摘要
3～5μm中红外波段激光在大气中具有较高的透过率,因此被广泛应用于光电对抗等领域.报道了基于掺杂氧化镁的周期极化铌酸锂(MgO∶PPLN)晶体的kHz、mJ量级的中红外光学参量振荡器(OPO).采用纳秒脉冲1064 nm激光泵浦基于多周期MgO∶PPLN晶体的OPO,OPO采用泵浦双通单谐振平凹腔结构.中红外激光的重复频率为1 kHz,4.08μm处输出的最高单脉冲能量达到1.041 mJ.最高光-光转换效率为16.8％,斜效率为19.3％,中红外激光脉宽约为9.53 ns.在最高能量输出时,OPO运转30 min时输出功率的均方根(RMS)为0.24％.通过温度-极化周期结合的调谐方式,OPO在3.49～4.18 μm的较宽范围内都能够保持0.9 W以上平坦的高能量输出.在极化周期27.5～29.6 μm以及温度25～200 ℃的调节范围内,闲频光波长的调谐范围为3.49～4.48 μm.实现了重复频率为kHz量级、单脉冲能量为mJ量级的可调谐中红外输出,其在光电对抗领域具有应用价值.
Abstract
Objective Nanosecond mid-infrared lasers with central wavelengths of 3-5 μm are used in various applications,such as environmental monitoring,LIDAR,and electro-optical countermeasures,owing to the high atmospheric transmission at this band.Thus,developing high-energy mid-infrared laser sources with high pulse repetition frequencies is crucial for practical applications because such lasers exhibit superior jamming effects and can effectively target an opponent's anti-jamming mechanism.An optical parametric oscillator(OPO)is suitable for efficiently realizing a tunable,high-pulse-repetition-frequency laser owing to its compactness,wide tuning range,and high conversion efficiency.The output characteristics of an OPO are primarily determined by the optical properties of the constituent nonlinear crystals.Among the various nonlinear crystals used in OPOs,periodically poled magnesium-oxide-doped lithium niobate(MgO∶PPLN)has attracted considerable attention because of its large effective nonlinear coefficient,high damage threshold,and flexible phase matching ability.However,to date,a pulse energy of the order of hundred microjoules has been realized for PPLN-OPOs with high pulse-repetition frequencies.In this study,a kilohertz-and millijoule-level mid-infrared OPO based on an MgO∶PPLN crystal is developed.The OPO,which is pumped by a 1064-nm nanosecond pulsed laser based on multi-period MgO∶PPLN with a double-pass single-resonance structure and flat-concave cavity,delivers a high output energy of up to 1.041 mJ at 4.08 μm with a pulse repetition frequency of 1 kHz.Subsequently,the output energy of the high-pulse-repetition-frequency mid-infrared OPO based on a PPLN crystal is increased to mJ range,which is suitable for mid-infrared electro-optical countermeasure applications.Methods The output energy of a mid-infrared laser with a high pulse-repetition frequency is significantly increased.First,the OPO is pumped by a high-performance,high-pulse-repetition-frequency 1064-nm laser with a double-pass single-resonance structure,which improves the utilization of the pump light and reduces the pumping threshold.Second,we evaluate the output characteristics for different OPO cavity types and compare them with the theoretical calculation results to determine the optimal conversion efficiency under these pumping conditions.With the combined periodic and temperature modulations,the OPO maintains a flat and high output energy over a wide range of 3.49-4.48 μm,wherein the energy exceeds 0.9 mJ.Results and Discussions In the experiment,a pump power higher than 6.2 W easily damages the surface of the crystal,indicating that maximizing the pump power to 6.2 W will result in a high output energy.We determine the relationship between conversion efficiency and suprathreshold multiplicity.The maximum conversion efficiency is obtained when the suprathreshold multiplier reaches 6.6(Fig.3).Further,the pumping threshold changes significantly with changes in the cavity type because a flat-concave cavity improves the mode matching of the three interacting waves in the OPO cavity,resulting in a low OPO threshold.However,as the radius of curvature(R)decreases,the input mirror affects the pump light dispersion,which in turn reduces the intracavity pump power density,resulting in a decrease in the conversion efficiency of the parametric process.Figure 4(b)shows that using a flat concave mirror with R=300 mm as the input mirror results in a low threshold of 0.9 W.In our case,the suprathreshold multiplier is the closest to the optimum value of 6.6,and thus,the highest output power is obtained for this cavity.Consequently,we maximize the use of pumping energy below the damage threshold,thereby further increasing the output energy.Conclusions We demonstrate tunable mid-infrared lasers based on MgO∶PPLN crystals with a pulse repetition frequency of the order of kilohertz and an output energy of the order of millijoules.At a pump power of 6.2 W,the single-pulse energy of the 4.08-μm mid-infrared laser is 1.041 mJ with a pulse repetition frequency of 1 kHz.The optical-to-optical conversion and slant efficiencies are 16.8％and 19.3％,respectively,and the mid-infrared laser pulse width is approximately 9.53 ns.The fluctuation in the average power measured within 30 min is 0.24％(root mean square,RMS).The final output is highly stable,and the observed small fluctuations are caused by the drifts in the central wavelength of the laser diode.Combined with periodic and temperature modulations,the OPO delivers a flat and high output energy signal over a wide range of 3.49-4.48 μm.The tunable range covers 3.49-4.48 μm within the polarization period range of 27.5-29.6 μm at 25-200 ℃.
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