| The common nonlinear optical effects include the second and the third order nonlinear optical effects. The second order nonlinear optical effects include second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG) and optical parametric oscillation (OPO), etc. The third order nonlinear optical effects include self-focusing, optical Kerr effect (OKE), four-wave mixing (FWM) and stimulated Raman scattering (SRS), etc. OPO is a frequently used and effective method for expanding the spectrum. In this process, the pumping wave, the signal wave and the idler wave need phase matching, so it is easy to realize the tunable wavelength operation by changing the phase matching angle. In the process of SRS, one pumping photon is converted into one Stokes photon and one phonon. As this process does not need phase matching, it is easier to operate. Nevertheless, the Raman shifts are always fixed for a certain crystal material, so it is hard to generate tunable Stokes wave in crystals by SRS.In this thesis, the generation of tunable Stokes wave based on the stimulated polariton scattering (SPS), another interaction between the material and the light, was mainly studied. SPS is a nonlinear optical phenomenon which includes both second and third order nonlinear effects. For the generation mechanism and experimental realization, SPS has both similarities and differences with OPO and SRS. Polaritons are coupled photon-phonon waves. Only the crystals which have both infrared and Raman active transverse optical modes can generate the stimulated polariton scattering and KTP is one of them. In terms of the generation mechanism, OPO is a second order nonlinear effect, SRS is a third nonlinear effect, and SPS is a process which includes both second and third order nonlinear effects. In terms of the experiments, compared with OPO, the frequency shifts between the pumping wave and the Stokes (signal) wave are much smaller in the process of SPS by using KTP crystal. Compared with SRS, it is much easier to realize the tunability of the Stokes wave based on the phase-matching in the SPS by using KTP crystal.The most important content of this thesis is the generation of the tunable Stokes wave in the KTP crystal by SPS. The pump source was a Q-switched Nd.YAG laser operating at 1064.2 nm with a repetition rate of 1 Hz. In addition, the intracavity second harmonic generation was realized by a LBO crystal. The characteristics of the Stokes wave and the frequency-doubled wave were studied and the fundamental theories of SPS were used to explain the experimental results. The main contents of this thesis include:1. The characteristics of the Stokes wave were investigated. Firstly, the Stokes pulse energies were compared when using Stokes output mirrors with different transmissions. The optimum transmission for the maximum Stokes output was found to be 60%. Secondly, the angle tuning characteristic of the Stokes wave was measured. The wavelength of the Stokes wave was tunable when the angle between the pumping beam and the Stokes beam was changed. When the angle (θext) between the pumping and the Stokes beams outside the KTP crystal was varied from 1.875° to 6.750°, corresponding to the range from 1.073° to 3.856° inside the crystal, the wavelength of the Stokes wave increased from 1076.5 nm to 1077.0 nm, from 1080.7 nm to 1081.3 nm, from 1082.6 nm to 1084.0 nm, from 1085.3 nm to 1087.2 nm and from 1090.6 nm to 1091.4 nm, respectively with four wavelength gaps. Thirdly, the relationship between the Stokes pulse energy and the Stokes wavelength was demonstrated. With a pumping pulse energy of 120 mJ, the maximum Stokes pulse energy and conversion efficiency were 46.5 mJ and 39%, respectively, corresponding to the wavelength of 1086.6 nm. Fourthly, the input-output relationships between the Stokes pulse energy and the pumping energy at different wavelengths were illustrated which showed that the energy of the Stokes wave was approximately linearly dependent on the pumping pulse energy. Lastly, the waveforms of the original pumping pulse, residual pumping pulse and Stokes pulse were recorded and the temporal relationship between these three waves was studied.2. The characteristics of the frequency-doubled wave (green light) were investigated. The dependence of the green light wavelength on θext was demonstrated. For the second harmonic generation, when θext was varied from 1.875° to 2.000°, from 2.250° to 3.000°, from 3.125° to 3.375°, from 3.625° to 5.625°, the green light wavelength increased from 538.5 nm to 538.6 nm, from 540.4 nm to 540.9 nm, from 541.5 nm to 542.1 nm and from 543.1 nm to 543.8 nm accordingly. Secondly, the relationship between the green light energy and its wavelength was illustrated. When the pumping pulse energy was 120 mJ, the maximum energy of the frequency doubled pulse was 15.8 mJ at the wavelength of 543.4 nm. Finally, the relationships between the energy of the green light and the energy of the pumping pulse were given at different wavelengths.3. The dispersion and absorption curve of polariton and the phase matching curves of the stimulated polariton scattering in KTP crystals were drawn. The theoretical Stokes wavelengths were given by calculating the intersections between the dispersion curve and the phase matching curves of polariton and the theoretical results were proved to match the experimental results very well. Based on the coupled wave equations of the pumping wave, Stokes wave, polariton and lattice vibration, the existence of the Stokes wavelength gaps was explained. |
PDF Full Text Request