The optically pumped far-infrared laser: Rate equations and diagnostic experiments

A four-level rate-equation model for an optically pumped far-infrared (FIR) laser is presented, and an expression is derived for the output power as a function of pump power and gas pressure. The theory is compared with measurements on the 570.5-μm line of CH3OH, pumped by a CO2laser. The width of the gain curve is measured and is shown to go below the Doppler width of the laser transition at low pressure. This indicates that the use of a monochromatic pump source leads to a velocity selective pumping process.     

High Efficient, Intense and Compact Pulsed D2O Terahertz Laser Pumped With a TEA CO2 Laser

A high efficient, intense and compact pulsed D₂O terahertz laser is presented, which is pumped by a multi-transverse mode TEA CO₂ laser. For D₂O gas as the active medium, with the cavity length of 120 cm, pulse energy of the THz laser has been investigated as the variation of pump energy and gas pressure. When the pump energy was 1.41 J, the maximum single pulse energy of 6.2 mJ was achieved at the wavelength of 385 μm. Photon conversion efficiency as high as 36.5% was obtained when laser operated at the maximum output energy. As the pump energy was raised from 0.57 to 1.41 J, the optimum pressure was slightly changed from 400 to 700 Pa. The THz pulse consisted of a spike pulse with pulse width of 120 ns and a tail pulse with pulse width of about 170 ns. The peak power of the spike pulse is about 44.3 kW. Comparing with the occurring time and pulse width of pump pulse, 70 ns delay and 10ns broadening were observed in the THz spike pulse.     

Performance and Spectral Tuning of Optically Overtone Pumped Molecular Lasers

A laser model was developed to predict the performance of optically pumped higher overtone molecular lasers under pulsed and continuous wave (CW) excitations. This model takes into account up to 30 rotational levels in each of the eight vibrational states considered. Collision-induced relaxation among rotational levels, vibrational levels, energy transfer to translational degrees of freedom, and interactions with buffer gas, and the temperature dependence of these processes are included. Using parameters for a second overtone pulse pumped HBr laser, a complete lasing cascade can be expected at a certain pump pulse fluence resulting in maximum achievable efficiencies approaching 80%. Optimum operational pressure and temperature conditions are determined by the gas kinetic rates and line broadening effects. Frequency tuning and spectral narrowing of the laser output is possible with the insertion of intracavity filters without sacrificing laser output power owing to efficient energy redistribution between rotational levels mediated by appropriate buffer gases. CW lasing using waveguide (WG) like geometries is possible with efficiencies approaching 92% for first overtone pumped lasers. Heat conduction through the active gas enhanced by buffer gases and subsequent heat dissipation through the cooled WG walls are expected to handle the thermal load up to kW output power levels.     

Study of Lasing Action in a Pulsed Optically Pumped D<Subscript>2</Subscript>O Gas Terahertz Laser

A modified three-level laser kinetics model for a pulsed high-power optically pumped gas terahertz laser is introduced and used to model the lasing kinetics process of a gas terahertz laser system. We, for the first time to our knowledge, investigated the time evolution dynamics process of the pump intensity, population distribution among the energy levels, pump and THz signal gain coefficient, and the THz laser intensity within the pulsed D₂O gas THz laser. High-power THz pulse with peak power of about 7.4 kW and pulse width of 145 ns at wavelength of 385 μm were obtained in the simulation, using an incident pump pulse with peak power of 2.2 MW and pulse width of 110 ns. THz pulse delay of 40 ns and pulse broadening of 35 ns were quantitatively analyzed. In addition, the experimental results for the pulse profile, pulse width, pulse broadening, pulse energy, and peak power are in agreement with the theoretical simulation results.     

A twin optically-pumped far-infrared CH3OH laser for plasma diagnostics

A twin optically-pumped far-infrared CH₃OH laser has been constructed for use in plasma diagnostics. The antisymmetric doublet due to the Raman-type resonant two-photon transition is reproducibly observed at 118.8 μm. With the 118.8-μm line, it is obtained from the frequency separation of the anti-symmetric doublet that CH₃OH absorption line center is 16±1 MHz higher than the pump 9.7-μm P(36) CO₂ laser line center. It is shown that the Raman-type resonant two-photon transition is useful in order to get several-MHz phase modulation for the far-infrared laser interferometer. Some preliminary performances of this twin laser for the modulated interferometer are described.     

Effects of Buffer Gas on the Output of Optically Pumped NH3 Far-Infrared Cavity Laser

By solving the semi-classical density matrix equations and developing a buffer gas mechanism model, the effects of buffer gas on the output power of optically pumped NH₃ far-infrared cavity laser were calculated and verified by experiment. Our results showed that: the output power of NH₃ far-infrared laser could be increased when certain buffer gas was added into the laser medium, and there existed an optimum ratio of gases mixture and an optimum operating gas pressure which could make the output power of far-infrared laser reach maximum.     

New FIR laser lines from fully deuterated dichloromethane:completion of perpendicular/parallel emission pairs

We report 16 new far-infrared (FIR) laser emissions of CD₂ Cl₂. These lines are obtained by optically pumping the active molecule by means of a large-tunability waveguide CO₂ laser. Our tunability of 290 MHz around each CO₂ laser emission allows for the excitation of large-offset absorption transitions of CD₂Cl₂, not reachable by conventional CO₂ Lasers. While the overwhelming majority of the known FIR laser emissions of CD₂Cl₂ are polarized parallel to the polarization of the pump CO₂ line, 13 of the 16 new lines reported in this paper are polarized perpendicularly to the corresponding CO₂ pump line. The presence of both perpendicular and parallel lines is extremely important for the assignment of laser transitions     

基于TDLAS测量HBr化学激光器气体温度

利用可调谐二极管激光吸收光谱技术（TDLAS）,基于吸收光谱的多普勒展宽原理,对D2/NF3燃烧驱动的HBr化学激光器,进行了光腔和扩压段的气体温度测量实验研究。为了有效地测量TDLAS吸收光谱,选用了主气流中吸收系数较大的HF分子（2-0）振动谱带的R2谱线作为研究对象。实验中利用一台中心波长1 273 nm的分布反馈式（DFB）二极管激光器,搭建了一套基于直接吸收法TDLAS的HBr化学激光器气体温度测量系统。通过对HF分子的吸收谱线进行Voigt线型拟合,获得了多普勒展宽宽度,从而给出了光腔和扩压段气体温度。在进行时域频域变换时,使用了一台自由光谱范围（FSR）为1.5 GHz的F-P标准具用于频率校准。实验测量结果表明,光腔温度约为280 K,扩压段温度约为400 K。实验过程中的碰撞展宽和多普勒展宽的比值小于0.1,表明多普勒展宽为主,能够方便地用HF吸收光谱的展宽来监测光腔和扩压段的气体温度。     

A 170 J electron beam pumped XeF(C→A) laser

A pulse output energy of 170 J has been achieved from an XeF(C→A) laser system, pumped by a pair of counterpropagating, three-meter-long electron beams. This represents a record for all types of pumping, for this excimer system. Energy was extracted from a volume of ~100 L, using a free-running stable oscillator. No evidence of laser oscillations on the competing XeF(B→X) transition was observed. Within the extraction volume the laser gas was pumped at a rate of 140 kW/cm³ (time average value), for a period of 1.7 μs. The optical cavity was folded, giving a gain length of 6 m. The optical pulse duration was 0.8 μs (full width at half maximum), and the observed flux buildup time of ~1 μs was consistent with modeling and a measurement of the net gain. The specific output energy was 1.7 J/L which is comparable to that achieved in previous, small scale experiments at somewhat higher pump rate. The results confirm the volumetric scalability of the electron beam pumped XeF(C→A) laser system to high output energy per pulse, and the feasibility of operating this system at a low electron beam pump rate which relaxes constraints on the design of the electron gun and pulse power subsystems in a high output energy device. Means for extending the laser pulse duration and increasing the output energy of the specific test device are discussed. An output energy of ~1000 J is projected for an optimized gas cell width, for full size resonator mirrors, and with injection     

缓冲气体对光泵远红外激光器能量交换的影响

基于分子振动弛豫的理论,完善了缓冲气体的作用模型。利用半经典密度矩阵理论与量子力学理论,研究了缓冲气体对光泵远红外激光(FIR)激光过程的作用机理,计算了缓冲气体作用下小型光泵FIR激光器的能量交换过程以及气压等工作参数对输出光强的影响,得出了优化规律,并进行了实验验证。结果表明,适当的缓冲气体可缩短工作气体分子的振动弛豫时间,提高光泵FIR激光器的能量转换效率,使FIR激光信号得到更大的输出。在最佳混合气体比例与最佳工作气体下,可以获得最大的FIR激光信号输出。     

Gamma-ray induced pulsewidth broadening and power loss inmodelocked Er-doped fibre laser

A modelocked erbium-doped fibre laser (EDFL) exhibited a 9.7% decrease in its pump laser output power and pulsewidth broadening from 9.2 to 300 ps following a 10.14 kGy(Si) gamma-ray irradiation. A decrease in the EDFL intra-cavity power from the combined effects of the pump laser degradation, and colour centre induced absorption in the fibre segments and other EDFL components was responsible for the broadening and long recovery times required for restoration of modelocking     

单纵模Nd∶YAG激光抽运H_2-He混合气体中的强后向受激拉曼散射

用单纵模Nd∶YAG二倍频激光(波长532 nm,线宽0.003 cm-1,脉宽(半峰全宽,FWHM)6.5 ns)抽运H2/He气体,观察到很强的后向一阶斯托克斯(BS1)受激拉曼散射。在H2的分压分别为1.0 MPa和1.5 MPa的H2/He(体积比)(3/7)混合气体中,当抽运能量为92 mJ时,后向一阶斯托克斯光的量子转换效率高达69%左右,而在纯H2中后向一阶斯托克斯光的量子转换效率分别只有15%和18%。这是因为加入He增强了后向拉曼散射多普勒线宽的Dicke压窄效应,使后向一阶斯托克斯光的拉曼增益系数与前向一阶斯托克斯(FS1)光的拉曼增益系数的比率提高,而且因为后向一阶斯托克斯光与抽运光在反方向传播,它可以提取大部分抽运光的能量,并且脉冲被压窄到1.1 ns,使后向一阶斯托克斯光峰值功率达到了抽运光的2.6倍。从激光光斑的强度分布可以观察到后向一阶斯托克斯光呈现为抽运光的相位共轭波。     

光纤气体激光光源研究进展及展望(Ⅱ):基于粒子数反转

粒子数反转和受激拉曼散射是实现光纤气体激光器输出的最常见的两种基本原理。与光纤气体拉曼激光光源不同,基于粒子数反转原理的光纤气体激光器是通过气体分子振转能级的本征吸收跃迁实现激光输出。由于绝大多数气体分子的振转能级对应的激射跃迁谱线都在中红外波段,这种激光器的输出波长基本都在中红外波段。简要分析了基于粒子数反转原理的光纤气体激光器在产生中红外波段激光方面的优势,重点回顾了其发展历史与研究现状,并对下一步的发展趋势进行了展望。     

1.27μm atomic argon laser parameters in fission-fragment excitedHe/Ar and He/Ne/Ar gas mixtures

The authors examine the power efficiency of the fission-fragment-excited atomic argon laser operating on the 1.27-μm (3d'[3/2]₁-4'_p[3/2]₁) argon transition as a function of pump power, gas mixture, and pressure. The maximum measured power efficiency was 1.1±0.3% for a gas pressure of 1300 torr and a He/Ar ratio of 99.88/0.12. Neon addition to the He/Ar gas mixture increased both the energy deposited in the gas and the energy output without decreasing efficiency for a neon gas fraction of less than 0.5. Small-signal gain and saturation intensity are between 0.15-0.27%/cm and 25-200 W/cm² for pump rates of 7.5-30 W/cm ³ in He/Ar and He/Ne/Ar gas mixtures. The laser threshold as a function of total pressure and argon concentration is presented     

基于D2O的光抽运腔式太赫兹波激光谱线竞争

从半经典密度矩阵理论出发,建立了光抽运太赫兹波谱线竞争的双三能级分子系统模型,理论推导了抽运信号吸收系数以及太赫兹波信号增益系数的数学表达式,采用迭代法数值计算了CO2激光-9P(32)抽运重水(D2O)气体分子腔式太赫兹激光66μm和116μm两条谱线之间的竞争。给出了抽运功率、工作气压及激光腔长等工作条件下谱线竞争的一般规律。     

利用多吸收峰降低全固态激光器温控功耗

半导体激光器（LD）的工作波长是随温度变化的,对LD进行温控是扩展全固态激光器（DPSSL）正常工作温度范围的常用方法,但常用的控温方法在-50~70℃的宽温区范围存在体积大、能耗高、效率低等问题。通过实验测试得到GaAs量子阱激光器的波长温度漂移系数为0.25 nm/℃,分析了Nd:YAG晶体吸收谱的多峰特性。提出采用高温时工作波长为808 nm的GaAs量子阱激光器作为泵浦源,利用Nd:YAG晶体的795.7 nm和808 nm的两个吸收峰,通过分段加热控温降低温控功耗的方案。实验结果显示:全固态激光器在两个吸收峰处得到的输出脉冲特性基本相同,在温度较低时,分段控温的加热功率减小了4.7 W,接近不分段最大加热功率的一半。     

A Revisory Mathematical Model on Optically Pumped Far-Infrared Laser

In an optically pumped far-infrared laser (OPFIRL) system, the infrared (IR) pumping laser with collision broadening which was closer to the practice was considered. A more precise mathematical model was set up. When the IR pumping laser line had the Lorentz function structure, by solving the semi-classic density matrix equations, the generalized form of FIR signal gain Gs and IR pumping signal absorption coefficient Gp were obtained.     

分布反馈光纤激光器自放大特性实验研究

通过相位掩膜板移动法,制作了非对称相移结构分 布反馈光纤激光器(DFB-FL)。为了获得更高的激光功率,利 用同样长度的掺铒光纤(EDF),分别采用同向泵浦和反向泵浦结构方式进行了窄线宽激光信 号的自放大实验,结 果表明,两种自放大结构的DFB-FL均可充分利用泵浦光功率,在保证激 光线宽展宽有限的前 提下,实现了激光信号至少23dB的功率放大。同时,实验结果对比 说明,同向泵浦放大结构更有利于保 持原激光信号的噪声特性。这为较大输出功率的DFB-FL提供了一种简单 可行的方案。     

宽带KrF激光抽运SF6受激布里渊散射的脉宽稳定性

从理论和实验上对利用宽带KrF激光抽运SF6产生的受激布里渊散射(SBS)的脉宽稳定性进行了研究。获得了受激布里渊散射脉宽稳定性随抽运功率密度稳定性、抽运脉宽稳定性及介质气压变化的规律。发现受激布里渊散射脉宽的稳定性与抽运激光的稳定性直接相关,抽运激光的稳定性越好,获得的受激布里渊散射脉宽相对稳定性也越好。抽运光脉宽和能量的不稳定都会造成所产生的受激布里渊散射脉宽不稳定。在较低抽运功率密度情况下抽运光脉宽和能量的波动对受激布里渊散射脉宽稳定性的影响都不可忽略,但在较高抽运功率密度情况下受激布里渊散射脉宽稳定性主要受抽运光脉宽波动的影响。对结果进行了分析和讨论,获得脉宽稳定性较好的受激布里渊散射输出的条件是,使用脉宽稳定性较好的抽运光和在保证没有其他非线性效应产生的情况下,尽可能提高抽运光的功率密度和介质气压。