| The electro-optic effect is a significant effect of second-order nonlinearity inferroelectric crystals, and can be divided into two parts on the basis of direction of appliedelectric field in crystal, one is longitudinal electro-optic effect(Pockels effect), and anotheris the transverse electro-optic effect. Periodically poled lithium niobate (PPLN) is a kind offerroelectric crystal whose nonlinear coefficients are modulated in space and extensivelyapplied in nonlinear processes such as second harmonic generation, optical parametersamplifying, and optical parameters oscillation under the condition of quasi-phase-matching.The main contents in this dissertation, based on the electro-optic effect in PPLN, areinvestigated theoretically and experimentally in three fields which can be listed as follows,To achieve the group velocity modulation of femtosecond pulse in Mg-doped PPLN,we specially choose the wavelength which is corresponding to non-phase-matching as anoperating wavelength, and the entire nonlinear process is carried out in the condition ofphase-mismatching. Moreover, we derive the second-order nonlinear coupled waveequations in the presence of DC electric field, and take into account the nonlinear shiftinduced by third-order nonlinearity effect including self-phase modulation and cross-phasemodulation to the modulation of group velocity of ultra-short pulse. Several impact factorssuch as: electrical field strength, intensity of injected pulse, temperature,phase-mismatching are contributed to the scheme for group velocity modulation. Since thelimitation of coercive field and anti-optical power threshold in Mg-doped lithium niobate,we cannot ignore the influence of high electric field and high power of injected pulse on the shape of output pulse, therefore, we confirm the maximum of the electric field strengthand the injected pulse which are applicable in the scheme for group velocity modulation. Inaddition, we compare the joint effect of electro-optic effect and third-order nonlinearitywith the influence of electro-optic effect only, and the results manifest that the formereffect is greater. So we can conclude that the modulation scheme with the assistance ofthird-order nonlinearity is a good and effective guidance for experiment.The magneto-optical Faraday effect is that a linearly polarized light propagated alongtransparent medium in one round trip, and its angle of rotation of plane of polarization istwice larger than the one in a single trip. But under the condition of quasi-phase matching,each domain in PPLN can be considered as a half-wave plate, the plane of polarization oflinearly polarized light which propagated in PPLN within the help of electric field mayrotate along c-axis. The further studies show that the parity of number of domains incrystal determines the category of optical activity. When a PPLN contains an even numberof domains, it exhibits a natural optical activity. However, when a PPLN contains an oddnumber of domains, it exhibits strongly non-reciprocal, that is similar to magneto-opticalFaraday effect. Therefore, we have a new proposal that we called it as “Faraday-like effect”in analogous to the magneto-optical Faraday effect, and show the value of “Verdet-likeconstant” of5mol%Mg-doped PPLN. We also propose a new structure, which contains anodd number of domains tailed with a semi-domain (O&HPPLN) and break through theimprisonment of the traditional viewpoint that the number of domains must be integer.Based on the new type of structure, an electro-optical isolator is presented.To realize the requirement of uniformity of ultra-high power of beam spot in ICF, oursimulation results show that the simultaneous use of smoothing by spectral dispersion(SSD) and random phase plate (RPP) can significantly improve the smoothing of beamsspot. Furthermore, we achieve the conclusion that the uniformity of spot will be furtherenhanced by involvement of multiple microwave frequencies in SSD. Finally, from modeling to experiments, we have fabricated successfully a kind ofelectro-optic modulator which can be adapted to multiple microwave frequencies. Theentire process mainly includes some steps as following, Firstly, based on theory ofquasi-velocity-matching, we design an appropriate aperiodic domains inversionarrangement by simulated annealing algorithm to verify theoretically the responses to eachmicrowave frequencies within the target bandwidth. Fortunately, the depth of modulationwe achieved are equal. Secondly, we achieve successfully large scale aperiodic domaininversion by room temperature poling technique, and introduce some details in polingstage to readers. At last, the sample test results manifest that the depth of modulation inexperiments can meet the demands of design. |
PDF Full Text Request