Study On The Laser Propagation, Control And Frequency Conversion In The Optical Superlattice

With the development of the electric poling technique at room temperature, it is possible to build various optical suprlattice in ferroelectric crystals such as LiTaO3, LiNbO3, KTiOPO4 and so on. Quasi-phase-matched(QPM) technique substantially extends the class of material available to various nonlinear optical interactions. Subsequently, optical suprlattice is widely used for optical parametric processed, pulse shaping and pulse compression. The structure of optical superlattice also changes from one-dimension to two-dimension, from single period to quasi-period, even to more complex structure. In recent years more and more requirements are demanded in Laser with the development of modern optics. To alleviate the limited wavelength coverage of available lasers, nonlinear optical frequency conversion is widely employed to extend the useful wavelength ranges of laser source.It is well known that lithium niobate crystal has a serious of outstanding properties, especially the nonlinear coefficient and the electro-optic coefficient. They change their sign due to the domain inversion. We emphasized our study on the structure design of the optical superlattice to modulate the parameters of laser. In this thesis, the main contents are following:The fabrication of optical superlattice crystal with special domain-inverted pattern is the foundation of our researches. From the micro structure of the ferroelectrics, we found four stages of the domain inversion under the room temperature in external field poling procedure. Some elements that affect the quality of the poling are discussed. Then the fabrications of the periodic and aperiodic optical superlattice are presented, and the aperiodica optical superlattice is successfully fabricated. The electro-optic effect and the nonlinear optical frequency conversion in periodically poled lithium niobate(PPLN) are studied. The wave-coupling equations including polarization coupling and frequency doubling in PPLN is proposed. Numerical solutions of the coupling equations show that there is a competition between the polarization coupling and second-harmonic generation(SHG) in a PPLN under the phase-matching condition. The influences of the external electric field on the polarizations and amplitudes of three interactive waves are studied. We found that the original energy coupling behavior among three waves can be broke by another light. The numerical results show that the magnitude of the three interactive waves can be controlled by the intensity of the controlled light. These results are beneficial in the design of optic controlled optic devices. In order to alleviate the limit of the wavelength and the crystal temperature, an aperiodic optical superlattice is engineered in the cascaded process. So both polarization and magnitude of the arbitrary second harmonic can be manipulated by an external DC electric field, which is useful when simultaneous frequency conversion and signal tuning are desired.The theory of pulse shaping by second-harmonic generation with nonuniform QPM gratings is presented. And the procedures of designing different gratings for different pulse shaping are introduced. A new procedure of designing the QPM gratings proposed by our group is introduced. For the purpose of pulse compression, Genetic algorithm is selected to search for the optimized structure of QPM gratings. In order to minimize the effect of the unconsidered frequency components on the generated compression pulses, phase-blanking effect is taken into account of designing the one-dimensional aperiodical optical superlattice. Hierarchic Genetic Algorithm(HGA) is chose to design the QPM grating's structure, which can modulate the spectrum and phase of the generated second harmonic simultaneously. Simulation results show that the quality of the output pulse is fairly improved.A temporal walk-off effect occurs as a result of large group velocity mismatch(GVM) in the frequency conversion of ultrashort pulse. Only thin nonlinear crystal can be used for femtosecond laser frequency conversion. The procedure of design the aperiodically quasi-phase-mathced gratings was extended to the difference-frequency generation(DFG) and sum-frequency generation(SFG) for wavelength shift of optical ultrashort pulses. Under the assumptions of plane waves, undepleted pump, unamplified signal, slowly-vary envelopes, the general derivation of the ouput idler light are presented. A cw pump with sharp line-widths is employed together with a Ti: sapphire laser during the DFG and SFG process. Because SFG between two interactive pulses during SHG is avoided in this situation, the effect of GVM can be eliminated and the conversion efficency can be significantly enhanced with a relatively long crystal.