Objective A green-pumped Kerr lens mode-locked alexandrite laser is reported.A commercial 532-nm solid-state laser is used as the pump source for the alexandrite laser.Utilizing the pump power of 10 W,stable mode-locked operation at a repetition rate of 92 MHz,with an average power of 369 mW,is obtained.In this case,the pulse width is 86 fs,center wavelength is 749 nm,and full width at half maximum is 6.3 nm.Methods The alexandrite crystal with a length of 3 mm and doping atomic fraction of 0.22％was selected as the gain medium in the experiment.Given that the pump laser was linearly polarized,the crystal was cut at the Brewster angle and clamped onto a copper heat sink that could pass through the circulating water.To test the conversion efficiency of continuous waves,we measured the continuous-wave output power using coupling output mirrors(OCs)with different transmissivities.Then,we conducted an experimental study on Kerr lens mode locking using an OC with 2％transmissivity.The mode-locking operation was realized by adjusting concave mirror M2 to find a suitable mode-locked position and by appropriately changing the insertion of prism P2.Results and Discussions When we explore the characteristics of the continuous wave,we obtain the continuous-wave output power curves for different pump powers,as shown in Fig.2(a).Among them,the highest continuous-wave output power of 1.33 W is realized by an OC with 2％transmissivity,and the output power threshold is 0.37 W,corresponding to a slope efficiency of 14.3％.The highest continuous-wave output powers of 918 mW and 1.03 W are obtained using OCs with 1％and 3％transmissivities,respectively.Furthermore,the output power thresholds are 0.42 W and 2.38 W,corresponding to slope efficiencies of 9.7％and 14.4％,respectively.Additionally,we use an output coupling mirror with 2％transmissivity to examine alexandrite laser Kerr lens mode-locking.Utilizing the pump power of 10 W,the stable mode-locked operation at a repetition rate of 92 MHz,with an average power of 369 mW,is obtained.The mode-locked spectrum of the laser oscillator is measured using a spectrometer,and the results are shown in Fig.3(a).The center wavelength of the mode-locked spectrum is 748 nm,and the spectral full width at half maximum(FWHM)is 6.3 nm.The pulse width of the mode-locked pulse is 86 fs,which is measured using a commercial autocorrelator as shown in Fig.3(c).The Fourier-transform-limited pulse width is calculated based on the optical spectra obtained from the measurements and differs from the pulse width fitted by the measurements,which indicates that the mode-locked pulse output from the oscillator has a certain amount of chirp.The central wavelength of the mode-locked spectrum obtained by the experiment is 748 nm,and the incident polarization direction of the pump light is parallel to the b axis.The second-order dispersion of the alexandrite crystal at the center wavelength is 61.2 fs2/mm,and the third-order dispersion is 39.0 fs3/mm.The spacing of the prism pair at the dispersion compensation position in the resonant cavity in this mode-locked state is 620 mm,and the prism insertion is 6 mm,which provides round-trip second-order dispersion of approximately-1110 fs2 and third-order dispersion of-1719 fs3 in the cavity.The second-order dispersion introduced by air is approximately 20 fs2/m,and the third-order dispersion is 10 fs3/m.Hence,the net round-trip second-order dispersion and third-order dispersion in the cavity are approximately calculated to be-679 fs2 and-1454 fs3,respectively.A significant level of negative dispersion exists in the cavity,which makes it easier to realize soliton mode-locking.However,this results in a larger pulse width.Conclusions Currently,domestic research on the all-solid-state Kerr lens mode-locking oscillator of alexandrite is still in the preliminary exploration stage,and there remains a gap in the advanced results reported globally.Moreover,the large fluorescence lifetime of the alexandrite crystal can easily generate self-tuning Q pulses,making it difficult to establish a Kerr lens mode-locking pulse,and the mode-locking threshold is high.By leveraging international research methods and theories,we successfully realize a Kerr lens mode-locked operation with green light pumping.This operation exhibits an average output power of 369 mW,pulse width of 86 fs,repetition frequency of 92 MHz,center wavelength of 749 nm,and full width at half maximum of 6.3 nm.The alexandrite crystal emission spectral width supports a pulse output with pulse width of less than 20 fs.
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