| The 1.5-μm and 3.5-μm lasers are located in the atmospheric transmission windows,which are highly penetrating to air and smoke,and less affected by the absorption of gas molecules and scattering of suspended matter,therefore these wavelengths have important applications in coherent radar,environmental monitoring,etc.,especially in military fields such as optoelectronic countermeasures,which have wide application prospects.Optical Parametric Oscillator(OPO)based on KTiOAsO4(KTA)crystal pumped by 1064 nm laser is one of the main methods to obtain high energy 1.5 μm and 3.5 μm lasers and it has been attracting extensive attention in the field of laser due to the high efficiency and compact structure.With the development of laser technology,medium-wave and short-wave infrared lasers are developing with higher pulse repetition frequency(PRF),higher energy and better beam quality.Achieving high energy medium-wave and short-wave infrared lasers by KTA-OPO requires high performance 1064 nm pump source,high efficiency frequency conversion,and effective beam quality control techniques.Firstly,the 1064-nm laser with good output performance and stable structure is the basis for exploring high-energy OPO.The side-pumped Nd:YAG master oscillator power amplifier(MOPA)is an important way to obtain high-energy 1064-nm laser,but there is still the problem of beam quality degradation caused by thermal effect during the amplification process,so thermal management needs to be optimized in the system to obtain high energy and high beam quality 1064-nm laser.Then,the high energy 1064 nm Nd:YAG laser with a PRF of 100 Hz is used as the pumping source of KTA-OPO to achieve 1.5 μm and 3.5 μm dual-wavelength laser output.To achieve high-efficiency and high-energy dual-wavelength laser output,OPO adopts a flat-flat cavity and the numerical simulations are usesd to optimize the length of nonlinear crystal and the transmittance of the output mirror.However,the OPO based on plane-plane cavity suffers from poor beam quality due to excess intracavity modes,back-conversion,and thermal effects under high-energy pumping.The unstable cavity based OPO with Gaussian reflectivity mirror(GRM)has attracted extensive research because of its excellent performance in improving the beam quality of high energy OPO.Meanwhile,ring cavity and Optical Parametric Amplifier(OPA)are also effective methods to improve beam quality,which are important methods that deserves further study.Based on the Nd:YAG MOPA,this paper presents theoretical and experimental research on the high repetition rate and high energy KTA-OPO system.Firstly,a side-pumped Nd:YAG 1064 nm laser with high-energy and high beam quality was established as a pump source for OPO;the high-energy KTA-OPO experiments were implemented,the 100 Hz dual-wavelength laser prototype was completed,with the highst pulse energy that has been publicly reported.Thirdly,the beam quality optimization was investigated using unstable cavity and ring cavity.The unstable cavity based on GRM has excellent performance in high-energy OPO.In addition,high-energy OPA was experimentally studied,and the experimental results of OPA with different structures were discussed and analyzed.Finally,high-energy 1.5-μm and 3.5-μm lasers with a PRF of 300 Hz were obtained by the KTA-OPO.The details of this paper are as follows:1.Based on the technology of thermal effect management,a high-efficiency 1064 nm laser was obtained by a Nd:YAG MOPA structure,which includes an unstable cavity oscillator and two-stage amplifiers.The pump light distribution of the side pump module and the thermal effect of Nd:YAG crystal were simulated.Unstable cavity structure of the oscillator provided high beam quality fundamental mode laser output,the beam quality factor was Mx2=1.14 and My2=1.10 at the output energy of 30 mJ.Finally,the total output energy of 1064-nm laser was 580 mJ,the PRF was 100 Hz,and Mx2 and My2 was 4.6 and 3.7,respectively.2.Theoretical and experimental studies were conducted on the 100 Hz high energy dualwavelength KTA-OPO.Theoretically,based on the three-wave coupling theory,the output characteristics of the KTA-OPO were simulated,including output efficiency,spectral linewidth,and pulse width.Experimentally,a 20 mm long KTA was used in OPO,the output energy was 62 mJ at a pumping energy of 250 mJ,corresponding to a conversion efficiency of 24.8%.Then,two 20 mm-long KTA crystals were used in the OPO cavity,and a total energy output of 102 mJ was obtained,which improved the conversion efficiency to 40.8%.In order to further improve the OPO output energy,the optimal length of the KTA crystal and the optimal transmittance of the output mirror were obtained with simulation.A 33 mm long KTA crystal and a short cavity of 65 mm were used.The highest pulse energies at signal(1.53 μm)and idler(3.47μm)were about 178 mJ and 64 mJ,respectively.The pulse widths of signal and idler were 13.7 ns and 11.8 ns,the conversion efficiency was 41.7%,and the RMS of output energy was 1.2%.Finally,a stable integrated prototype was obtained.3.Beam quality improvement of OPO has been investigated by adopting an unstable cavity.Firstly,the theoretical study of the unstable cavity OPO was conducted,and the unstable cavity based on the non-confocal and confocal were introduced,respectively.Then,a nonconfocal unstable cavity based on GRM was used instead of a flat-flat one.The maximum pulse energies at signal(1.53 μm)and idler(3.47 μm)were about 75 mJ and 26 mJ,respectively.The corresponding beam quality factors were Mx2=9.8 and My2=9.9 for signal,and Mx2=11.2 and My2=11.5 for the idler.Compared with a plane-parallel cavity,over 8 times and 3 times brightness improvements were realized for the signal and idler,respectively.Finally,a confocal unstable cavity based on GRM was used to further improve the beam quality,and the output characteristics of unstable cavities with different magnifications were investigated.When the magnifications factor(M)was 1.5,The pulse energies at signal(1.53 μm)and idler(3.47 μm)were about 56 mJ and 20 mJ,respectively..The M2 were measured to be Mx2=5.7,My2=5.9 for signal and Mx2=8.4,Mt2=8.1 for idler accordingly.4.The output characteristics of the OPO with ring cavity were investigated.The output energy and beam quality of the ring cavity OPO with different pumping structures were investigated.Firstly,ring cavity based on two-pass pumping had higher output energy but the beam quality was not significantly improved.Then,the output characters of single-pass pumped ring cavitity OPO with different numbes of KTA crystals were investigated.The best beam quality was obtained with one KTA crystal,but the output energy was only 55 mJ,and the M2 factor of the signal was Mx2=4.4 and My2=4.5.Finally,sigle-pass pumping ring cavity based on two KTA crystals obtained 70 mJ output energy with a signal M2 factor of Mx2=6.2 and My2=6.4.5.The OPA based on the confocal unstable cavity was investigated.A confocal unstable cavity with an M of 1.5 was used to obtain an energy output of 15 mJ,with the M2 of 3.7 and 3.5 for the horizontal and vertical,respectively.The 1.5 μm signal with an output energy of 87 mJ was obtained after one-stage OPA with an extraction efficiency of 20.6%,and M2 of the signal was 5.9 and 6.8 in the horizontal and vertical directions,respectively.The output energy of 110 mJ was obtained by a two-stage OPA experiment,in which the M2 of the signal in the horizontal and vertical directions was 10.5 and 11.3,respectively.6.Based on the ring cavity OPO,the investigation of OPA was carried out.A ring-cavity OPO with a cavity length of 330 mm was used to obtain an energy output of 10 mJ,the M2 of OPO was 3.0 and 3.1 in the horizontal and vertical directions,respectively.The output energy of 55 mJ was obtained after OPA with time-delay,the extraction efficiency of OPA was about 15%,and the beam quality of signal was Mx2=4.0 and My2=4.1.7.In order to increase the PRF of output laser,a high-energy KTA-OPO was achieved at 300 Hz PRF.First,a 1064 nm pump source with a PRF of 300 Hz was obtained by the Nd:YAG MOPA system.The output energy was 210 mJ and M2 factors of 19.1 and 19.7 in the horizontal and vertical directions,respectively.Then,a flat-flat cavity was used in KTAOPO,a total energy of 80 mJ was obtained for 1.53 μm(signal)and 3.47 μm(idler),the pulse widths were 17.8 ns and 16.4 ns,respectively.The conversion efficiency was 38.1%,and the M2 factors were 11.1 and 12.5 in the horizontal and vertical directions,respectively,and the corresponding M2 of the idler was 14.6 and 13.8. |
PDF Full Text Request