Lasers And Nonlinear Effects In Waveguides Of Optical Crystals

The integrated photonics, which refers to the fabrication and integration of several photonic components on a common planar substrate, is a swiftly developing and synthesized technical domain ever since the end of last century. Based on the combination of photonic components, complex miniature devices with a wide range of functions could be realized and have been used as optical communication, environmental monitoring, biological and chemical sensing, etc. Waveguides are fundamental and key elements of integrated photonics that perform guiding, coupling, switching, splitting and multiplexing of optical signals. And the quality of integrated devices is determined by the performance and optical property of waveguide. Hence, the fabrication of low loss waveguides and the investigation of optical properties of waveguide are a continuous interesting topic in integrated optics.At present, several techniques have been developed to fabricate waveguides in optical materials, including ion implantation, proton beam focusing, swift ions irradiation, ultrafast direct laser writing (DLW), ions exchange, diffusion and film deposition etc. Ion implantation and DLW are two widely used methods for waveguide fabrication, due to their non-sensitivity of the structure of substrate materials. During ion implantation, the losses of incident ions include two different kinds of physical mechanisms called nuclear energy deposition and electronic energy deposition. Those energy depositions would cause the lattice distortion of the substrate material near the surface region and induce the variation of refractive index. At the end of the ion range, an optical barrier with a relatively low refractive index compared with the substrate is formed by the nuclear energy deposition. Meanwhile, refractive index would increase or be slightly disturbed between the air and the optical barrier, which is mainly decided by electronic energy deposition. The propagation light would be well confined by air and optical barrier in optical waveguide. For DLW, femtosecond lasers are used for buried three-dimensional waveguide fabrication due to nonlinear absorption of focused pulsed laser beam. The technique for writing waveguides can be sorted into two strategies. One relies on the direct refractive-index increase at the laser focus, and another one causes the refractive index decrease at the laser focus while indirectly increases on its surroundings. Both processes depend on the material and on the writing conditions and have a complex physical origin which involve several processes, such as ionic rearrangement, lattice stress, heat accumulation, etc. Although kinds of waveguide structures have been formed in masses of optical materials(such as optical crystals, glass, semiconductors etc), fabrication of waveguide with high quality in new materials is still a focus in integrated optics according to the benefit of wide applications of ion implantation and DLW.Waveguide structure formation is based on the modification of refractive index of bulk materials by fabrication techniques. The modification would change properties on the bulk, which are of great importance to the practical applications. Hence, investigation of property modification in waveguide is significant. On the other side, the waveguide is designed in micrometers and confines light diffraction in one or two dimensions. The specialties in the waveguide structure lead to the unique features of waveguide compared with bulk material. In such structure, the light is confined within a small volume and optical density could reaches a high level with low input power. As a result, the corresponding features of the bulks, such as nonlinear response or ability for laser generation, may be improved to some extent within waveguides. Based on the investigation of fabrication and properties, waveguide could be appropriately designed for integrated photonics devices with different functions in various research fields.In this dissertation, we report fabrication, characterization and application of waveguide. Ion implantation and DLW were used to fabricate channel, planar and ridged waveguides in variety of materials, which include SBN, KTP, Nd:CBN, Nd:YVO4, Nd:GdVO4, Nd:YLiF4, rare earth doped LiNbO3, Nd:YAG ceramics and Er3+/Yb3+ co-doped phosphate glass. Prism coupling method was introduced to investigate the refractive index distribution of waveguides. The propagation modes in waveguide were analyzed by end-facet coupling method. A confocal microscope was applied to study the fluorescence properties in rare-earth doped waveguides. Photorefractive property in waveguide was investigated by two-wave mixing method. Besides, we analyze the thermo-optic and etching properties in implanted waveguide. According to previous work, kinds of waveguide laser were generated, such as single-wavelength laser and simulated dual-wavelength laser. A new method was developed to form reconfigurable optical waveguide array. And we analyzed binary gap solitons in the waveguide array formed by this method. We also found a new method to fabricate ridged waveguide by wet etching.Neodymium-doped yttrium orthovanadate Nd:YV04 is one of most used gain media for solid state laser generation owing to its outstanding features high emission cross section, broad absorption bands, good mechanical and thermal properties. We fabricate the channel waveguides in Nd:YVO4 crystal by ultrafast direct laser writing method. The confocal fluorescence images revealed that the waveguide is constituted by a locally compressed area in which the original fluorescence of the Nd:YV04 system are preserved. Waveguide laser was generated at 1064nm with pumping at 808nm. Through the well design of input and output mirrors, dual-wavelength laser at 1.06μm and 1.34μm were also found in the channel waveguide.Neodymium doped gadolinium vanadate (Nd:GdVO4) is a well known crystal that is of special relevance for the development of compact near-infrared solid state lasers, which is owing to its excellent spectroscopic properties, high thermal conductivity, high damage threshold, high Raman gain. We report high efficiency continuous wave laser oscillations at～1.06μm from an ultrafast laser written Nd3+:GdVO4 channel waveguide under the 808 nm optical excitation. A record 17 mm/s writing speed was used while the low propagation loss of the waveguide (-0.5 dB/cm) enabled laser performance with a threshold pump power as low as 52 mW and a near to quantum defect limited laser slope efficiency of 70%.Neodymium-doped yttrium aluminum garnet (Nd:Y3Al5O12 or Nd:YAG) ceramics have emerged during the last years as an outstanding laser material capable of providing serious competition to the traditional single crystals. Indeed Nd:YAG ceramics show several advantages over their crystalline partners while retaining the outstanding fluorescence properties of neodymium ions. We report on the generation of continuous wave lasers at a wavelength of～1.06μm in a Nd:YAG ceramic waveguide at room temperature. The waveguide was fabricated by using 6 MeV carbon ion implantation at a fluence of 3×1014 ions/cm2. Laser operation has been realized with a slope efficiency as high as～11%. The pump threshold of an 808-nm laser beam for the waveguide laser oscillation is 19.5 mW.The rare-earth doped LiNbO3 system combines the excellent laser features of rare-earth with the electro-optical and nonlinear properties of LiNbO3, allowing the fabrication of many attractive devices for optical and photonic applications, such as Q-switched and self-doubling-frequency lasers. We report on rare-earth doped LiNbO3 active planar waveguides produced by ion implantation. The extraordinary refractive index of the sample surface experiences positive alternations constructing enhanced-well confined waveguide structures. After the implantation, the sample was annealed to reduce loss of waveguide The propagation loss of the waveguide was measured to be less than 2 dB/cm, which means acceptable quality for further guide-wave applications. The micro-luminescence spectra of the waveguide show fairly good potentials for laser action.Lithium niobate (LiNbO3) is one of the most favorite materials for integrated optical applications owing to its combination of many intriguing features, e.g., excellent thermo-optic, electro-optic and nonlinear optical properties. The thermo-optic (TO) properties of the lithium niobate waveguide fabricated by oxygen ion implantation at three different energies (2,3 and 6MeV) have been investigated. It is found that, as the electronic stopping power (Se) of the O ions is below a threshold of 2.2 keVnm-1, the TO features are well preserved in the waveguide regions. When Se is above this value, the TO coefficients of the waveguides are considerably modified, which is attributed to the increased defect generation in the crystal.We report on a new, simple method to fabricate optical ridge waveguides in a z-cut LiNbO3 wafer by using proton implantation and selective wet etching. The measured modal field is well confined in the ridge waveguide region, which is also confirmed by the numerical simulation. With thermal annealing treatment at 400℃, the propagation loss of the ridge waveguides is determined to be as low as～0.9 dB/cm. In addition, the measured thermo-optic coefficients of the waveguides are in good agreement with those of the bulk, suggesting potential applications in integrated photonics.Photorefractive (PR) materials, in which rather large light-induced refractive index changes can be obtained at low optical power levels, have attracted great attention for their potential applications in holographic storage and optical communications. The photorefractive properties of optical planar waveguides in Fe:LiNbO3 crystals fabricated by ion implantation are investigated. Two-wave mixing experiments are carried out for both the waveguide and the bulk. The results show that the measured gain coefficients are almost identical for the waveguiding layer and the substrate. In the waveguide, the response time could be reduced by one order of magnitude, with respect to the bulk, at the same power level of the incident light.Photovoltaic photorefractive binary waveguide arrays are fabricated by implantation and selective light illumination on top of an iron-doped near stoichiometric lithium niobate crystal. Linear discrete diffraction and nonlinear formation of gap solitons were investigated by single-channel excitation using Gaussian light beams coupled into either wide or narrow waveguide channels. The results show that, at low power, linear light propagation leads to discrete diffraction, whilst for higher input power the focusing mechanism dominates, finally leading to the formation of gap solitons in the binary waveguide arrays. Our simulation of light propagation based on a nonlinear beam propagation method confirms the experimental findings.Neodymium-doped yttrium lithium fluoride (Nd:YLiF4) is an excellent candidate for low-threshold continuous-wave (CW), mode-locked laser operation due to its many advantages, such as low thermal lensing, large fluorescence linewidth, and natural birefringence. Commercial lasers based on Nd:YLiF4 crystals have been available at wavelength of both 1047 and 1053 nm. Strontium barium niobate (SBN) is a well-known photorefractive crystal that has been successfully used to realize optical amplification, holographic storage, and self-pumped phase conjugation, and also exhibits promising potential applications for optical information processing and optical computing. Recently, another crystal from the same family, calcium barium niobate (CBN) with higher a Curie temperature 280℃has attracted rapidly growing attention for its outstanding ferroelectric and optical properties. We report on the fabrication and characterization of waveguides in Nd:YLiF4, Nd:CBN and SBN crystals by ion implantation. The guided-mode profiles are analyzed by the end-coupling method and numerical simulations. Room-temperature microluminescence investigations reveal the features of waveguides.