Fabrication And Investigation Of Optical Waveguide Devices In Lithium Niobate And Yttrium Aluminum Garnet Crystal By Femtosecond Laser Writing

The integrated optics is a concept proposed in the 1960s as similar to integrated circuits It can limit optical information into highly integrated space for light transmission and processing.And it has faster processing capabilities,the advantages of small size,low losses,and stability compared with integrated circuits.Therefore,the traditional integrated circuit is gradually replaced by integrated optical circuit.Besides,compared with the bulky and poor stability of traditional optical systems,integrated optics can integrate optical devices with different functions and different degrees of integration(such as beam splitters,directional couplers,lasers,phase modulators,polarization modulators,etc.).It can realize the integration and miniaturization of optical information processing system,and its anti-electromagnetic interference,large amount of information,and good stability are in favors of transfer information and information processing.The optical waveguide is the most basic element of the integrated optical circuits.It is composed of a region with a higher refractive index surrounded by a region with a lower refractive index.Based on the principle of total reflection,the optical confinement can be limited to transmission in a region with a high refractive index.Traditionally,the size of the area is generally in the order of micro-or sub-micro scale.Thus,the optical density in the waveguide cavity can be enhanced to a large degree,leading to enhancement of optical properties,such as laser characteristics and nonlinear optical characteristics.According to the waveguiding structure,it can be divided into one-dimensional(ID)optical waveguides,such as planar optical waveguides;two-dimensional(2D)optical waveguides,such as channel optical waveguides;and three-dimensional(3D)optical waveguides,such as branched optical waveguides.The planar optical waveguide can only restrict the light transmission in one direction,and the channel optical waveguide can limit the light transmission in two dimensions with confining most of the incident light in the transmission direction.Thus,compared with one-dimensional optical waveguides,the optical density of two-dimensional waveguides is higher,and the size is smaller,which is advantageous for the integration and miniaturization of integrated optical circuits.The three-dimensional optical waveguides have various shapes,and common ones such as waveguide splitters and directional couplers have more complicated functions and applications in integrated optics.Therefore,two-dimensional and three-dimensional optical waveguide structures have higher research value and broader application prospects than one-dimensional optical waveguides.Optical dielectric crystals are important materials for many aspects of modern life.For example,electrooptic crystals are ideal platforms for the modulation of light phase,energy and polarizations.Nonlinear crystals are important frequency converters of light from different wavelength regions.Laser crystals are the favorite gain media for solid-state laser systems with lower lasing thresholds than glasses.Based on dielectric crystals,various optical applications can be realized through a broad variety of devices and components.With the combination of compact geometry of waveguides and useful features of crystals,crystalline waveguides become unique platforms for versatile photonic applications.So far,people have used a variety of techniques to realize the fabrication of optical waveguides in dielectric crystals,such as ion beam implantation/irradiation technology,proton exchange technology,thin film deposition technology,epitaxial layer deposition technology,metal ion diffusion,femtosecond laser writing,etc.Among them,the femtosecond laser writing technology has many advantages such as high processing precision,small thermal effect,and three-dimensional processing,and is widely applied to the three-dimensional processing of transparent optical materials.During preparation,the femtosecond laser system can easily generate pulse energy with a peak power of up to watts.When focusing on the surface or inside of the material,it induces strong nonlinear optical effects such as multiphoton absorption,avalanche ionization,or tunneling ionization.This creates a localized,high-temperature,high-density plasma near the focal point.Finally,the high-temperature plasma is rapidly quenched and cured,causing its refractive index to change.By designing femtosecond laser processing parameters(pulse energy,repetition frequency,scan rate,depth of focus,etc.),different types and structures of optical waveguides can be fabricated.Based on the optical waveguide structure induced by femtosecond lasers in dielectric materials,a variety of micro-photonic devices can be fabricated,such as beam splitters,directional couplers,waveguide lasers,frequency converters,and other active or passive devices.As the basic components of the integrated optical circuit,diversified photonic devices play an indispensable role.In addition,femtosecond laser writing technology combined with ion implantation technology can produce a fiber-like optical waveguide structure with stronger light-restricting ability,and femtosecond laser writing technology combined with acid etching can prepare photonic crystals and other photonic devices.The main contents of this paper include the fabrication of different types of channel waveguides in dielectric crystal materials by femtosecond laser writing,and the preparation of photonic devices based on optical waveguides,including beam splitters,directional couplers,waveguide lasers and photons crystal.And the properties of waveguides and photonic devices are studied through experiments.According to the different dielectric crystal materials and the different types of optical waveguide devices fabricated,the main work of this paper can be summarized as follows:Different types of channel optical waveguides were fabricated in z-cut LiNbO3 crystals by femtosecond lasers writing,including dual-line waveguides,cladding waveguides,and Type I waveguides.First,the dual-line waveguides and the cladding waveguide are fabricated in a z-cut MgO:LiNbO3 crystal by femtosecond laser writing,including three dual-line waveguides with the same preparation parameters but different scan rates and a cladding optical waveguide with diameter of approximately 30 ?m.The near-field intensity of each waveguide was measured based on end-face coupling at 632.8 nm,and it was found that the dual-line optical waveguide only supports single mode transmission along TM polarization;and the cladding waveguide supports multimode transmission along TE and TM polarization.However,the mode along TE polarization transmission is better than TM polarization.Otherwise,the measured propagation loss of the cladding waveguide is as low as 0.94 dB/cm.Based on the refractive index reconstruction,it was found that the refractive index change values of these two types of optical waveguides are on the order of 10-3.For the dual-line optical waveguides,the refractive index change becomes smaller and smaller as the scan rate increases.For the cladding waveguide,the refractive index change along the TE polarization is higher than that for the TM polarization.Then based on the refractive index distribution of the reconstructed dual-line waveguide,the transmission mode was simulated at the wavelength of 632.8 nm,which is nearly identical to the experiment.Based on the optical damage resistance of Mg2+ ions doped LiNbO3 crystal,the optical damage threshold of the optical waveguide was measured.It was found that the optical damage threshold(?105 W/cm2)of the dual-line waveguide was an order of magnitude higher than the cladding waveguide(?104 W/cm2).In addition,compared with the previously reported optical damage threshold of ion-implantation preparation optical waveguide,it was found that the optical waveguide manufactured by femtosecond laser writing technology has better optical damage resistance.In addition,we fabricated Type I optical waveguide with an elevated refractive index in z-cut Er,MgO:LiNbO3 crystals by femtosecond laser writing.It has been found that the refractive index-raising optical waveguide only supports fundamental mode transmission along TM polarization at 1064 nm and 1550 nm.Based on the Er3+ ion characteristics,the waveguide achieved upconversion fluorescence effects at 550 nm and 528 nm and down-conversion fluorescence effects at 1450-1625 nm with 980 nm laser pumping.And the waveguide emitting fluorescence intensity was higher than that of the bulk material.The final structure shows that femtosecond laser writing technology has the ability to fabricated various types of optical waveguides,providing a powerful technology for the development of integrated optical circuits.We fabricated various types of waveguide photonic devices in MgO:LiNbO3 crystals by femtosecond laser writing,including the Type I waveguide beam splitter,optical-lattice-like cladding structures waveguide beam splitter and waveguide directional coupler.For the Type I waveguide beam splitter,it is a single scan of femtosecond laser,where the scanning trace is the waveguide region,which includes 2D(1 × 2)and 3D(1 × 4)branch structures.All the Type I waveguide beam splitters only support fundamental mode transmission along TM polarization at 632.8 nm and 1064 nm.Based on the reconstructed refractive index profile,the 1 × 4 beam splitter transmission mode was simulated,which is consistent with the experimentally measured mode distribution.And the propagation loss of this type of beam splitter is less than 4 dB/cm.For the cladding-like waveguide beam splitter,it is composed of a plurality of femtosecond laser writing multiple hexagonal microstructures,due to the role of stress,the waveguide region is located in the hexagonal center region.We also fabricated 2D(1 × 2)and 3D(1 × 3)construction.And it only supports the fundamental mode transmission along TE polarization at 1550 nm,which is consistent with theoretical simulations.The propagation loss of the 1 × 3 waveguide splitter is approximately 2.1 dB/cm,and the optical power distributions of the branches are almost equal.For the Type I waveguide directional coupler,it is based on the existence of evanescent mode fields outside the waveguides.In this case,directional couplers are most frequently constructed from two coupled transmission lines set close enough together such that energy passing through one is coupled to the other.In this paper,we also designed and fabricated 2D and 3D waveguide couplers in MgO:LiNbO3 crystals by femtosecond laser writing based on the Type I waveguide.The near-field modal profile and beam propagation model of 3D waveguide coupler are experimentally and numerically investigated,and the simulations are in good agreement with the experimental results.The splitting ratio of 3D waveguide coupler is approximately equal,which implies that our fabrication approach to waveguide couplers has a potential application in integrated optics and optoelectronics devices.The results show that femtosecond laser processing technology is an effective method for preparing micro-photonic devices in integrated optical circuits.We fabricated the surface cladding-like waveguide by the cooperation of ultrafast laser writing and ion irradiation.This cladding-like waveguide was proved to have higher stronger ability in confining light compared to the monolayer waveguide,which is convenient for the laser oscillation.Under the pump at 810 nm,the stable CW and the Q-switched waveguide lasing at 1064 nm were realized.The stable CW waveguide laser has a maximum output power of 28.4 mW and the slope efficiency of 27.8%.Using the WS2 as the saturable absorber,the Q-switched pulsed laser was achieved with pulse duration of 45 ns.This work suggests that the technique with cooperation of the ultrafast laser writing and the ion irradiation may be an efficient method to fabricate diverse waveguides in a large range of optical materials.We have fabricated 3D hexagonal photonic-crystal-like waveguides in YAG crystal by femtosecond laser writing with H3PO4 acid etching.The etched rate of the fabricated microstructures is decreased with etched microchannel depth.The function of this photonic-crystal-like structure was analyzed by the beam propagation method,which indicates the photonic-crystal-like structure is an excellent single-mode waveguide in the mid-infrared range.The guiding properties are numerically and experimentally investigated,and the simulations are in good agreement with the experimental and calculational results.The measured near-field modal profiles of photonic-crystal-like waveguides exhibit fundamental transmission along both TE and TM polarizations at the wavelength of 4 ?m,and the propagation loss is measured to be as low as?0.5 dB/cm.This work suggests that the technique of the fs-laser inscription with acid etching may be an efficient method to fabricate diverse photonic crystal devices in various optical materials.