| Since the birth of lasers,ultrafast lasers have always been a hot topic in laser research.All solid-state lasers have the advantages of compact structure,high power,stable performance,good beam quality,and long life.With the development of laser technology,all solid-state passively mode-locked ultrafast lasers are widely used in military,medical diagnosis,laser communication,nonlinear frequency conversion,and industrial processing.In this paper,a semiconductor laser with a center wavelength of 808 nm was used as the pump source,Nd:GYSGG and Nd:YSAG crystals were used as gain media.An all-solid-state passively mode-locked ultrafast laser was constructed by using periodic polarized lithium niobate(PPLN)based on quasi-phase matching technology and a dichroic mirror partially reflected the second harmonic against the fundamental wave.In the experiment,the nonlinear mirror mode-locking was realized by building a "Z"-type laser resonator,and the performance of the mode-locked laser in terms of continuous wave,pulse,spectrum,and frequency spectrum was studied.The main contents are as follows:1.The development history and advantages of all-solid-state lasers were introduced.A brief introduction to the basic principle of the mode locking technology and several common mode locking technologies was givin: including active mode locking,passive mode locking and self mode locking.2.Take the second harmonic generation process as an example to introduce the concept of phase matching.The basic principle of nonlinear mirror mode locking and two phase-matching methods in the nonlinear mode locking process were introduced.Commonly used phase matching methods were birefringent phase matching(BPM)and quasi-phase matching(QPM).A theoretical analysis of nonlinear mirror mode locking was carried out.The performance of the nonlinear mirror was similar to that of an effective saturable absorber resulting in mode locking.The research progress of nonlinear mirror mode-locked lasers were briefly introduced.3.A 808 nm fiber-coupled diode laser was used as the pump source and the core is 400μm.The performance of the nonlinear mirror mode-locked Nd:GYSGG laser was studied by building a "Z"-shaped folded cavity.In the experiment,PPLN was used as a nonlinear crystal to achieve nonlinear mirror mode locking,and a 1061 nm ultrafast laser output was obtained.The average output power was 450 m W,the repetition frequency was 97.1MHz,and the spectral bandwidth of the mode-locked pulse was 1.3nm.Taking into account the PPLN The group velocity is mismatched.According to the Fourier transform limit,the pulse width was about 9.3ps.4.Using Nd:YSAG crystal as the gain medium and PPLN as the frequency doubling crystal we realized the nonlinear mirror mode locking.The emission wavelength of the Nd:YSAG laser was 1061 nm and 1063.5nm.We analyzed the mode-locking mechanism of the nonlinear mirror and found that the 1063.5nm wavelength does not contribute to the mode-locking,and only the1061 nm wavelength participates in the mode-locking process.The maximum average output power of the mode-locked laser was 710 m W,the slope efficiency was 14.6%,the repetition frequency was 101.7MHz,and the signal-to-noise ratio of the mode-locked pulse was 45 d B.According to the spectral bandwidth of the mode-locked pulse,the pulse width was calculated to be about 9ps. |
