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Research On High-energy Optically Pumped Gas Terahertz Laser

Posted on:2013-01-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:L MiaoFull Text:PDF
GTID:1118330371480955Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
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.
Keywords/Search Tags:terahertz laser, transversely excited atmospheric CO2laser, metal grid polarizer, etalon, Fabry-Perot interferometer, ammonia, methyl alcohol, deuterium oxide
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