Research On1.3-μm Short Pulse Laser Characteristics Of LD-pumped Nd³⁺-doped Vanadate Crystals

Laser-diode-pumped solid-state laser has the advantages of high conversion efficiency, good reliability and noise characteristics, frequency stability, simple structure and miniaturization. So it has been widely applied in the fields of industrial cutting and welding, laser ranging, laser guidance, surgery and so on. The lasers at1.3μm have important applications in fiber-optic communications, laser medicine, laser micromachining, eye-safe regions. In this dissertation, by using the fiber-coupled laser-diode as the pump source, we have studied the continuous-wave (CW), passively Q-switched and mode-locked1.3-μm laser performances of Nd3+-doped vanadate crystals (Nd:GdVO4, Nd:GdYVO4,Nd:LuYVO4and Nd:GdLuYVO4). In the experiments, acoustic-optic (AO) is employed as the active Q-switch component, V:YAG, Co:LMA and SWCNT saturable absorbers are employed as the passive Q-switch components. We have studied the Q-switched characteristics of fundamental frequency and frequency doubling lasers with Nd:GdVO4/V:YAG, and theoretically simulated the experimental results. The CW laser operations for a class of mixed Nd:GdYVO4crystals have been realized, and the relevant parameters have been experimentally measured. Meanwhile the Q-switched characteristics of fundamental frequency and frequency doubling lasers with V:YAG for different Nd:GdYVO4crystals have been compared. LD-pumped Nd:LuYVO4and Nd:GdLuYVO4CW lasers have been studied, thermal effects have been measured, and the researches on passively Q-switched performances have also been done. By using Nd:LuYVO4, we have done the researches on the output characteristics of passively Q-switched mode-locked lasers with V:YAG, Co:LMA, SWCNT and actively Q-switched mode-locked laser with AO. At last, we have simultaneously employed V:YAG, Co:LMA, or SWCNT with AO in cavities of LD-pumped Nd:LuYVO4laser to study the performances of hybrid mode-locked laser operations.The specific research contents of this dissertation include:(Ⅰ) The research on LD-pumped Nd:GdVO41.3-μm laser has been done. We have used input mirrors with different curvature radii and output mirrors with different transmissions to compare the CW laser output characteristics. By inserting V:YAGs with different initial transmissions into the plano-concave cavity, the passively Q-switched operation is carried out. The variations of pulse width, repetition rate, single pulse energy and peak power versus pump power have been measured. The theoretical analysis on the variation of repetition rate has been done. The experimental results have shown that the thermal focal length can affect the beam radii and weaken the output characteristics. By considering the Gaussian distributions of pump beam, intracavity photon density and population-inversion density into the rate equations, we have simulated the experimental results.(Ⅱ) The Nd:GdVO4/KTP intracavity frequency doubling red laser has been demonstrated in a V-shape cavity. KTP with type-Ⅱ phase matching has been employed to realize the intracavity frequency doubling laser operation, and V:YAG has been employed to obtain pulse laser outputs. Along the pump power increasing, the output characteristics have been measured. The experiments have shown that, with Nd:GdVO4/KTP, the system can generate stable red laser pulses. The rate equations that simultaneously consider the Gaussian distributions of the pump beam, the intracavity photon and the population-inversion densities, have been solved to simulate the experimental results.(Ⅲ) By LD pumping a class of mixed Nd:GdxY1-VO4(x=0.19,0.3,0.63,0.83) crystals, we have presented the1.3-μm CW lasers in plano-concave cavities. The passive losses, stimulated emission cross-sections and thermal focal lengths as well as variations of beam radii have been experimentally measured. The CW output powers for Nd:GdxY1-xVO4crystals are poor due to the relatively small stimulated emission cross-sections. The passively Q-switched operations have been carried out with V:YAG saturable absorber, and the variations of output characteristics versus Gd-doped proportion have been demonstrated. As been shown, the Nd:GdxY1-xVO4mixed crystals can generate stable laser pulses with shorter pulse widths, larger energies and higher peak powers than Nd:GdVO4.(Ⅳ) The performances of passively Q-switched intracavity frequency-doubling Nd:GdxY1-xVO4red lasers in V-shape cavity with KTP and V:YAG have been demonstrated. We have measured the output characteristics, including the average output powers, pulse widths, single pulse energies and peak powers, and testified that Nd:GdxY1-xVO4are suitable laser crystals for red lasers. (Ⅴ) The output of1.3-μm laser with LD-pumped Nd:Lu0.15Yo.ssVO4crystal has been realized. We record the CW output powers with simple plano-concave cavity and different output transmissions, experimentally measure the thermal effects, calculate the variations of beam radii versus pump power during the laser operation and finally confirm that the thermal effect weakens the output characteristics. In addition, V:YAG is used to realize the passively Q-switched laser operation and the output performances with different V:YAG initial transmissions are compared. The research shows that Nd:Lu0.15Y0.85VO4crystal can be a good medium to generate1.3-μm pulse lasers.(Ⅵ) We have demonstrated the output of1.3-μm laser with Nd:GdLuYVO4crystal. Under different output transmissions, the CW laser output performance has been compared. The influences of the passive loss, stimulated emission cross-section and thermal effect on the laser characteristics have been experimentally demonstrated. The relatively small thermal focal length deduces serious thermal effect and weakens the laser output performance. The passively Q-switched operation with V:YAG has been carried out and the laser pulse property has been depicted and compared.(Ⅶ) It is viable to use Ⅴ-shape cavity and LD pumped source to realize the Q-switched mode-locked laser operation for Nd:Lu0.15Y0.85VO4crystal. At first, the CW laser output characteristics in V-shape cavity under different output transmissions have been displayed. Then V:YAG, Co:LMA and SWCNT saturable absorbers are located near the output mirror to realize the passively Q-switched mode-locked operations. We measure the output powers, Q-switched pulse widths, Q-switched pulse energies and mode-locked widths as well as the repetition rates. The modulation depths can be near90%, and the mode-locked widths are under picosecond order.(Ⅷ) In a Z-shape cavity, we study the actively Q-switched mode-locked performance of Nd:Lu0.15Y0.85VO4laser with AO modulator. Under different AO repetition rates, the variations of the output powers, Q-switched pulse widths, Q-switched pulse energies and mode-locked widths as well as the repetition rates versus pump power have been measured. With AO modulator, the actively Q-switched mode-locked operation of Nd:Lu0.15Y0.85VO4laser can generate short pulse with large energy.(Ⅸ) The V:YAG/AO actively and passively Q-switched mode-locked operation at1.3 um with Nd:Lu0.15Y0.85VO4crystal has been presented. In the Z-shape cavity, the AO modulator is located near the laser crystal, the V:YAG is located near the output mirror, and the characteristics have been recorded as the pump power increasing. In contrast to single passively Q-switched mode-locked, the hybrid Q-switched mode-locked operation owns shorter pulse width, larger pulse energy and better pulse symmetry. So the actively and passively Q-switched mode-locked operation can enhance the laser characteristics.(X) By employing Co:LMA and AO in a V-shape cavity, we obtain stable short laser pulse in doubly Q-switched mode-locked Nd:Lu0.15Y0.85VO4laser. The experimental results show that the laser performance has been distinctly enhanced-the pulse energy is9times, the pulse width is only20%and the symmetry has been improved-after the AO being located into the single passively Q-switched mode-locked cavity. This operation can effectively enhance the pulse energy and the beam quality.(Ⅺ) Using SWCNT and AO in a V-shape cavity is an effective way to improve the Q-switched mode-locked pulse performance. The research has shown that the insertion of the AO modulator has caused the increase of pulse energy and the decrease of pulse width. Moreover, the modulation depth can be90%and the amplitude fluctuation is largely reduced. It is demonstrated that by using SWCNT/AO hybrid Q-switched mode-locked operation, we can obtain laser pulse with better stability, shorter pulse width and larger energy.The main innovations of this dissertation are as follows:(I) We have firstly compared the output laser performances at1.3μm with a class of Nd:GdxY1-xVO4mixed crystals (x=0.19,0.3,0.63,0.83). The CW lasers have been realized at first. We experimentally measure the passive losses, stimulated emission cross-sections and thermal focal lengths as well as variations of beam radii. The passively Q-switched operations have been carried out with V:YAG saturable absorber, and the intracavity frequency-doubling passively Q-switched operations have been realized in V-shape cavity with KTP and V:YAG. The variations of the output characteristics, including average output powers, pulse widths, single pulse energies and peak powers, versus Gd-doped proportion have been demonstrated. As been shown, Nd:GdxY1-xVO4mixed crystals can generate stable pulses with shorter pulse widths, larger energies and higher peak powers than Nd:GdVO4.(II) The CW and passively Q-switched laser operations at1.3μm with Nd:Lu0.15Y0.85VO4crystal have been realized for the first time. We experimentally measure the thermal effect, analysis the influence of thermal effect on output characteristics, and calculate the variations of beam radii versus pump power during the laser operation. V:YAG is used to realize the passively Q-switched laser operation and the output performances with different V:YAG initial transmissions are also compared. The research shows that the Nd:Lu0.15Y0.85VO4crystal can be a good medium to generate1.3-μm lasers and obtain stable pulses.(Ⅲ) We have firstly realized the stable1.3-μm laser output with Nd:GdLuYVO4crystal. The CW laser performances with different output transmissions have been compared. We experimentally measure the influences of passive loss, stimulated emission cross-section and thermal effect on the laser characteristics. With V:YAG, the passively Q-switched operation has been carried out and the laser pulse properties have been depicted and compared.(Ⅳ) We have firstly studied the Q-switched mode-locked operation of Nd:Lu0.15Y0.85VO4crystal. Using V or Z shape cavity, and V:YAG, Co:LMA, SWCNT saturable absorbers or AO modulator, the outputs of the Q-switched mode-locked laser pulses have been realized. The output powers, Q-switched pulse widths, Q-switched pulse energies and mode-locked widths as well as the repetition rates have been measured. It is demonstrated that these types of Q-switched mode-locked operations can generate stable and short pulses with large energies.(Ⅴ) We have firstly realized the doubly Q-switched mode-locked operations of Nd:Lu0.15Y0.85VO4crystal by using active and passive Q-switch components. The V:YAG/AO, Co:LMA/AO and SWCNT/AO hybrid types are employed to obtain doubly Q-switched mode-locked operations. The output characteristics, including the Q-switched pulse widths and single pulse energies, have been recorded and compared with those obtained by single V:YAG, Co:LMA, or SWCNT passively Q-switched mode-locked operations. The hybrid Q-switched mode-locked operations can obtain shorter pulse widths, larger pulse energies and better pulse stabilities.