百赫兹大能量KTA双波长光参量振荡器

100Hz High-Energy KTA Dual-Wavelength Optical Parametric Oscillator

报道了百赫兹大能量KTiOAsO_4(KTA)晶体光参量振荡器(OPO)系统,并研制了脉冲重复频率为100 Hz、输出能量达580 mJ、波长为1064 nm的Nd∶YAG主振荡功率放大器。OPO谐振腔采用平平腔结构,用在X方向切割的KTA晶体作为非线性晶体。在脉冲重复频率为100 Hz时,得到1.53μm信号光和3.47μm闲频光的单脉冲能量分别为178 mJ和64 mJ,脉冲宽度分别为13.7 ns和11.8 ns,泵浦光到参量光的光-光转换效率为43.6%。

Objective Laser sources at 1.5μm,providing high pulse energies and short pulse durations are used in various applications,such as electro-optical countermeasures and high precision ranging.Lasers at 3--5μm are used as lighting sources for active remote sensing and gas detection,which show important potential applications.Therefore,lasers with high energy at both wavelength bands have become research hotspots.The optical parametrical oscillators(OPO)are effective ways to generate lasers with wavelengths at 1.5μm and interval 3--5μm due to their compactness,wavelength-tunable property,and potential for generating high energy and short pulse width.Presently,the major nonlinear crystals with high-quality include biaxial crystals such as KTiOAsO4(KTA),KTiOPO4(KTP),ZnGeP2(ZGP),and periodically poled crystals such as PPKTP,PPLN,PPLT,etc.KTP crystals are used to obtain lasers at 1.5μm,which is affected by severe absorption in the mid-infrared region.To obtain lasers at 3--5μm,ZGP crystals have been under investigation for a long time.However,2μm pump sources are more in need,which is technically more difficult than their 1-μm counterpart.PPLN crystals are used to obtain mid-infrared lasers.Compared with crystals such as KTP,the damage threshold of PPLN crystals is lower.KTA and KTP crystals belong to the same crystal group and have a high damage threshold(>600 MW/cm2),large nonlinear coefficient(d24=3.2pm/V),large acceptance angle,a wide temperature range,and stable physical and chemical properties.The transmission performance of KTA crystals in the mid-infrared band(3--5μm)must be better than that of KTP crystals.These characteristics make KTA crystals suitable for high energy mid-infrared laser applications.In this study,we report a 100Hz high energy KTA crystal-based OPO system.Methods The 100 Hz high energy KTA-OPO system is composed of 1064nm Nd:YAG main oscillator power amplifier(MOPA)and KTA crystal-based OPO.The Nd∶YAG MOPA laser at 1064nm served as the pump source.To obtain high beam quality,the Nd∶YAG MOPA system adopted the"unstable cavity oscillator+twostage amplifiers"scheme.Both the oscillator and the two-stage amplifier used a double rod structure connected in series,and a 90°quartz rotator between the two Nd∶YAG crystal rods to compensate for the thermal depolarization effect.To prevent self-excited oscillation and spontaneous radiation between the stages while protecting the optical components of each stage,isolators are placed between each stage.The X-cut KTA crystal is used in the experiment,and the dimension of the KTA crystal is 10mm×10mm×33mm.The cavity is designed as a signal resonant oscillator with a cavity length of 65mm.The input mirror is coated to be highly reflective for the signal and high transmittance for the pump light.The output mirror is coated with a partial reflectivity of 50%for the signal and high transmittance for the idler.The pump light passed the OPO twice.An isolator protects the pump laser from the remaining pump light that comes back from the OPO cavity.Results and Discussions A homemade 1064nm Nd∶YAG MOPA with a pulse energy of 580 mJ at 100 Hz repetition rate is employed as the pumping source.After two-stage amplification,580 mJ of 1064nm laser is obtained with the extraction efficiency of the primary amplifier and secondary amplifier at 6.7%and 10.8%,respectively(Fig.3).The beam quality factor of the 1064nm laser is Mx2=4.6and My2=3.7(Fig.4).The pulse width of the laser from the oscillator and primary amplifier and secondary amplifier are 15.3,16.9,and 18.0ns,respectively(Fig.5).In the OPO experiment,the optical-to-optical conversion efficiency is optimized by increasing the cavity length and KTA crystals length.The output energy and conversion efficiency of the KTA crystal with a length of 33mm are higher than that of the KTA crystal with a length of 38mm(Fig.6).Then,experiments with different OPO cavity lengths are performed on the 33-mm KTA crystal.The results indicated that the output energy and conversion efficiency are higher for short cavity length(Fig.6).The threshold of the OPO is about 20mJ.When the pump energy is 580mJ,64mJ idler is obtained at 3.47μm and associated signal at 1.54μm is 178mJ(Fig.7).The OPO efficiency is 46.3%high,and OPO output stability is 1.2%rms(Fig.7).The pulse width of the output laser at 1.54and 3.47μm is 13.7and 11.8ns,respectively(Fig.8).The beam quality factor of the 1.54μm laser is Mx2=30.5and My2=28.2(Fig.9).In addition,the center wavelength of the signal laser is 1.535μm(Fig.10).Conclusions A 100-Hz,high-energy KTA crystal-based OPO system is reported.A homemade 1064nm Nd∶YAG MOPA with a pulse energy of 580mJ at a 100Hz repetition rate is used as the pumping source.We adopted plane-plane cavity configuration for the OPO,and an X-cut KTA crystal as the nonlinear crystal.The obtained pulse energies at a signal wavelength of 1.53μm and idler wavelength of 3.47μm are 178and 64 mJ at a pulse repetition rate of 100Hz,respectively.Furthermore,the pulse durations are 13.7and 11.8ns,respectively,and the optical-to-optical conversion efficiency is 43.6%.