气流方向Z型折叠离轴混合非稳腔研究

Off-Axis Hybrid Unstable Resonator with Z-Fold in Gas Flow Direction

提出并研究了一种气流方向Z型折叠离轴混合非稳腔,应用在小型化DF气流化学激光装置中(增益体积为30 mm×40 mm×125 mm),实现了平均功率550 W、光束质量β值1.8(稳定腔方向β值为1.5,非稳定腔方向β值为1.9)、光谱覆盖3680~4089 nm的中红外连续激光输出。该腔在气流方向是平凹稳定腔,并且进行了Z型折叠以减小稳定腔方向上的增益尺寸;在高度方向是正支共焦非稳腔。针对稳定腔方向上远场光斑从高阶横模振荡到基横模振荡的物理过程进行了理论计算与实验研究,光腔振荡的计算结果与实验结果吻合较好。计算与实验结果表明,气流方向Z型折叠离轴非稳腔可用在小型化、紧凑型DF气流化学激光装置中,实现高功率、高光束质量中红外连续激光输出。

Objectivee t Hybrid unstable resonators have been widely used in slab solid-state lasers and gas laser systems in recent years duo their high extraction efficiency and high beam quality.So far,hybrid unstable resonators have not been used in DF gas flow chemical laser devices to achieve high-power,high-beam quality mid-infrared continuous wave(CW)laser output.In addition,the beam quality in the direction of stable resonator is always poor and hard to handle,and the physical process of far-field spot from high-order transverse mode oscillation to single fundamental transverse mode oscillation should be studied theoretically and experimentally.Methods A kind of off-axis hybrid unstable resonator with Z-fold in the flow direction is proposed and studied.The Z-folded off-axis hybrid unstable resonator is a flat concave stable resonator in the gas flow direction,and Z-folded to reduce the gain size in the direction of the stable resonator.In the height direction,it is a positive branch confocal unstable resonator,as shown in Fig.1.The magnification in the direction of unstable cavity is 1.2.The gain medium is generated by the DF gain generator driven by combustion.Nitrogen is used as diluent,and the static pressure of the gain air flow is generally less than 666.7 Pa.The gain size is about 30 mm in the air flow direction and 40 mm in the height direction,and the gain length of the laser single pass is 125 mm,while after Z-folding the gain length becomes 375 mm.The virtual assembly diagram of the gain generator module and the optical cavity module is shown in Fig.2.Figure 3 shows the schematic diagram of the monitoring optical path,with which the output power,far-field spot,output spectrum and near-field spot are monitored.We calculate the oscillatory self-reproducing modes of Z-folded off-axis hybrid unstable cavities by means of fast Fourier transform based on the diffraction theory of plane-wave angular spectra.In the calculation,the seed beam used for the optical cavity oscillation is a rectangular parallel plane wave conforming to the cross section of the gain medium(30 mm×45 mm).There are four mirrors in the optical cavity,and the seed beam oscillates back and forth between the concave spherical mirror 4 and the convex cylindrical mirror 1.After the gain saturation,a self-reproducing mode with stable intensity and phase distribution is formed.The calculation process is shown in Eqs.(1)--(10).Results and Discussions The mid-infrared CW laser output with average power of 550 W,beam qualityβvalue of 1.8(theβvalue is 1.5 in the stable resonator direction and 1.9 in the unstable resonator direction),and spectral coverage of 3680--4089 nm is realized in a miniaturized DF gas flow chemical laser device(size of gain medium:30 mm×40 mm×125 mm),as shown in Figs.4--6.The calculation results of the far-field spot are shown in Fig.4.Figures 4(a),4(b)and 4(c)respectively correspond to the gain size in the direction of the air flow,which is determined by the rectangular apertures in front of the concave spherical mirror 4,being limited to 10 mm,8 mm,and 6.5 mm.High-order transverse mode oscillations appear in the X direction for the far-field light spots in Figs.4(a)and 4(b).Figure 5 shows the experimental results of the far-field spots.When the gain size in the air flow direction is limited to 10 mm and 8 mm,the intensity distributions of the near-field spot(10 mm×6.6 mm,8 mm×6.6 mm)and the far-field spot in the experiment have the characteristics of three bright spots and two bright spots,respectively.The bright spot appears as a HermitianGaussian high-order transverse mode,and the beam qualityβvalues of the far-field spots in the air flow direction are 4.2 and 3.4,respectively.As the gain size in the air flowdirection is limited to 6.5 mm,we experimentally obtained a high beam quality CW laser output with a beam qualityβvalue of 1.8,and the beam quality in the air flow direction,i.e.,the direction of the stable cavity,is 1.5.Conclusions In order to achieve high-beam quality and high-power mid-infrared laser output based on a miniaturized DF gain generator(gain volume is 30 mm×40 mm×125 mm),we propose and study a Z-folded off-axis hybrid unstable resonator in the gas flow direction as the optical resonator of DF chemical laser device,and the beam quality in the gas flow direction of the resonator is studied theoretically and experimentally.Finally,mid-infrared CW laser output with an average power of 550 W,a high beam qualityβvalue of 1.8(theβvalue is 1.5 in the stable cavity direction and 1.9 in the unstable resonator direction),and a spectral coverage of 3680--4089 nm is achieved.