The Technology And Application Of Optical Frequency Conversion In The Mid-infrared And Terahertz

Terahertz（THz）wave,lying between the microwave and the far-infrared wave,has a unique spectrum and magnetic properties.Yet due to technical limitations,it has not been fully developed.For more than a century,the THz technology has experienced substantial development.The THz radiation and detection technology has been the main line of the development.Although at present there have been many commercialized THz sources and detection and imaging devices,it is still difficult to meet the requirements for high-power and highly-stable THz sources and high sensitivity in detection and imaging.With the rapid development of nonlinear optical theory and technology in recent years,the high-power radiation and highly-sensitive detection of the mid-infraed and THz waves via optical frequency conversion has become an important research direction.Aiming at the inadequacy in practical and effective THz wave radiation sources and detectors so far,this dissertation conducts research on THz generation and high-sensitivity imaging in the mid-infrared based on a variety of nonlinear optical frequency conversion technologies,through applying such star technologies in the field of nonlinear optical frequency conversion as optical parametric oscillation（OPO）,different frequency generation（DFG）and quasi phase-matching（QPM）.It discusses the features of these conversion technologies theoretically and experimentally so as to lay the foundation for further research on the THz wave generation and detection.The following presents the main framework of this dissertation:1.THz-DFG is an important development direction of optical THz source.As one of the most important indicators in most applications of THz source,the stability of THz-DFG has been scarcely discussed in reports.Therefore,based on the three-wave mixing model containing absorption,with the Zinc germanate phosphate（ZnGeP₂）crystal as an example,we study the high-power stability conditions of THz-DFG.Results show that output stability of the THz-DFG is more dependent on the stability of the pump wave.When the parametric gain is similar to the absorption loss,the stable condition is hardly reachable in the attainable crystal length.This is more likely to appear when the absorption coefficient of pump wavelength is high in nonlinear crystals.2.THz wave parametric oscillator（TPO）operating under room temperatures is one of the earliest commercialized optical parametric THz sources.Usually,this type of THz sources do not set the precision temperature controller.Therefore,the best room temperature for TPO applications is an essential factor in researches and production.In this study,a single-resonance TPO at the infrared frequency with continuous tunable wavelength from 104μm to 226μm is established.The experimental results show that the TPO output performance declines quickly as the temperature increases.At the room temperature of 20℃,compared with the performance under 25℃,the initial TPO threshold is reduced by 20%,and the peak THz wave output is increased by 3.5 times,when the peak power of 220 mW is achieved at the wavelength of 146.2μm.3.Combined with the low threshold and high stability of TPO,and the easy tunability and high output of DFG,a compact cascaded parametric system for THz wave generation is demonstrated.The cascade system of a single-pump laser source with synchronously tuned THz wavelength is designed and set up.By adjusting the resonant wavelengths of TPO and the phase-matching angles of DFG,the output THz wavelength of the system is tuned,and the THz wave radiates from two optical parametric devices.Results show that the TPO output characteristic fluctuates with the pump instability.The time domains of the resonance light and the rest pump vary synchronically,which directly affects the output power and the stability of the THz-DFG.4.Given the technical problem that it is difficult to improve the sensitivity of the imaging device in the mid-infrared and THz wave,the upconversion imaging and detection technology on incoherent signals is studied.Based on enhanced optical upconversion imaging and detection in the cavity,the incoherent mid-infrared upconversion imaging and detection with few photons are verified.Via LD pumped Nd:YVO₄ crystal,a circulating power up to 70 W is generated in the laser resonant cavity equipped with the PPLN crystal.The original images of the light emission from a 40 W heat light bulb with a voltage regulator are used for the intracavity optical parametric upconversion.Incoherent images are detected by the near infrared EMCCD detection at the single-photon level via transferring the mid-infrared wave to near-infrared wave.The detection sensitivity of the mid-infrared imaging is greatly improved.The quantum conversion efficiency of the polarized signal-wave achieved is 28%,and the efficiency may theoretically reach around 100%by improving the power of cavity and the effective crystal length.In the 1-second exposure time,the optimal mid-infrared detection sensitivity at 0.57 photon/space element/second is achieved.Meanwhile,the minimum signal power able to respond to the mid-infrared imaging system is 31 fW,which is much higher than the performance of the commercialized mid-infrared camera.