Experimental Studies On Beam Quality-improving Of8μm ZGP OPO

The wavelength range of8-12μm is in the atmospheric window, so the lightsources in this wavelength range are widely used in the areas concerned with theatmospheric transmission including optoelectrionic countermeasure, environmentalmonitoring and LIDAR. As a direct indicator of transmission performance and theability of beam focussing, beam quality is a vital parameter for the light sources in thiswavelength range.8μm ZGP OPO is one of the most useful laser sources in thewavelength range of8-12μm, but high energy output from ZGP OPO is oftenaccompanied by poor beam quality. So this project is aimed at the beam quality-improving of8μm ZGP OPO.Firstly, to meet the need of8μm ZGP OPO for the pump source, the2.09μmHo:YAG laser with high energy and high repetition rate output was applied. Throughthermal field theory and rate equation theory, the thermal properties and outputproperites of the Ho:YAG laser was calculated and analyzed, and the preliminaryscheme and further optimum proposal was also base on this. Then the experimentalstudy was conducted. Through the pumping structure of polarization isolation fromdouble ends of Ho:YAG crystal, the problem of power scaling was solved. Throughdesigning laser radius in the resonator and changing the structure of the resonator tocompensate the thermal lens effect, the damage with the high energy output wasavoided. At last, the output energy of31mJ at the pulse repetition rate of1kHz wasachieved with the pulse width of28ns and the beam quality of M2~1.14, meeting therequirement of ZGP OPO for the pump source.Secondly, with the designed2.09μm Ho:YAG laser above as the pump source, thetheoretical and experimental research on the output properties and beam quality of8μmZGP OPO was done in depth. Theoretically, with the equations of momentum andenergy conservation and other conditions to achieve the operation of OPO, throughcalculation, the signal wavelength of2.829μm and the phase matching angles of51.39°and65.12°for type I and type II phase matching respectively were got under the pumpwavelength of2.09μm. The pumping threshold, conversion efficiency, acceptant linewidth and acceptant angle were also calculated. The acceptant angles for type I and typeII phase matching were3.93mrad and6.76mrad respectively. Experimentally, thereexisted a problem. As the output power from the ZGP OPO is not stable, it was hard tomeasure the beam quality with the method of moving blade. To overcome this problem,we put forward the “Power Contrast Method”, and achieved the accurate and reliablemeasurement of the beam quality of the output beam with fluctuant power. Thenthrough a series of comparative experiments, the effects of resonator length, type ofphase matching, resonant wavelength and structure of the resonator on the beam quality of ZGP OPO were studied respectively. Through analysis, the effects were as follows.The increase of resonator length contributes to the improvement of the beam quality,and the effect of resonator length on the beam quality in type II phase matching is lessthan that in type I phase matching. At the condition of long resonator length, such as50mm, the beam quality in type I phase matching is better than that in type II phasematching. The OPO with the longer resonant wavelength has better beam quality thanthe one with shorter resonant wavelength. Compared with linear cavity, the ring cavityhas the output with much better beam quality. In the structure of ring cavity with theresonator length of120mm, the type I phase matching ZGP OPO with longer resonantwavelength achieved the output with the beam quality of M2~1.20and1.22in thehorizontal and vertical direction respectively, which was rather good.In conclusion, after the demenstration for the operation of high-energy Ho:YAGlaser theoretically and experimentally, the simple analysis was done on the outputperformance for the ZGP OPO. Then the output beam quality fo the ZGP OPO wasstudied and improved experimentally, and the beam quality of M2~1.20and1.22in thehorizontal and vertical direction was achieved respectively, having met the requirementof the actual use.