Theoretical And Experimental Studies On Passively Mode-locked Tm,Ho:YVO₄Lasers

2μm ultrafast laser has significant application in laser spectra technology and nonlinear frequency conversion. In this dissertation, theoretical and experimental research were made on2μm ultrafast laser, including the generation of mode-locking laser, measurement of pulse width and cavity dispersion compensation.Firstly, in this dissertation, continuous-wave mode-locking threshold criteria was obtained from the rate equation of passively mode-locked laser. According to the criteria, Tm,Ho:YVO4, which has a large stimulated emission cross section was employed as the laser media. Then, according to the principle of Newton telescope, a telescope-shaped cavity was designed and some primary parameters were analysed. The analysis showed that the cavity was hardly affected when the thermal focal length of the crystal was larger than200mm and the collimating distance was smaller than500mm. The location of the output coupler and the absorber, whose maximum permissive deviation were20mm and15mm respectively, had significant influence on the cavity stability and the beam distribution. Moreover, the deviation made the beam radius in the meridian plane and sagittal plane unequal, wich brought astigmatism in the cavity. In order to minimize the astigmatism, the folding angle of each mirror should be less than5°. With the designed cavity, continuous-wave mode-locked Tm,Ho:YVO4laser was achieved for the first time in this dissertation. At the maximum absorbed power of5.6W,146mW mode-locking laser was obtained. With the pump power increasing, Q-switching, Q-switched mode-locking and continuous-wave mode-locking pulses were achieved successively. The continuous-wave mode-locking threshold was106mW. The radio frequency spectra showed a clean peak at64.2657MHz without any side peaks, which proved the continuous-wave mode-locking output. The central wavelength was2041.5nm and the beam quality factor was3.8.Secondly, in order to measure the pulse width, intensity autocorrelation measurement was taken, employing MgO:PPLN as the second-harmonic crystal. Pulse width of14.3ps was measured in this scheme. In order to measure the laser pulse without chirp, background-free autocorrelation device was set up employing type-Ⅱ KTP as the nonlinear crystal. Theoretical calculation proved that GVM has almost no influence on the measuring result (less than2%). The experimental measuring result was14.6ps, accordant with that of intensity autocorrelation measurement. At last, cavity dispersion was calculated and compensated with prism pairs so as to obtain transform-limited pulses.14.6ps was measured with background-free autocorrelation after the dispersion compensation, which was the same as the direct output laser. In order to explain this phenomenon, pulse width and chirp coefficient at different cavity dispersion were calculated theoretically, whose results showed that the original cavity dispersion was so small that it could only broaden the pulse width to14.6ps.