Study And Application On Optical Multi-wavelength Conversion In Periodically And Aperiodically Domain Inverted Ferroelectric Crystal

In all-optical wavelength division multiplexing communicationnetworks, frequency conversion is the key technology to resolve thenode frequency blocking and competition. Nowadays, the frequencyconverter based on periodically domain inverted ferroelectric crys-tals have attracted much more favors and are believed to be the mostpromising one due to its pure nonlinear optical interaction, such asstrictly transparency to signal format and transmission rate, flexi-bility to realize frequency broadcasting. Nevertheless, the period-ic quasi-phase-matched structure has a negative impact by limitingthe bandwidth of the frequency conversion on the demand of multi-wavelength response for ultrafast signal processing, dual-band opti-cal coherence tomography, THz generation and so on. In this dis-sertation, we use the periodic and aperiodic domain inverted ferro-electric crystal to solve this problem, make a deep study on the op-timization process of the inverted domains’ distribution to achievetarget frequency conversion response which is then applied into spa-tial modulation of the nonlinear Raman-Nath diffraction.Quasi-phased-matching (QPM) theory is the foundation of our researches, from which we deduce the quadratic nonlinear opticalcoupling process inside of periodic domain inverted ferroelectric crys-tals, and then bring it into aperiodic crystals.After an introduction of the optical properties, especially thequadratic nonlinear optical effect, of domain inverted ferroelectriccrystals, we review the common technique to produce domain in-versions, the poling at room temperature by external electrical fieldtechnique. The samples used in the experiments of this dissertation,periodic and aperiodic, are both produced in this way.Using type I QPM in periodic domain inverted ferroelectric crys-tals, multi-wavelength second harmonic generation (SHG) at the com-munication band has been achieved. Nevertheless, sum frequen-cy generation (SFG) takes place simultaneously, and consumes thepumps’ energy together with the SHGs, which leads to the compe-tition between SHGs and SFG, resulting in a big fluctuation in thefrequency conversion response. In the dissertation, we analyze thiscompetition process, theoretically predict and experimentally veri-fy the change in the response curves of both SHGs and SFG, whichhelps to better understand the second-order nonlinear parametric pro-cesses.Type0QPM, due to its advantage of using the largest nonlinearcoefficient d33, has attract more interesting in the ferroelectric crys-tals, and various engineering has been used to offer a benefit of broadbandwidth for multi-wavelength conversion such as linearly chirpedgrating (LCG), step chirped grating (SCG) and their apodizations. However, they still bring two problems: noticeable ripples on theconversion efficiency curves lead to a bad flatness response in LCGand SCG; the small chirp step in LCG (i.e. a few hundred picome-ters), and the smallness of the initial inverted domain attributed fromapodization (i.e. one or two micrometers) lead to fabrication diffi-culties. In the dissertation, we use engineered MgO-doped lithiumniobate (MgO:APPLN) to solve these problems. Each domain ofthe crystal has an uniform width of3μm for easy fabrication, andthe arrangement of the inverted domains is optimized by the simu-lated annealing algorithm to achieve a flattop broadband for multi-wavelength conversion. Based on this engineered crystal, the multi-wavelength conversion is not only flexibly tunable through chang-ing the wavelength, wavelength spacing, and power of two employedpumps, but also selective for output by tuning the operation tempera-ture. We then numerically simulate the transmission process of seriesof picosecond pulses at1.5μm in MgO:APPLN crystal with engi-neered structure, compare its benefit with the results in other periodicor aperiodic gratings.Through a detailed analysis of the nonlinear Raman-Nath d-iffraction, we find that the frequency doubling output angles dependon the distribution of the second-order nonlinear coefficient, namelythe arrangement of inverted domains in ferroelectric crystals. Sincethe frequency doubling via the nonlinear Raman-Nath diffraction canbe used to generate and modulate the vortex beams, it is important tospatially modulate its output distribution. Therefore, we theoretical- ly deduce the connection between its output angles and the inverteddomain structure. By using the self-adjusted algorithm for optimiza-tion, we demonstrate the spatial modulation of the frequency dou-bling via the nonlinear Raman-Nath diffraction. The experimentalresults are in good accord with theoretical predicts, and parameters,such as fundamental frequency beam waist and its position of inci-dence that affect the nonlinear Raman-Nath diffraction, are discussedin detail, indicating that this type of frequency doubling is not a lo-cal reaction but should take all domains of the crystal into accounts.These analyses and discussion will help with better understanding ofthe nonlinear Raman-Nath diffraction and its further applications incontrolling orbital angular momentum (OAM) of vortex beams.