Study Of Compact High-repetition-rate Terahertz Sources Based On Nonlinear Optical Difference Frequency Generation

Terahertz(THz)waves have important application values for the spectroscopy and medical imaging.The compact high-repetition-rate terahertz source can reduce the size and the cost with the advantages of easy operation,high average power,and high sam-pling rate,which has already been an important research focus in the terahertz field.Difference frequency generation(DFG)is regarded as an important way to achieve compact high-repetition-rate terahertz sources with low cost,simple experimental ap-paratus,wide tunable range,no threshold and room-temperature operation.This disser-tation mainly focuses on this topic to carry out the related research work,with the main work and innovations given as follows:1.By using the end-pumped intracavity optical parametric oscillators(OPOs),where the Nd:YVO₄ and Nd:YLF crystals were employed as the gain mediums,and the KTP and PPLN worked as nonlinear crystals,the highest output power of 2-?m dual-wavelength lasers could reach up to 4.15 W.The repetition rate and the tunable range were around 25 k Hz and 1.92–2.38?m,respectively.Compared to others similar laser systems,the optical-to-optical conversion efficiency of 22.5%was the best results.Such compact efficient 2-?m dual-wavelength lasers were applied to QPM-GaAs and Ga Se crystals to generate THz by DFG.The maximum output power was 1.8?W and the tuning band covered the range of 0.24–3.78 THz.The whole length of terahertz source was within 180 mm,and it was successfully applied in spectroscopy and multi-spectral imaging.2.A hybrid ridge waveguide structure based on GaAs was designed.By using 2-?m and 10-?m dual-wavelength lasers as the pump sources,the output characteristics of THz waves was calculated.The monochromatic terahertz in the band of 1.59–2.66THz was achieved by changing the dimensions of the waveguides.The maximum out-put power could reach up to 56.12?W with the efficiency of 5.62×10^-5 W^-1 which was about two orders higher than the previous results based on the bulk crystals.The wave-guides working as the DFG medium can be applied in the integrated optics and designed with high flexibility,which are the ideal candidates for compact DFG THz sources.3.Based on the reasonable design of the quantum wells,the energy level of DFG was induced in the mid-infrared dual-wavelength quantum cascade lasers(QCLs).Two different active regions of bound to continuum(BTC)and dual-upper states(DAU)were used in the experiment.By using the techniques of external cavity system and Cherenkov radiation,terahertz waves operating at 30 k Hz with the highest peak power of 65.2?W and the tuning range of 2.25–4.5 THz were generated.The intracavity DFG-THz QCLs can work at room temperature,which provide reliable terahertz radiations for integrated practical terahertz systems.