Study On 3.8 μm Narrow-linewidth Mid-infrared Optical Parametric Oscillator Based On Composite Intracavity Pumping

The mid-infrared band of 3-5μm is widely used in atmospheric environment monitoring and laser radar because it contains the absorption peak of a large number of gases and water molecules,and which has strong penetration ability to particles and fog in the air.At present,there are many methods to realize mid-infrared laser.Optical parametric oscillator(OPO)has been studied and developed continuously with the progress of nonlinear optical material technology due to the advantages of easy tuning for output wavelength and high conversion efficiency.However,the mid-infrared laser spectrum obtained by the optical parametric oscillation technology is wide due to the influence of factors such as the linewidth for fundamental wavelength and the crystal gain spectral width,which is difficult to meet the market demand for the accuracy of the mid-infrared laser.In order to solve the problem that 3.8μm laser output spectrum is wide and lack of direct narrow linewidth devices,the theoretical and experimental studies are carried out respectively.Finally,3.8μm narrow linewidth mid-infrared laser output based on compound cavity pump is realized.On the theoretical aspects:(1)The crystal period and temperature tuning characteristics for the Mg O:PPLN are simulated,and the working parameters of crystal with 3.8μm laser output are determined,which lays the foundation for obtaining a 3.8μm narrow linewidth laser.(2)The factors and relations affecting the spectral linewidth are analyzed based on the optical dispersion theory to provide methodological guidance for the narrow linewidth experimental operation.(3)The factors that affect the model selection of the F-P etalon are analyzed based on the model selection principle and transmittance curve for the F-P etalon.(4)The structure parameters of 3.8μm mid-infrared optical parametric oscillator pumped by composite intracavity are determined based on the cavity structure design theory and ABCD matrix model,and in the narrow linewidth experiment,the structure with F-P standard is determined.On the experimental aspects:At the first,an experimental platform of 3.8μm mid-infrared optical parametric oscillator based on composite intracavity pumping is built,and the three-wave spectral linewidth of free oscillation is measured.On this basis,the experiment of fundamental wavelength with single F-P etalon,the experiment of fundamental wavelength and signal wavelength with F-P etalon at the time are carried out.The linewidth at 3.8μm in free oscillation,the linewidth compression at fundamental wavelength,the linewidth compression at fundamental wavelength and signal wavelength are 3.962 nm,2.236 nm and 0.735 nm,respectively.In order to suppress the adjacent mode,a narrow linewidth mid-infrared laser output of 0.729μm without adjacent mode was obtained indirectly in the experiment of double F-P elaton act on the signal wavelength,the output power of 1.06 W at 3.8μm was obtained.The conversion efficiency of pump to ider up to 2.41%,and the pulse width is 11.17 ns,the beam quality in x and y directions is M_x~2=1.92 and M_y~2=1.87,respectively.The experiment shows that the linewidth at idler wavelength depends not only on the linewidth at fundamental wavelength,but also on the linewidth at signal wavelength in the intracavity pumped optical parametric oscillator.Meanwhile,the double F-P elaton with different thickness can help to suppress the adjacent mode.