高光束质量闲频光谐振中红外MgO:PPLN光参量振荡器

High-beam-quality idler-resonant mid-infrared optical parametric oscillator based on MgO:PPLN

报道了采用纳秒脉冲激光器泵浦基于掺杂氧化镁周期极化铌酸锂(MgO:PPLN)晶体的高光束质量、闲频光谐振中红外光参量振荡器。通过选取曲率半径为200 mm的凹面输入镜和平面输出镜来建立平凹腔,实现了高光束质量的近-中红外激光输出。当输入的最大泵浦能量为21 mJ时,输出信号光和闲频光的最大能量分别为3.2 mJ和1.12 mJ,对应信号光和闲频光的斜效率分别为24%和9%。通过在25~200℃范围内改变MgO:PPLN晶体温度,实现信号光波长1.505~1.566μm和闲频光波长3.318~3.628μm的激光调谐输出。由于闲频光单谐振的光参量振荡器具有较大的衍射损耗和光束发散角,可以提高输出闲频光的光束质量。采用刀口法测量得到中红外激光在两个正交方向的光束质量因子分别为M_(x)^(2)≈1.2和M_(y)^(2)≈1.2。

Objective Widely tunable,high-energy,stable,compact,high-beam-quality,mid-infrared 3-5μm light sources based on optical parametric oscillator(OPO)and optical parametric amplifier(OPA)systems,known as the fingerprint region,are of considerable importance in applications including remote sensing,atmospheric monitoring,spectroscopy analysis,and photoelectric detection surveys.In particular,it is desirable to utilize highenergy,mid-infrared light sources with large wavelength tunability for highly sensitive and selective photoacoustic trace-gas sensing,in which most molecules have strong vibrational transitions.At present,the technologies available that can achieve the desired laser output in the widely tunable and highly-energized midinfrared region of 3-5μm are primarily quantum and inter band cascade lasers(QCLs)and OPOs.Although OPO technology has been around for a long time,it is still an excellent light source choice for the widely tunable midinfrared region.It provides selectivity owing to its large wavelength tunability,high energy,increased beam quality,and compact,cost-effective devices for the generation of mid-infrared light in the 2-5μm spectral range.Methods Experimental setup for the high beam quality,idler-resonant MgO:PPLN-OPO is shown(Fig.1).A solid-state Nd:YAG laser(pulse duration:25 ns,wavelength:1.064μm,PRF:50 Hz,maximum pulse energy:21 mJ,spatial form:Gaussian profile)was used as the pump source of the OPO.The pump beam was observed by a conventional CCD camera.The spatial forms of the signal and idler outputs were measured by using a Spiricon pyroelectric camera III(Fig.2).The energy scaling of the compact idler-resonant OPO has been investigated with the increasing pump energy(Fig.3).Spectral properties of a compact idler-resonant OPO have been measured by spectrometer(SpectraPro HRS-500,300 line/mm)(Fig.4).Idler output energies as a function of the idler wavelength at a pump energy of 21 mJ was shown(Fig.5).To validate the high beam quality idler output,the beam quality factor(M²)of the mid-infrared idler output was measured by means of the knife-edge method(Fig.6).Results and Discussions The maximum signal and idler output energies of 3.2 mJ and 1.12 mJ were obtained at a pump energy of 21 mJ,corresponding to the slope efficiency of 24%and 9%,respectively(Fig.3).The wavelengths of the signal and idler outputs were tuned in the ranges of 1.505-1.566μm and 3.318-3.628μm by changing the MgO:PPLN crystal temperature in the range of 25-200℃(Fig.4),and the beam quality factor of the mid-infrared idler output was measured by means of the knife-edge method,resulting beam quality factors were estimated as 1.2 and 1.2 along the horizontal and vertical directions,respectively(Fig.6).Conclusions We have successfully demonstrated the high-beam-quality,idler-resonant tunable optical parametric oscillator based on single-grating MgO:PPLN crystal.A maximum idler output energy of 1.12 mJ and signal output energy of 3.2 mJ was achieved at a pump energy of 21 mJ,and the beam quality factors of 1.2 and 1.2 in the two orthogonal directions,respectively.The wavelengths of the signal beams and idler beams outputs were tuned in the ranges of 1.505-1.566μm and 3.318-3.628μm by changing the MgO:PPLN crystal temperature in the range of 25-200℃.In the future work,by using a PPLN crystal with multiple gratings,a broader range of wavelength tuning is expected.