| THz-wave parametric oscillators(TPOs)based on parametric effect of crystal can produce high power,widely tunable THz wave at room temperature,especially the TPO coupled with Si-prism array(Si-TPO),with the advantages of high efficiency,compact and unidirectional,with important value in THz-wave spectroscopy applications.In the past 20 years,although THz-wave parameter sources have been developed continuously,there are still some problems need to be solved urgently.Especially the strong crystal absorption of the THz-wave in high-frequency range seriously restricts their output energy and tuning range at the high frequency end.In addition,frequency agility as a prerequisite technology for TPOs' application has been widely concerned.In view of this,this paper mainly studies the reduction of crystal absorption loss in Si-TPO to improve THz-wave extraction efficiency and output energy,and studies the key technology to realize automatic control tuning of Si-TPO.Theoretically,the dispersion characteristics of the crystal are analyzed according to the interaction between the electromagnetic wave and the ion crystal TO phonon.The Stokes and THz waves gain expressions of stimulated polariton scattering(SPS)are deduced.According to the lattice parameter of Li Nb O3 crystal,theoretical curves of the dispersion,absorption and gain coefficient in the THz-wave reign are calculated.Based on this,the threshold expression of TPO based on Li Nb O3 crystal is deduced.The influences of pump spot size,resonant cavity length,and other parameters on TPO threshold are analyzed.Theoretical work provides theoretical basis and data reference for the following experiment.Experimentally,an experimental setup of Si-TPO was established.The dispersion and Stokes gain coefficient curves of Mg O: Li Nb O3 crystal are measured by parameteic generation experiment.Some basic characteristics of the THz-wave generated in Si-TPO,such as spatial distribution,crystal absorption,angle tuning,tuning output and cascaded Stokes process,are studied.The influence of the pump spot on the output THz-wave energy is studied in detail.It is concluded that the TPO with pump of thickness of about 1mm in the y direction provides higher efficiency.The propagation characteristic of THz-wave in the crystal is studyed and the principle to reduce crystal absorption in Si-TPO is inferred.Enhancement of THz-wave output efficiency was realized through optimizing pump spot shape and crystal installation.By cutting the pump and Stokes lights,we obtain that the THz wave is coupled with the pump light in the crystal during its transmission,triggering new THz waves and Stokes lights.The crystal with strong pump can effectively transmit THz wave energy.To reduce crystal absotption,strong pump light in the crystal near the output surface of THz wave is needed,but it does not necessarily require the occurrence of Stokes light oscillation.A circular pump spot with a diameter of 2 mm was optimized for a 1 mm thick elongated shape,the low frequency limit of the tuning range was extended from 1.15 THz to 0.58 THz.Output energy at 1.15 THz compared to that from the diameter of 1 mm round spot increased by 2.5 times.The specific positon of crystal locates on the rotation axis,ensuring that the pump output position is consistent,which avoids crystal damage or additional crystal absorption loss.During the tuning process,the THz-wave output surface of the crystal is kept parallel to the pump light to solve the crystal absorption loss due to the tilt of the crystal.The output energy near 1.6 THz is improved by about 3.2 times.Si-TPO with the pump reflected at the THz wave exit surface(PR-Si-TPO)is constructed,Enhancement of THz-wave output energy of several tens of times is realized,and high frequency tuning range is also expanded.According to the theory of evanescent wave of total reflection,it is proposed to realize the total reflection of the incident pump light by making an air gap with specific thickness between the crystal and the prism,the pump total reflection is realized and THz-wave highly effective coupling can be ensured meanwhile.Using the air gap with a thickness of about 0.8 ?m,the PR-Si-TPO is constructed.Compared with the traditional Si-TPO under the same pumping conditions,the THz output energy is increased by about 20-50 times,the oscillation threshold is reduced by 10 %-34%,high frequency tuning range extends from 2.5THz to 3.66 THz,and the linewidth is nearly compressed by 70%,from 0.23 nm compressed to 0.07 nm.Using the crystal with precision polishing at the output end,the cut angle of the crystal was not damaged under 2 Hz and 8 m J pumping with 1 mm in diameter,and the low frequency limit of the PR-Si-TPO was equivalent to that of Si-TPO.The tuning range is 0.6-3.5THz.When the pump light energy is 15 m J,the maximum THz wave output energy of 2.4?J is obtained at 2THz,the peak power is 0.3k W and the energy conversion efficiency is 1.6×10-4The key technology to realize TPO automatic control tuning is studied.By using a reflector together with a 1: 1 telescope system,frequency tuning is realized just by rotating the angle of reflective mirror in the PR-Si-TPO,and the high efficiency of the PR-Si-TPO can be meatained.Based on the Porro prism as the angle reflector,an asymmetric cavity type is used to realize output-mirror tuning Si-TPO under double-pass pumping configuration.THz-wave output in the range of 0.8-2THz is obtained,and the threshold value is about 23% lower than that of the traditional plane-parallel cavity.Replacing the output mirror with another Porro prism that intersects the angular reflector to form a cross ed Porro-prism cavity,comparative study shows that the ability to withstand endoscopic detuning is 1-2 orders of magnitude higher than that of the traditional Si-TPO with plane-parallel cavity,which greatly enhances its adaptability to the working environment. |
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