| Terahertz wave is located between the microwave and infrared wave.It has many special properties because of its special position in the electromagnetic spectrum.It has many applications in the fields such as spectroscopy,imaging,nondestructive testing,security checking,radar and communication.Terahertz source is critical to the development of terahertz technology.Terahertz parametric source based on the stimulated polariton scattering is an outstanding method to generate the terahertz radiation.It has many advantages such as good spatial and temporal coherence,narrow line width,tunable operation,long lifetime,no need for frequent maintenance,room temperature operation and it has attracted much attentionThere are still some problems in the terahertz parametric source by now.The most critical deficiency is that both the pulse energy and the average power generated from the terahertz parametric source are small.On one hand,MgO:LiNbO3(MgO:LN)has been used as the practical nonlinear crystal for a long time.With the growing requirements for the terahertz pulse energy and average power,the low optical damage threshold of the MgO:LN crystal becomes a bottleneck and limits the terahertz parameter source output.And the common terahertz frequency range generated from the MgO:LN parametric source is in the range of 0.6 THz to 3.6 THz.On the other hand,almost all the intracavity-pumped terahertz parameter sources which are used to generate high average power adopt the diode-end-pumped configuration.In this diode-end-pumped system,the thermal effect in the laser medium is serious and the fundamental beam size in the nonlinear crystal is small,which will limit the average output power of the terahertz parameter source in some degree.In this paper,a series of efforts are carried out to improve the output energy and average power of the terahertz parametric source based on the RbTiOPO4(RTP)crystal Firstly,an extracavity-pumped terahertz parametric oscillator based on the RTP crystal is realized.Secondly,we adopt the diode-side-pumped configuration to alleviate the serious thermal effect in the laser medium and increase the fundamental beam size in the nonlinear crystal.The terahertz parametric oscillators with high pulse repetition rate and high average power based on the KTiOPO4(KTP)and RTP crystals are realized respectively.Thirdly,the stimulated polariton scattering and stimulated Raman scattering are successfully combined in the Stokes resonator,and a novel wavelength-tunable Raman laser is obtained.The concrete contents of this dissertation are as follows.1.The dispersion curve and the absorption curve of the polariton in the RTP crystal were calculated and analyzed.The crystal used for the surface-emitted configuration was designed according to the non-collinear phase matching condition in the RTP crystal.2.An extracavity-pumped terahertz parametric oscillator based on the RTP crystal was realized.The extracavity-pumping source was a 1064 nm Q-switched Nd:YAG laser,and the terahertz wave was abstracted using the surface-emitted configuration.The terahertz wave obtained in the experiment could be tuned from 3.01 THz to 3.09 THz,from 3.53 THz to 4.17 THz,from 4.54 THz to 4.66 THz,and from 5.35 THz to 5.89 THz.When the pump energy was 120 mJ,the obtained maximum terahertz pulse energy was 1.77 ?J at 3.85 THz.3.In order to improve the average power of the terahertz parametric source,we proposed the method of diode-side-pumped configuration to alleviate the serious thermal effect in the laser medium and increase the fundamental beam size in the nonlinear crystal.A preliminary experiment of the intracavity diode-side-pumped terahertz parametric oscillator was conducted using a KTP crystal as the nonlinear medium,and the obtained terahertz frequency-tuning range was from 3.19 THz to 5.94 THz.The fundamental beam size in the KTP crystal was measured as about 1.3 mm.When the diode pump power was 98 W and the pulse repetition rate was 6 kHz,the terahertz output power of 166 ?W was obtained at 5.74 THz.4.We used the RTP crystal as the nonlinear medium to further increase the terahertz output power based on the intracavity diode-side-pumped configuration.The fundamental beam sizes in the RTP crystal were measured as 1.2 mm before consumption and 1.3 mm after consumption.When the diode pump power was 105 W and the pulse repetition rate was 7 kHz,the maximum terahertz output power of 367 ?W was obtained at 3.85 THz.As far as we know,367 ?W is the highest output power for terahertz parametric sources based on the stimulated polariton scattering5.The stimulated polariton scattering(SPS)and the stimulated Raman scattering(SRS)were combined by elaborately designing the fundamental and Stokes resonators.The fundamental wave was firstly transformed into Stokes and terahertz waves through the non-collinear stimulated polariton scattering.When the intensity of the oscillating Stokes wave was high enough,it had potential to interact with the vibration mode to generate further Raman conversion.A cascaded SPS-SRS tunable Raman laser with high conversion efficiency was realized in the intracavity diode-end-pumped system based on the RTP crystal.The SPS-SRS Raman wave could be tuned in the range of 1107.7 nm-1108.1 nm,1109.0 nm-1112.7 nm,1114.3 nm-1115.1 nm,and 1117.8 nm-1121.1 nm,respectively.The peak wavelength was at 1118.8 nm.When the diode power was 7.9 W and the pulse repetition rate was 10 kHz,the obtained maximum output power of SPS-SRS Raman laser was 970 mW,and the conversion efficiency from diode to SPS-SRS Raman wave was 12.3%6.The output power of this cascade SPS-SRS tunable Raman laser was improved by adopting the diode-side-pumped configuration.When the pump power was 93 W and the pulse repetition rate was 5 kHz,the obtained maximum average power of SPS-SRS Raman was 3 W.In addition,we tried to design the Stokes resonator output mirror with relatively high reflectivity in a larger spectral range,and obtained the cascade Raman wave output of up to 5 orders.With the wavelength tuing of the Stokes wave,the wavelength of any order Raman wave was tuned accordingly.The main innovations of this dissertation are as follows.1.For the first time,an extracavity-pumped terahertz parametric oscillator based on the RTP crystal was realized.The terahertz wave obtained in the experiment could be tuned from 3.01 THz to 3.09 THz,from 3.53 THz to 4.17 THz,from 4.54 THz to 4.66 THz,and from 5.35 THz to 5.89 THz.When the pump energy was 120 mJ,the obtained maximum terahertz pulse energy was 1.77?J at 3.85 THz.2.For the first time,we proposed the average power improvement of the terahertz parametric source by adopting the diode-side-pumped configuration.An intracavity diode-side-pumped terahertz parametric oscillator based on KTP crystal was conducted for the first time.When the diode pump power was 98 W and the pulse repetition rate was 6 kHz,the maximum terahertz output power of 166?W was obtained at 5.74 THz.3.An intracavity diode-side-pumped terahertz parametric oscillator based on RTP crystal was realized for the first time.When the diode pump power was 105 W and the pulse repetition rate was 7 kHz,the maximum terahertz output power of 367?W was obtained at 3.85 THz.367 ?W is the highest output power for terahertz parametric sources based on the stimulated polariton scattering.4.A novel cascaded SPS-SRS tunable Raman laser was realized by combing the stimulated polariton scattering and the stimulated Raman scattering.The obtained SPS-SRS Raman wave could be tuned in the range of 1107.7 nm-1108.1 nm,1109.0 nm-1112.7 nm,1114.3 nm-1115.1 nm,and 1117.8 nm-1121.1 nm,respectively. |
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