kW were obtained respectively with pulse repetition frequency of 5kHz. The reason why the satellite pulse emerged was given and we also gave a method to reduce it. In addition, the Nd:LuVO₄/KTP pulsed green laser was performed for the first time.The maximum pulse energy and peak power were 66μJ and 2.6kW, respectively.