【作者】 苗亮；

【导师】 程祖海； 左都罗；

【作者基本信息】 华中科技大学 ， 光学工程， 2012， 博士

【摘要】 太赫兹波具有透视性、安全性、光谱分辨本领等特点,在无损检测、安全检查、光谱分析等领域有重要的应用前景。光泵太赫兹气体激光器是获得高能量高功率相干太赫兹波的重要技术,通过改变增益介质和泵浦光波长可以获得数千条太赫兹谱线。很多太赫兹增益介质的吸收谱都在CO2激光的波长范围内,横向放电激励大气压(TEA) CO2激光器的脉冲能量和峰值功率很高,是脉冲光泵太赫兹激光器最有效的泵浦源。本论文利用TEA CO2激光器泵浦NH3、CH3OH和D2O产生脉冲太赫兹激光,主要研究内容包括提高太赫兹激光能量和转换效率的方法,以及太赫兹激光参数和特性的测试技术。为了获得多条太赫兹谱线,需要泵浦激光实现波长调谐输出,本论文首先研究高能量TEA CO2激光器的光栅选支技术。探讨了光栅选支的原理,光栅参数的设计依据,以及光栅的Littrow安装结构。利用闪耀光栅实现两种型号TEA CO2激光器的选支输出,其中TEA-30型激光器输出58条支线,最高能量13J,TEA-300型激光器输出49条支线,最高能量54J。为了实现单支线振荡,激光谐振腔必须足够长以增大相邻支线间的线色散。在高气压工作的TEA-300型激光器的混合气体中加入少量三乙胺可以稳定辉光放电和激光脉冲能量,但会降低9μm带支线的能量。合理设计太赫兹激光振荡器可以有效提高太赫兹激光的转换效率,本论文设计了两套太赫兹激光振荡器真空系统。其中低真空系统由石英玻璃管激光腔、机械泵、热偶规等组成,以O型橡胶圈密封；高真空系统由不锈钢管激光腔、涡轮分子泵、复合高真空规等组成,主要以刀口法兰和铜垫圈密封。提出用Ge标准具作为振荡器的太赫兹高反镜,其太赫兹反射率可达78%,太赫兹部分反射镜使用石英晶体。为了获得高能量太赫兹激光,需要高增益的太赫兹增益气体。利用前面设计的选支TEA CO2激光器和太赫兹激光振荡器,研究了NH3、CH3OH和D2O的太赫兹谱线特性。利用TEA-300的10P(32)支线泵浦NH3产生的151.5μm谱线能量高达204mJ,9R(16)支线泵浦产生的90μm谱线能量为130mJ。利用Ge标准具充当太赫兹激光振荡器的高反镜时,151.5μm谱线的光子转换效率高达20.8%。CH30H蒸汽由TEA-30的9P(16)支线泵浦产生的570.5gm谱线能量为0.39mJ,9P(36)支线泵浦产生的118.8μm谱线能量为0.27mJ。D2O蒸汽由TEA-30的9R(22)支线泵浦产生高增益的385μm谱线,使用Ge标准具高反镜的效率是金属线栅高反镜的3.5倍。3851μm谱线的最高能量为4.44mJ,光子转换效率为7.0%。对太赫兹激光各项参数的研究有利于充分掌握光泵太赫兹激光的特性,本论文最后研究了脉冲光泵太赫兹激光的波长、光束质量、偏振和大气传输特性的测试技术。提出用两个全同Ge标准具组成法布里-玻罗(F-P)干涉仪测量太赫兹激光波长,Ge标准具F-P干涉仪的理论精细度(相邻透射峰的相位差2π与透射峰的相位半宽度之比)大于12.5,实际的波长测量误差小于±1%。利用激光模式仪分析了脉冲光泵NH3太赫兹激光的光束质量,151.5μm谱线的远场发散角约为衍射极限的7倍。检测了TEA-30的9R(22)支线和D20的385μm谱线的偏振特性,结果显示9R(22)支线基本是光栅的TM线偏振光(偏振方向与光栅刻槽方向垂直),而385gm谱线在垂直于泵浦光的偏振方向上增益较强。通过测量太赫兹激光传输10m后的剩余能量,发现90μm谱线的大气吸收严重,151.5μm和385μm谱线的大气吸收相对较小。本论文建立了完整的脉冲光泵太赫兹激光器及其测试系统,太赫兹脉冲能量高达204mJ,提出的Ge标准具太赫兹高反镜使太赫兹光子转换效率达20.8%,研制出新型太赫兹波长计,测试了太赫兹光束质量、偏振和大气传输等特性。本论文实现的高能量高效率光泵太赫兹激光器系统将有助于推动太赫兹波在透视成像和无损检测领域的应用。更多还原

【Abstract】 The unique characteristics of terahertz (THz) waves include:1) THz waves are transparent to most dry dielectric materials;2) THz waves are safe to biological tissues;3) many molecules have spectroscopic fingerprints in THz range. These characteristics make THz waves have promising applications in nondestructive evaluation, safety inspection, spectral analysis, etc. Optically pumped THz lasers (OPTLs) are important techniques to obtain high-energy high-power coherent THz waves. OPTLs can emit thousands of THz lines when the active medium and the pump wavelength are changed. Many THz active media can be pumped by CO2lasers, and high-energy high-peak-power TEA (Transversely Excited Atmospheric) CO2lasers are the most efficient pump sources for OPTLs.In this paper, TEA CO2lasers are used to pump NH3, CH3OH and D2O to emit pulsed THz laser radiation. To increase THz energy and conversion efficiency, and measure the parameters of the laser lines are the main research contents.The wavelength of the pump laser must be tuned to obtain various THz lines, so a blazed grating is used to realize wavelength tuning of high-energy TEA CO2lasers. The line-tuning theory, designing principles of grating parameters and the Littrow configuration are discussed. A blazed grating is used to tune the wavelengths of two TEA CO2lasers. The TEA-30laser emits58lines with the highest energy13J, and the TEA-300laser emits49lines with the highest energy54J. In order to ensure single line oscillating, the laser cavity length must be large enough to increase the dispersion between adjacent lines. A little triethylamine should be added in the high-pressure gas mixture of TEA-300to stabilize the glow discharge, but the triethylamine will decrease the energy of9μm lines.The configuration of the THz laser oscillator is designed to obtain high energy conversion efficiency. A low vacuum system and a high vacuum system are designed. The low vacuum system, sealed with O-type rubber rings, is composed of a quartz glass tube, a mechanical pump, a thermocouple gauge, etc. The high vacuum system, sealed with knife-edge flanges and copper rings, is composed of a stainless steel tube, a turbo molecular pump, a combined high vacuum gauge, etc. We propose that a Ge etalon is used as the high-reflectivity mirror of the THz resonator, and the THz reflectivity can reach78%. Crystal quartz is used as the output coupler.THz media with high gain are essential for realizing high-energy THz lasers. The TEA CO2lasers and THz laser oscillators mentioned before are used to pump NH3, CH3OH, and D20to emit pulsed THz laser lines. When the10P(32) line of TEA-300pumps NH3, the151.5μm line reaches204mJ. When the9R(16) line is the pump wavelength,130mJ90μm radiation is obtained. When the Ge etalon acts as the high-reflectivity mirror, the photon conversion efficiency of151.5μm reaches20.8%. When the9P(16) and9P(36) lines of TEA-30pump CH3OH vapor,0.39mJ570.5μm line and0.27mJ118.8μm line will be emitted, respectively. When D2O vapor is pumped by the9R(22) line of TEA-30, intense385μm THz line will be emitted. Both a Ge etalon and a metal grid polarizer are used as the high-reflectivity mirrors, and the Ge etalon mirror is3.5times as efficient as the metal grid mirror. The highest energy of the385μm line is4.44mJ, and the corresponding photon conversion efficiency is7.0%.Research on the parameters of THz laser lines are important for understanding the characteristics of OPTLs. The parameters of the THz laser lines are investigated, including THz wavelengths, beam quality, polarization and atmospheric transmission character. Fabry-Perot interferometers (FPIs) composed of two metal grid polarizers or two identical Ge etalons are presented to measure the THz wavelengths. The theoretical finesse, defined as the ratio of2π to the phase halfwidth of the transmission fringes of the Ge FPI, is larger than12.5. The practical measuring errors of THz wavelengths are within±1%. A pyroelectric camera is used to analyze the beam quality of NH3THz laser lines. The far-field divergence angle of151.5μm line is7times of the diffraction limit. The polarization characteristics of the9R(22) line of TEA-30and the385μm line of D2O are measured, and the results show the9R(22) line is almost linearly polarized with a horizontal polarization direction and the gain of the385μm line is larger in the vertical polarization direction. By measuring the transmitted energy of THz lines at10m off the output window of the laser oscillator, the atmospheric transmission characteristics can be analyzed. The results show the90μm line is absorbed strongly by the air, and the absorption coefficients at151.5μm and385μm are less than90μm.In this paper, a complete OPTL and THz measuring system has been set up. The highest THz pulse energy reaches204mJ, and the photon conversion efficiency reaches20.8%with a Ge etalon high-reflectivity mirror. A novel THz wavemeter has been designed and the parameters of the THz laser lines are investigated, including THz wavelengths, beam quality, polarization and atmospheric transmission character. The high-enery high-efficiency OPTL system obtained in this paper can be useful for the applications of transmission imaging and nondestructive evaluation.更多还原