Optical Waveguides By Femtosecond Laser Direct Writing Of Lithium Niobate,Lithium Triborate And Silicon Carbide Crystals:Fabrication And Applications

Integrated optical circuit can achieve high-speed transmitting and processing of optical signals,which plays a vital role in optical communication and quantum information processing.Optical waveguide,which is one of the most basic components in integrated photonics,can be applied to construct versatile and miniaturized devices,such as waveguide lasers,beam splitters,electro-optic modulators,frequency converters,and waveguide arrays.In optical waveguides,the light could be tightly confined within a micro-or nanoscale volume and transmitted through total internal reflection.By using low-loss optical waveguides,both optical density and nonlinear interaction can be greatly enhanced.Therefore,high-performance optical waveguides are of great significance for many promising applications in integrated optics and nonlinear optics.A number of techniques have been utilized to fabricate optical waveguides in crystals,such as proton/ion exchange,ion implantation/irradiation,and metal ion thermal-indiffusion.However,the optical waveguides fabricated by above techniques are limited to a planar geometry.The embedded 3D waveguide configurations,which are located at a certain depth below the surface of sample,may be fabricated by femtosecond laser direct writing.Femtosecond laser direct writing is a flexible,maskless,and truly 3D micro-and nanofabrication technique,which has been widely employed to produce photonic and microfluidic devices(e.g.,photonic quantum chips,waveguide sensors,and optofluidic chips).It also plays an important part in data storage,glass bonding,and ferroelectric domain inversion.The great success of this technique may be attributed to the ultrashort pulse width and extremely high peak intensity of a femtosecond laser.The ultrashort pulse width can suppress the formation of heat-affected zones,which results in the ultrahigh-precision material processing.The extremely high peak intensity leads to occurrence of nonlinear interactions(e.g.,multiphoton absorption and tunneling ionization)in transparent materials.In order to realize the 3D precise fabrication of crystals,a microscopic objective is usually applied to focus NIR femtosecond laser onto the surface or into the bulk of sample.Femtosecond-laser-induced nonlinear interactions are related to the creation of free electron plasmas,which may result in the material modifications in focal areas.The femtosecond-laser-induced modifications in crystals are mainly classified into two categories,i.e.,Type-I and Type-II modifications.As for Type-I modification,the refractive index changes are positive.The negative refractive index changes have been found in regions with Type-II modification.Based on these two types of modifications,a variety of optical waveguides have been created in crystals,such as single-line,dual-line,and depressed-cladding optical waveguides.These optical waveguides have been employed to construct versatile photonic devices,such as electro-optic modulators,beam splitters,waveguide lasers,and frequency converters.More intriguing waveguide-based devices will be fabricated with the indepth investigations on the interactions between femtosecond laser and crystals.Lithium niobate (LiNbO₃ or LN) is a multifunctional crystal,possessing many distinguished properties,such as large electro-optic as well as nonlinear coefficients,wide transparency range,and excellent ferroelectric effects.LiNbO₃ crystal plays a vital role in frequency conversion,electro-optic modulation,and optical parametric oscillation(OPO).Lithium triborate(LiB₃O₅ or LBO)is an important nonlinear optical crystal with high damage threshold and wide transparency range(160-2600 nm).LBO crystal can be employed for many applications in nonlinear optics and quantum photonics,such as second harmonic generation(SHG),third harmonic generation(THG),optical parametric amplification(OPA),OPO,and spontaneous parametric down conversion(SPDC).As one of the most attractive third-generation semiconductor materials,silicon carbide(SiC)crystal is of great importance for high-power and highfrequency applications in electronics and photonics.SiC crystal also has potential applications in quantum optics,such as the fabrication of efficient and bright singlephoton sources.Compared with the bulk crystal,the enhanced optical density and strengthened nonlinear interaction may be achieved in LiNbO₃,LBO,and SiC optical waveguides,making it possible to construct miniature and high-performance novel integrated photonic devices.The contents of this thesis mainly include:1)femtosecond laser direct writing of optical waveguides in LiNbO₃,LBO,and SiC crystals;2)investigations of the confocal micro-Raman spectroscopy on formation mechanism of optical waveguide;3)studies about guiding properties of optical waveguides by using end-face coupling system;4)analysis and characterization of waveguide-based photonic devices.According to the selected crystals and the types of photonic devices,the main research contents of this paper could be summarized as follows:We have achieved mode tailoring by using femtosecond-laser-written LiNbO₃ multi-line waveguides,which are fabricated by multiscan technique.The effects of pulse energy and focusing depth for fabricating multi-line waveguides have been investigated.The guiding properties of these waveguides have been studied at 632.8nm or 1550 nm.The results indicate that mode profiles of multi-line waveguides at telecommunication band can be tailored by optimizing femtosecond laser parameters.This work has potential applications for producing novel integrated photonic devices with LiNbO₃ multi-line waveguides.By using birefringent phase matching at room temperature,SHG of 1064 nm has been realized in femtosecond-laser-written LiNbO₃ waveguides.The guiding properties(i.e.,mode profiles and propagation losses)of multi-line,vertical-dual-line,and depressed-cladding waveguides have been investigated at 1064 nm based on end-face coupling system.We have also studied the effects of guiding properties on frequency doubling of 1064 nm.The results demonstrate that guiding properties along ne polarization are more important to SHG process.The maximum conversion efficiency⁰.87 % has been obtained in depressed-cladding waveguide,corresponding to maximum SHG peak power of 40.40 W.Our work paves a way for fabricating novel frequency converters by using femtosecond-laser-written waveguides in LiNbO₃ crystals.The depressed-cladding waveguides have been fabricated by femtosecond laser in PPLN crystal.Based on third-order quasi-phase matching(QPM),the SHG of 1064 nm has been realized in these waveguides.The temperature tuning curve of SHG output has been obtained as well.At the phase-matching temperature(⁹4.9?),the guiding properties of waveguides are investigated at 1064 nm.We have also studied the SHG peak power and conversion efficiency as a function of input peak power in depressedcladding waveguides.The results indicate that the maximum conversion efficiency is⁰.075% when the SHG peak power reaches to 9.64W.This work may pave a way for constructing versatile and miniaturized frequency converters.We report on 1D and 2D depressed-cladding-waveguide arrays written by femtosecond laser in z-cut LiNbO₃ crystals.The 1D waveguide array is composed of three depressed-cladding waveguides which are closely arranged along horizontal direction.The output mode profiles have been obtained when continuous-wave(CW)laser at 1550 nm or 1064 nm is incident on each waveguide.The 2D waveguide array(honeycomb structure)consists of seven depressed-cladding waveguides,which are closely arranged along both horizontal and vertical directions.The output intensity distributions have been obtained when excited with 1064 nm CW laser.Confocal microRaman experiments have been implemented to characterize depressed-cladding waveguides.Our results may enrich the contents of LiNbO₃ waveguide arrays,which are of great significance for describing discrete systems.A novel waveguide-based polarization beam splitter(PBS)has been fabricated in LiNbO3 crystals by femtosecond laser direct writing.The tailored waveguide structures have been employed to construct this monolithic PBS,in which the linearly-polarized light along no and ne polarizations can be well separated.When measured at 1064 nm along ne and no polarizations,the polarization extinction ratio(PER)can reach to 16.60 dB and 16.18 dB respectively,corresponding to insertion loss(IL)of 3.86 dB and 4.15 dB.This kind of PBS has potential applications for fabricating compact polarization conversion systems in integrated photonics and quantum optics.We report on femtosecond-laser-written dual-line and depressed-cladding waveguides in LBO crystals.Both confocal micro-Raman and end-face coupling experiments have been implemented in this work.The micro-Raman spectra obtained at waveguide cores are nearly the same as bulk,which indicates that bulk properties can be well preserved in guiding regions.In addition,these waveguides are almost polarization-independent,in which the laser at 405 nm and 810 nm can be well guided.Based on type-I phase matching,both SHG of 810 nm and SPDC of 405 nm have been obtained in laser-written waveguides.Our work may have potential applications in integrated nonlinear optics and quantum photonics.The dual-line and depressed-cladding waveguides have been produced in 6H-SiC crystals(one of SiC family,hexagonal system structure).Based on end-face coupling system,the guiding properties of waveguides have been studied at 1064 nm.Confocal micro-Raman spectroscopy(under 532 nm excitation)has been utilized to analyze femtosecond-laser-induced modifications in 6H-SiC crystals.The results indicate that the mode profiles can be tailored by optimizing femtosecond laser parameters.Blueshift of spectrum(at⁷87.05 cm-1)mainly takes place in laser-irradiated regions.According to obtained images of Raman intensity and spectral shift,we can draw a conclusion that,compared with the bulk,the lattice structures in waveguide cores remain almost unchanged.This work may pave a way for producing novel waveguide-based integrated quantum photonic devices.