| Tunable infrared laser sources operating at the atmosphere window have attracted great attention for its potential applications in environment monitoring, biomedical diagnostics, infrared remote sensing and laser radar. Due to the maturity of the CO2 laser technology, the laser source running at the mid-infrared range of 9～11μm could work effectively. However, there is a great desire to achieve efficient and widely tunable laser sources working at the mid-infrared range of 3-5μm and the terahertz region. While in this dissertation, we would present our research on the infrared tuning techniques and the frequency conversions from mid-infrared region to the far-infrared terahertz range. The main contents and innovations are as follows.The laser generation dynamics of pulsed CO2 laser has been theoretically analyzed based on the six temperature model theory. The impacts of working gas composition, gas pressure, dissociation of CO2 molecules, reflectivity of the output coupler and the ambient gas temperature on the laser pulse characteristics have been investigated. The theoretical calculations and experimental measurements of the CO2 laser pulse are in good agreement. Moreover, we also simulated the laser pulse of longitudinal mode structure, and studied output characteristics of the tunable CO2 laser running at different wavelength. This theoretical model could help us optimize the design for the pulsed CO2 laser.Based on the theoretical study of the laser dynamics, a short pulsed tunable CO2 laser has been built up in our laboratory, which could generate the maximum pulse energy of 520 mJ with a tuning range of 9.2-10.8μm. Meanwhile, the output characteristics of this laser and the tuning techniques are also investigated. Secondly, we modified the tunable CO2 laser with two gratings, and achieved dual-wavelength output in the same resonator. The dual-wavelength pulses could generate independently and synchronously. On the other hand, the current pulse discharged CO2 laser can be transformed into a CO laser. By utilizing the CO laser, we achieved pulsed CO laser radiation at 5.3μm at room temperature.Since the harmonics of CO2 laser frequencies located in the mid-infrared region, it is an effective way to generate mid-infrared radiation by the harmonic generations of CO2 laser radiation. Second harmonic generation (SHG) was investigated in three nonlinear crystals (AgGaS2, AgGaSe2 and GaSe). The maximum conversion efficiency of 6.3% was obtained in the AgGaSe2 crystal when the pulse energy of pump radiation was 100 mJ. By taking advantage of the second harmonic generation in AgGaSe2 crystal, we achieved the third-harmonic generation (THG) and the fourth-harmonic generation (FHG) in AgGaSe2 and GaSe crystals, respectively. The THG conversion efficiency in AgGaSe2 crystal was 0.3%, and the conversion efficiency for THG and FHG in GaSe crystal were measured to be 0.1% and 0.02%. Besides, the same frequency conversions in these crystals have been done at 10.6μm. By tuning the output wavelength of the laser, the harmonics of the CO2 laser radiation could cover the whole mid-infrared domain from 2.3μm to 5.4μm.We investigated the theoretical phase-matching conditions of the difference frequency conversion in several terahertz crystals, including the calculations of the phase-matching angles, walk-off angles and the angle tuning bandwidths. All these calculations could help us evaluate the performances of the terahertz crystals as a difference frequency generator. Finally, by utilizing the self-made dual-wavelength tunable CO2 laser, efficient, coherent and widely step-tunable terahertz radiation has been generated by noncollinear difference frequency generation in GaAs crystals. This terahertz source system can generate THz pulses in the range of 72.3-2706μm (0.11-4.15 THz) with maximum pulse energy of 2.1μJ (corresponding to a peak power of 35 W) and yield a average power of 10μW at 236.3μm when the laser working repetitively. Compared with previous results, we have achieved a wider tuning range of THz generation in GaAs crystal at room temperature and this THz source is more compact than others pumped by two CO2 lasers before.
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