Study On Low-duty-cycle Pulse Pumped Laser Base On Nd Doped MgO:LiNbO₃

At present,the quasi-phase-matched optical parametric oscillator（QPM-OPO）based on superlattice materials can cover the near and mid-infrared bands of 1.5-5μm,and it is widely used in the fields of atmospheric monitoring,radar detection and space ranging.However,the system contains laser gain medium and nonlinear crystal at the same time,which has the problems of large size and poor portability.Compared with the traditional superlattice material Mg O:PPLN,the periodically poled Nd:Mg O:Li Nb O₃can realize the simultaneous gain process and frequency conversion in a single crystal.Therefore,the Nd:Mg O:Li Nb O₃has become a new research hotspot for miniaturized lasers.However,under high-power continuous pumping,serious thermal effects are exposed,which leads to a series of problems such as incomplete suppression of non-matched polarized fundamental frequency light,low effective conversion efficiency,and energy inverse conversion.So the thermal effect of Nd:Mg O:Li Nb O₃is worthy of further investigation.Generally,pulse pumping technology can effectively reduce the thermal effect of laser crystals.This technology is widely used in alleviating the thermal effects of Nd:YAG,Nd:YVO₄and other laser crystals.But so far,there are few reports on pulsed end-pumped Nd³⁺ions doped Li Nb O₃crystals.Therefore,this paper uses low-duty-cycle pulse pumping technology to alleviate the thermal effect generated by the crystal gain process under high power pumping.By selecting a suitable combination of pump pulse parameters,the problems of simultaneous start-up of 1084 nm and 1093 nm dual-wavelength fundamental frequency light,uncontrollable output range,and low conversion efficiency under high-power pumping are solved.In terms of theory,firstly,the basic characteristics of Nd:Mg O:Li Nb O₃crystals are deeply studied,including the physicochemical properties,laser characteristics and energy level transition theory of Nd:Mg O:Li Nb O₃crystals.The relationship between the stimulated emission cross section of two wavelengths of 1084 nm and 1093 nm and the internal temperature rise of the crystal is analyzed.Secondly,a detailed numerical simulation of the thermal effect of Nd:Mg O:Li Nb O₃crystal is carried out,the change of crystal temperature rise under different repetition frequency and different duty-cycle parameters is obtained,and the thermal spherical aberration of the crystal under different pumping methods is simulated.Finally,the thermal focal length of the pulse-pumped crystal is numerically simulated,and a reasonable resonant cavity structure is designed based on this,so that theπ-polarized 1084 nm fundamental frequency light can be output more efficiently and stably.In terms of experiments,the output characteristic experiments of continuous pumping,pulse pumping and pulse pumping passive Q-switched fundamental frequency light are carried out respectively.Theπ-polarized 1084 nm single-wavelength laser with the highest average output power of 1.27 W is obtained under the pulse pumping repetition frequency of 10 Hz and the duty cycle of 20%,corresponding to a single pulse energy of 127.2 m J;Theπ-polarized 1084 nm single-wavelength laser with the highest average output power of1.40 W is obtained under the pulse pumping repetition frequency of 5 Hz and the duty cycle of 20%,corresponding to a single pulse energy of 279.2 m J.Theπ-polarized 1084 nm fundamental frequency optical pulse train output with a repetition frequency of 12.22 k Hz,a pulse width of 140 ns and an average output power of 471 m W is obtained under the condition of passive Q-switching by pulse pumping.Experiments have verified that low-duty-cycle pulse pumping technology can alleviate the thermal effect of Nd:Mg O:Li Nb O₃crystals,the thermal management of the crystal is achieved.By selecting the optimal combination of pump pulse parameters,theσ-polarized 1093 nm fundamental frequency light output can be suppressed.A more efficient single-wavelengthπ-polarized1084nm fundamental frequency light output is obtained.