Study On Nd³⁺ And Mg²⁺ Co-doped LiNbO₃ Crystal And Its Application In Self-optical Parametric Oscillator

The 3～5μm mid-infrared wavelength range serves as a primary transmission band within the atmospheric transparency window,containing numerous absorption peaks for gases and water molecules.This range holds significant potential and application value in fields of gas monitoring,medical diagnostics,and military operations.At present,Optical Parametric Oscillators（OPOs）utilizing non-linear frequency conversion technology have become a popular method for generating mid-infrared lasers,thanks to their easily tunable wavelengths and high conversion efficiency.Based on quasi-phase-matching（QPM）theory,periodically poled magnesium-doped lithium niobate（Mg:PPLN）was widely applied in mid-infrared lasers owning to the maximum non-linear coefficient d₃₃ of lithium niobate,a higher optical damage threshold,and an absence of walk-off effects.Generally,there existed gain crystal and frequency-conversion crystal in the OPO resonator,which was not favor for the laser miniaturization.However,the miniaturization of mid-infrared lasers will have revolutionary significance to expand its application field,especially in the field of military photoelectric countermeasures.Based on the research of idea of complex multifunctional crystals,periodically poled neodymium,magnesium co-doped lithium niobate（Nd:Mg:PPLN）combines the luminescent properties of rare earth ions and the nonlinear optical effects of Mg:PPLN.This combination allows for fundamental light resonance and optical parametric oscillation within a single crystal,which was named for self-Optical Parametric Oscillators（SOPO）.Obviously,the number of crystals is reduced for the SOPO,which is beneficial to realize the miniaturization of laser.Therefore,the following work was carried out on the growth of Nd:Mg:LN and the performance of mid-infrared laser in this thesis:1.Growth and characterization of Nd:Mg:LN crystal.The 3-inch Nd:Mg:LN crystal with the concentration of 0.5 mol%neodymium and 5 mol%magnesium was grown by the Czochralski method,and the polarization process was improved to obtained homogeneous single-domain crystal.X-ray fluorescence elemental analysis was used to test the concentrations of Nd and Mg ions in Nd:Mg:LN crystals,and the segregation behavior of two ions were analyzed accordingly.The basic optical properties of Nd:Mg:LN crystals were studied including absorption and emission spectra.2.Period polarization of Nd:Mg:LN.Based on the refractive index equation of the crystal and QPM theory,the relationship between the polarization period and output wavelength was obtained by MATLAB software.The high-quality Nd:Mg:PPLN with the domain period of 29.8μm was successfully obtained by the electric field periodic poling method.3.Self-pumped optical parametric oscillator.The compact Q-switched SOPO resonator was built,and the output of 3.86μm mid-infrared laser with a maximum output power of 1.2W was obtained by the Nd:Mg:PPLN crystal.The tuning ability of mid-infrared laser was studied,which can be tuned within the range of 3.862μm to 3.831μm.An ideal heat-dissipation model of double-ended bonding,Mg:LN/Nd:Mg:PPLN/Mg:LN was constructed to resolve the thermal effect of the Nd:Mg:PPLN to increase the conversion efficiency.The bonding length of the end face was determined by COMSOL simulation,and a double-ended bonded Mg:LN/Nd:Mg:PPLN/Mg:LN crystal was obtained for the first time.