| The integrated photonics system is composed of different optical components with different functions(such as laser light sources,optical couplers,polarizing elements,and gratings)onto smaller substrates to form some complex optical systems.Due to the good properties,such as the characteristics of miniaturization,diversified functions,and fast light response speeds,the optical systems have been widely applied to optical fiber communication,chemical analysis,medical detection,and spectral investigation.With the development of science and technology,the research of new integrated photonics systems has been moving towards miniaturization and multifunctionality.The designs of the new product of integrated photonics devices is not only to reduce the volume of structures,but also improve the optical properties as much as possible(reducing optical transmission loss,as well as increasing the resistance of light damage)So that the properties of the optical components are supposed to be improved for the applicationOptical waveguide is one of the basic components of integrated optical systems The waveguide structure is able to limit the light transmission in a very small volume where can easily achieve high light density.The propagation loss can also be reduced and in the small volume.In addition,the optical properties of the material could be improved,such as the laser properties of the dielectric material,and the nonlinear characteristics of some nonlinear optical crystals,which can be used to achieve signal amplification and reduce the threshold of laser excitation.As for the integrated optics application,the optical waveguide structure can be used as a suitable absorber with high integration and high stability to induce the generation of waveguide lasers.The structures can also be used as a high-precision sensor which can achieve optical integration on the micro scale.Because of these outstanding properties,optical waveguides structures have been widely accepted in the integrated optics.So far,the new types of photonic devices,based on optical waveguide microstructures on optical materials with different properties,have gained much more interest of researchers.Such as electro-optic modulator,waveguide directional coupler,frequency and converter,waveguide laser and so on.In order to obtain high-performance optical waveguides at the micro scale,A variety of techniques which are used to produce waveguides in photonic crystals have been well developed,including Ti ion diffusion,proton exchange,ion beam irradiation and ion implantation,chemical surface deposition technology and femtosecond laser direct writing,etc.Femtosecond laser direct writing has gradually developed into a standardized and precise waveguide processing technology due to its short laser response time,high precision,low thermal effect,and controllable effects on the properties of bulk materials.Through femtosecond laser direct writing,researchers have successfully obtained a variety of waveguide structures in glass,organic materials,ordinary optical crystals,and doped optical crystals.Among the most dielectric crystals,optical waveguides or waveguide structures fabricated by femtosecond lasers direct writing can be simply classified into two types,including so-called Type Ⅰ modification(refractive index increases inside the irradiated region,serving as the waveguide core)and Type Ⅱ modification(index decreases at irradiated points whereas increased at the vicinal regions).By changing the parameters of femtosecond laser(femtosecond laser pulse energy,pulse width,scanning depth,scanning speed,etc.),the two types of waveguide structures can be produced in some optical crystals.Among the dielectric crystals used to produce the optical waveguides by femtosecond laser direct writing,lithium niobate crystals(LiNbO3)have been considered as one of the outstanding waveguide materials due to its excellent electro-optical and nonlinear optical properties.Femtosecond laser-induced Type I and Type II modification can both be obtained in this material.In addition,lithium niobate crystals can also be doped with rare earth ions,such as neodymium ions(Nd3+)or europium ions(Er3+),which can effectively improve the light damage threshold and fluorescence properties of the crystals.In addition,the confocal micro-photoluminescence technology(μ-PL)and micro-Raman technology(μ-Raman)can be used to characterize the local lattice changes of the lithium niobate matrix induced by femtosecond lasers,and compare their fluorescence and Raman properties.(including peak intensity,position,and peak width).Through that the lattice changes of the Type Ⅰ and Type Ⅱwaveguide regions can be compared,and the possible mechanism for their refractive index changes is supposed to be well investigated.The main work of this thesis is based on lithium niobate crystals.Through the femtosecond laser direct writing technology,optical waveguide structures based on Type Ⅰ modification and Type Ⅱ modification are written in the same lithium niobate crystal.By comparing the light-guiding properties,fluorescence properties,and Raman characteristics of waveguide structures,the mechanism of changes in the lattice structure and refractive index of two different types of waveguide structures is revealed,and the application value of the two types of waveguides in the field of nonlinear optics is expected.The main research work and results of this paper are summarized as follows:Based on z-cut neodymium-doped magnesium oxide-doped lithium niobate crystals,optical waveguides based on Type Ⅰ single-line structure and Type Ⅱ dual-line structure were produced in the same crystal by femtosecond laser direct writing technology.In the 633nm laser end-coupling experiment,the light-guiding properties were studied,and the two optical waveguides proved that they can only restrain the TM polarization with weak sensitivity to TE polarization.Through confocal micro-spectroscopy(fluorescence and Raman tests),the lattice structure changes of the two waveguiding regions have been obtained,and direct evidence was found for the change in refractive index of the two waveguide regions induced by femtosecond laser.It was found that through experiments that the fluorescence and Raman performance of the femtosecond laser in the light-guiding region of the Type Ⅰ and Type Ⅱ waveguides were basically preserved,but the fluorescence and Raman properties in the Type Ⅱfemtosecond laser irradiation region were decreased by more than 50%.In addition,the Raman experiments have found that the different Raman peak shifts of the two types of waveguides are observed to be significantly different.Compared to the lithium niobate matrix(the bulk region),the Raman peak of 250cm-1 and 633cm-1 are observed to be both blue-shifted at Type Ⅰ and Type Ⅱ waveguiding regions.Whereas the Raman position of peak at 270cm-1 remains unchanged;however,the 250cm-1 and 633cm-1 peaks of the Type Ⅱ optical waveguide are both red-shifted,while the the Raman peak of 270cm-1 is blue-shifted.Based on the analysis,it is concluded that the changes in the lattice structure of the Type Ⅰ single-line and Type Ⅱ dual-line optical waveguide regions are due to the effect of stress due to the generation of slight lattice defects and laser-induced severe lattice damage,respectively. |
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