| Mode-locked pulse has the characteristics of high peak power and short pulse. Ultrafast lasers emitting in2μm spectral region have wide application prospect and value in many fields, such as micromachining, laser sprctra technology, medicine, exciting high harmonic and nonlinear frequency conversion. We have obtained mode-locking pulses with active modulation in this dissertation and did some research on the generation of2μm actively mode-locked pulses on the theory and experiment.First, this paper analyzed the advantages and disadvantages of several common2μm lasers. In the dissertation, the process of AML was simulated in the case of phase-locked through the definition of mode-locked. In the experiment the device for mode-locked was acousto-optic modulator (AOM). So we simulated the output of continue-wave mode-locked (CML) from the equation of amplitude modulating to oscillating light in cavity. Then the parameters of AOM as geometric characteristics, center frequency, diffraction type and Bragg angle and so on were obtained. And the relationship between driving frequency, drive power, angle of incidence and diffraction efficiency was obtained by experiment. The results indicated that the AOM was Bragg type, the center frequency was41.38MHz, the diffraction efficiency of zero-order was27.4%and the angle of incidence was0.3°loss with the maximum loss.Secondly, a Tm: YLF solid laser with the center wavelength from1098.1~1098.5nm was established as the pump of Ho: YAG laser based on in-band pumped. The output characteristics of Tm: YLF laser have been measured such as power character, beam quality and spectral character. Then it designed a double Z-cavity with six mirrors according to the principle of telescope and analyzed the effect of several vital parameters in detail. The analysis indicated that the location of0°totally reflecting mirror and output mirror had little effect on the stability of cavity, while both changing tens of millimeters the cavity was stable. However, they had significant impact on the oscillating beam radius and the astigmatism generating by the spot size in the sagittal plane and meridian plane were unequal. Both had best locations when the astigmatism was minimal. Also the folding angle of cavity mirrors had obvious effect on astigmatism, and the angle was10°in the experiment.Then, in this paper the CML Ho: YAG laser established according to the designed parameters was be obtained for the first time. By tuning the length of cavity and the loss causing by AOM, Q-switching, QML and CML were achieved respectively. The RF spectrum was measured in QML and CML regimes respectively and the center frequencies both were82.76MHz. For CML operation, there was nothingness of edge frequencies near the peak and the ratio of intensity of output pulse to background light was58~60dB. With the maximal pump power of13.2W, for QML operation the output average power was1.04W, center wavelength was2097.25nm and M2was1.146.Lastly, it measured the width of QML pulse by taking a second harmonic autocorrelation measurement using KTP as the double frequency crystal. The phase-matching angle of KTP was52.124°by solving the refractive index equation at type-II phase matching. The pulse width of102ps was measured, achiving the method of background-free measure Moreover,.the theoretical analysis showed that the influence of GVM between o-light and e-light in the nonlinear crystal to the experimental result was little, widening1.6ps only. |
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