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Second Harmonic Generation Of Periodically Poled Microstructure Using Ultra-short Laser Pulses

Posted on:2010-09-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:S G ZhangFull Text:PDF
GTID:1118360302957508Subject:Optics
Abstract/Summary:PDF Full Text Request
Nonlinear frequency conversion technology has being an attractive research topic with the advent of laser.And Quasi-phase Matching(QPM) based on periodically poled crystals becomes one of the most common and the most effective method to achieve tunable wavelength emission,to some extent,being the decisive point involving the further development of traditional laser sources.Nowadays, Quasi-phase Matching Second harmonic generation(QPM-SHG) is widely used to generate blue or green light radiation using bulk crystal and microstructure waveguides,especially the appearance of fiber-format SHG light source represents the most ideal scheme,which is compact,stable,and with higher beam quality.Femtosecond laser is born with ultra-short pulse duration and high intensity, therefore,bringing forth extensive applications.The most outstanding application is combination with microminiature technology,which has opened a new avenue to three-dimensional nonlinear photonic device fabrication.Focusing femtosecond laser into transparent materials,refractive index occurred to be locally changed under suitable parameters,which may be acted as waveguide.Comparative with traditional waveguide fabrication method,femtosecond laser direct writing has been regarded as a novel,simple,efficient method,and especially provides possibility to three-dimensional photonic device fabrication.On the basis of our laboratory facilities and the research project supported by the National Natural Science Foundation of China(60677013),the Specialized Research Fund for the Doctoral Program Foundation of Institute of Higher Education of China (20060055021),Open fund of the Key Laboratory of the Ministry of Education for Low-Intensity Nonlinear Photonics,and the Excellent Doctoral Dissertation Engagement Fund,this dissertation work focuses on the research of ultra-short laser frequency doubling and dielectric waveguide fabrication using femtosecond laser pulses.And the main contents of the dissertation are as follows:1.Theoretical study of QPM-SHG and farication of PPKTP crystal The tuning and tolerance of poling periodicity of the inversion domain,crystal temperature and fundamental wavelength were analyzed using differential method. Aspects from the pre-processing of crystal,fabrication of pulse electric pole, monitoring of domain inversion process online and evaluation of poling quality, domain inversion process of KTP crystal was discussed in detail.And the corresponding poling parameters were optimized.Under high pulse electric field of 2.8 kV,high quality periodically poled KTP crystal was fabricated with the duty cycle close to 50%.2.Experimental research on QPM-SHG in bulk crystalsQuasi-phase-matched second-harmonic generation in periodically poled lithium niobate is used to generate 90.6 mW at 775.9 nm with a single-pass conversion efficiency of 14.7%.A tuning bandwidth of 2.1 nm,and a tuning temperature range of 150.6±1.7℃can be achieved.The Er-doped seed fiber source is amplified by a clad-pumped Er3+/Yb3+-codoped fiber laser with a high output power up to 2.18 W over a tunable wavelength range from 1535 nm to 1570 nm.QPM-SHG with power of 150 mW,bandwidth of 2 nm,tuning temperature range of 150±1.8℃was obtained under mode-locking operation.In addition,green radiation with tuning bandwidth of 1.2 nm was obtained by a Q-switch Nd:YAG laser with a low repetition rate of 10 Hz and with pulse duration of 5 ns.3.Simulative research on ultra-short laser propagation in QPM crystalThe nonlinear propagation equation was firstly built,and numerically solved using Split-step Fourier Method.Considering the contribution of group velocity dismatch, group velocity dispersion,self-phase modulation(SPM) and self focusing effect to the propagation of laser pulse,the corresponding time domain,frequency domain characteristics of pulse and influence on conversion efficiency were obtained,which is used to optimize QPM-SHG system.4.Research on femtosecond laser written LN waveguidesSelf focusing and spontaneous repetitious self focusing phenomena were simulatively obtained,which is a feasible technique to parallel fabrication,a potential laser manufacturing trend.Additionally,femtosecond laser pulse propagation characteristics and ablation mechanism in transparent dielectric crystal were theoretically analyzed,which is completely different to the case of long pulse propagation.Finally,femtosecond laser direct writing waveguide fabrication in LN crystal was carded out.And the influence of laser pulse energy and laser scanning speed on width and depth of written channels was experimentally obtained.The 1550 nm band fiber loop laser was set up to measure the written channel's propagation loss,and the results indicate that the channels acted as waveguides, which were fabricated using about 20μJ pulse energy and scanning speed between 300μm/s and 400μm/s.The guiding property may be induced by outward diffusion of Li+ under thermal effect.5.Research on QPM-SHG of periodically poled crystal waveguides using femtosecond laser pulsesThe fabrication and characterization of thermally stable typeⅡwaveguides in Z-cut periodically poled lithium niobate crystal by using a femtosecond laser was presented,and utilized for QPM-SHG,which pumped by an Er-doped seed fiber source amplified by a clad-pumped Er3+/Yb3+-codoped fiber laser.A quasi-phase matching wavelength of 1548.2 nm,a tuning bandwidth of 2 nm,maximum power of 230 mW and a tuning temperature range of 150.4±1.6℃were achieved,yielding conversion efficiency of 34.8%.When pumped by a Ti:Sapphire combined with OPA fs laser system,emitting 1550 nm band radiation with pulse width of about 40 fs,the SHG power of 2.26 mW was obtained,yielding conversion efficiency of 8%, and single mode propagation can be observed using CCD camera.The fabrication and characterization of typeⅡwaveguides in c-cut periodically poled potassium titanyl phosphate crystals was presented by using a femtosecond laser,and were utilized for second harmonic generation at 1064 nm.Experiments have shown that single mode propagation was observed at optimal waveguide width of 14.5μm. Maximum SHG pulse energy of 1.26μJ,and corresponding conversion efficiency of 39.6%can be achieved,which is higher than that of bulk crystal 26.1%.
Keywords/Search Tags:quasi-phase matching, second harmonic generation, femtosecond laser, optical waveguide, fiber laser, periodically poling, lithium niobate, potassium titanyl phosphate
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