Study Of Tm:YAP/Nd:GdYNBO₄ Passive Q-switched Solid-state Laser Based On Saturable Absorber

Due to the advantages of small size,compact structure,simple operation and good output beam quality of laser diode（LD）end-pumped passive Q-switched（PQS）solid-state lasers,they have played an important role in industrial processing,military weapons and medical fields in recent years.Therefore,PQS solid-state lasers have become a hot research topic in recent years.Tm³⁺-doped crystals exhibit excellent physicochemical properties and are commonly used for 2μm lasers.Nd:Gd YNBO₄crystal is more suitable for generating 1066nm laser compared with other commonly used Nd³⁺-doped crystals.In this paper,the PQS solid-state laser based on these two crystals at 1.06μm and 2μm wavelength was studied respectively.The specific research was carried out from three aspects:theory,simulation and experiment.In theory,actively Q-switched and passively Q-switched were introduced.The physical properties and energy level structure of Tm:YAP crystal were analyzed.The quasi-three-level rate equation model of Tm:YAP was established.The PQS rate equation under the plane wave approximation and the Gaussian approximation was studied,and the variation of particle number with time in cavity was simulated.The stability conditions of the resonant cavity were investigated by the ABCD matrix theory.Theoretically discussed the side,end,direct and indirect pumps distinction.In simulation,aiming at the thermal effect produced by Tm:YAP crystal,a thermal analysis physical model with LD end-pumped and side constant temperature cooling was established,and the temperature distribution equation inside the crystal was derived.The end face deformation,temperature gradient,and thermal stress birefringence caused by thermal effects were analyzed,and then carefully studied the relationship between the thermal focal length of the crystal and pump power.Based on the boundary conditions and initial conditions of the crystal,the effects of pump spot radius and pump power on the crystal temperature were simulated.Some methods were proposed to reduce the thermal effect of the crystal.In the experiment,we investigated the output performance of Tm:YAP crystal at 2μm and Nd:Gd YNBO₄crystal at 1.06μm,using Bi₂O₂Se prepared by hydrothermal method and In₂Se₃prepared by Molecular Beam Epitaxy（MBE）as saturable absorbers（SA）.The two materials were characterized by Raman spectroscopy,atomic force microscopy and P-scan to obtain information on their structural properties,layer numbers,surface morphology and saturation absorption characteristics.A Tm:YAP laser with a resonant cavity length of 31 mm was built.As for PQS operation,the maximum average output power was 1.62 W.A 1.24μs pulse duration and a pulse repetation rate of 112.1 k Hz with a 7.3%optical-optical efficiency were obtained at 1988.29 nm,corresponding to a single pulse energy of 14.46μJ and a peak power of 11.66 W.And the beam quality factor was M_x²=1.24,M_y²=1.36.In the Nd:Gd YNBO₄solid-state laser at 1.06μm,the maximum average output power under PQS mode was 253 m W with a optical-optical efficiency of 3.7%.The pulse width and repetition rate were 0.43μs and 102 k Hz,respectively,corresponding to a peak power of 5.76 W and a single-pulse energy of 2.48μJ.The beam quality factors were M_x²=1.29 and M_y²=1.40.These results indicate that In₂Se₃and Bi₂O₂Se have broad application prospects in solid-state lasers at 1.06μm and 2μm wavelengths.