Research On 879 Nm Laser Diode Endpumped Passively Q-switched Mode Locked Nd:GdTaO₄/Cr⁴⁺:YAG Laser

As an important part of laser,the laser medium directly affects the output performance.It is of great significance to study the laser characteristics of the new laser medium.The new Nd:GdTaO₄ crystal has a large absorption bandwidth,suitable for laser diode（LD）pumping,and a wide fluorescence spectrum suitable for mode-locked laser operation.Q-switched mode-locked laser combines Q-switching with mode-locking technology to obtain pulsed lasers with high repetition rate,narrow pulse width,and high peak power.In view of the above research background,this paper studies the Q-switched mode-locked laser characteristics of Nd:GdTaO₄ crystal by using direct pumping.In this paper,the theories of passive Q-switched and Q-switched mode locked lasers are studied.Based on the fluctuation mechanism,the peak power of passive Q-switched mode locked Nd:GdTaO₄ lasers is discussed,the parameters that affect the width of Q switched envelope and the modulation depth of mode locking pulse are analyzed.The Vtype cavity and Z-type cavity for fundamental Q-switched mode locked laser and harmonic Q-switched mode-locked lasers are designed based on the principle of ABCD transfer matrix.The beam radius in the cavity is analyzed,the thermal focal length of Nd:GdTaO₄ crystal and astigmatism in the cavity are studied.By studying the energy level structure and absorption spectrum of Nd:GdTaO₄ crystal,the feasibility of using 879 nm LD to pump Nd:GdTaO₄ crystal is discussed.The loss coefficient and the focal length of Nd:GdTaO₄ crystal are measured.The influence of thermal effect and crystal loss on laser performance is analyzed.The continuous wave output performance of the 879 nm LD pumped Nd:GdTaO₄ laser is experimentally studied.The laser wavelength is ～1066.8 nm,the maximum output power is 5.6 W,and the optical efficiency is ～43.7%.The effect of Cr4+:YAG initial transmittance on the output performance of passive Q-switched pulsed laser is studied.Using the Cr4+:YAG crystal with initial transmittance of T0=85%,the average output power of the pulsed 1066 nm laser is 3.6 W,the pulse repetition frequency reaches 185.0 k Hz,the beam quality factor are Mx2=2.2,My2=1.5.In V-type cavity,the output performance of the 879 nm LD Pumped Nd:GdTaO₄ / Cr4+:YAG Q-switched mode-locked laser is experimentally studied.Using Cr4+:YAG crystal with T0 = 90%,the maximum average power of the Q-switched mode-locked 1066 nm laser is 4.8 W,the repetition frequency of mode-locked pulse is ～ 200 MHz,and the pulse width is ～ 1.9 ns.According to the basic principle of harmonic mode locking,the high-order harmonic mode locking is realized in the Z-type cavity by changing the structure of the cavity,which solves the contradiction between the high repetition frequency and the narrow pulse width in the fundamental mode locking.The repetition frequency of the Q-switched third-order harmonic mode locked 1066 nm laser is ～ 164.5 MHz,the maximum average output power is 3.2 W,the narrowest mode-locked pulse width is ～ 600 ps,the maximum Q-switched repetition frequency is 58.8 k Hz,and the narrowest Q-switched pulse width is 257.7 ns.The ninth order harmonic mode-locked 1066 nm laser pulse with a repetition rate of 655.3 MHz is realized,and the output performance of Q-switched harmonic mode-locked laser varies with the cavity length is discussed.