Investigation On KTiOPO₄ Optical Waveguides Fabricated By Cs-K Ion Exchange And Assistant Ion Implantation Techniques

High-performance optical materials are indispensable to the developments of laser technology. Potassium titanyl phosphate (KTiOPO₄, KTP) is one of the most attractive materials. KTP is a nonlinear optical crystal with superior properties, such as high optical damage threshold, large linear electro-optical coefficient, high thermal stability etc. Due to the high non-linear optical coefficients, KTP is an important and necessary material in the non-linear optics field. Moreover KTP has been widely used in many fields, for examples, electro-optical modulations, acoustic-optical modulations, frequency doubling and optical parametric oscillation. Especially the quasi-phase match technology extends the applications of KTP crystals in non-linear optics field. On another hand, KTP crystal can be used as an integrated optical device such as an electro-optical waveguide or a frequency doubling waveguide, which are benefit to achieve the miniaturization of optical devices. As the most fundamental integral unit of the integrated optics circuits, the optical waveguide structure is of great importance in the field of modern optical communication.A waveguide is characterized by a region of high refractive index surrounded by regions of lower index. In this structure the optical energy can be restricted in a small space, thereby enhancing the optical energy density and make better use of the non-linear optical properties of crystal. Optical waveguide is the basic unit of integrated optics and the all-optical network. It also plays an important role in the fabrication of various optical devices.Many researchers apply themselves to explore the effective method to fabricate the waveguide on crystal. Due to the special lattice structure of KTP, the ion exchange method is commonly used to fabricate waveguide on KTP crystal. Ion exchange process results in a refractive index increment at the surface thus forming waveguide on crystal. The ion implantation is an effective method to fabricate optical waveguide. Ion implantation can generate a barrier with a reduced refractive index in the depth region of high nuclear energy deposition and the region between the barrier and the atmosphere composes a waveguide structure.Most optoelectronics devices such as optical coupler, modulator, optical switch and waveguide laser are based on channel waveguide structure. Investigation on the fabrication of channel waveguide is the foundation of application of waveguide device, and is also helpful to the application in optic-electric field.Compared with Rb-K ion exchange, few research reports about fabricating waveguides by Cs-K ion exchange can be found, especially about fabricating channel waveguide. Higher ion exchange temperature is needed for Cs-K exchange, which put very high demands for the selection of lithography mask material and conditions of the film deposition.In this dissertation, we report the formation and the properties of the planar and channel waveguide on KTP crystal fabricated by Cs-K ion exchange. Furthermore, the modulation of ion exchange waveguide structure by ion implantation is also investigated. The main contents include:(1) Planar waveguides are fabricated on KTP crystal by ion exchange in molten CsNO₃. The exchange temperature ranges from 420℃to 470℃and the time from 15 minutes to 2 hours. The prism coupling method is used to measure the dark mode spectra of the waveguide at the wavelength 633 nm and 1539 nm, respectively. The samples exhibit multi-mode waveguide at 633 nm and the number of guide modes increases with the exchange time increasing. Based on the effective index of the guide mode measured, the refractive index profiles are obtained by inverse WKB method. The exponent distribution is used to fit the refractive index profile of the waveguide, and the effective depth and the increment of the refractive at the surface are obtained. The results show that along with the ion exchange increase, the refractive index increment of the waveguide at surface will increase gradually and achieve saturation. But raising the temperature is helpful to enhancing the ion exchange.After exchange at 430℃for 15 min, the sample exhibits a single mode waveguide at 1.54μm. Higher temperature leads a larger number of TE modes. The refractive index profiles of this single mode waveguide are investigated by ellipsometry method. The WKB approximation is used to simplify the calculating process. The results show that the effective depth of the waveguide is 2.8μm and the refractive index increments in three principal directions are 0.0151, 0.0127 and 0.0139, respectively. This method can be also used to reconstruct the profile of other waveguide.The annealing behavior of samples exchanged for 60 min and 22 min are studied. The anneal temperature rises gradually from 200℃to 410℃, and process last for 3 hours. The prism coupling method is used to measure the dark mode spectra of the waveguide after each anneal. The effective index of the first mode decreases with the enhancement of annealing temperature and time, and the interval of the modes also decreases. The index profiles of waveguides are reconstructed by iWKB method. The refractive index increment at surface reduces and the effective depth increases gradually, which make the refractive distribution smooth.(2) The influence of ion implantation on the ion exchange waveguide is also studied. H, C, Cu ions are implanted into KTP crystal with the energy of 0.3keW 5MeV and 1.5MeV, respectively. After annealing, Cs-K ion exchanges are performed on these samples. The dark mode spectra of samples are measured by prism coupling method. Compared with the results of sample without suffering ion irradiation, we can find that the damage layer generated by ion implantation act as a barrier to block the diffusion of the Cs ions. On the other hand, the ion exchange exerts an anneal-like process to the samples. This anneal-like process can induce the diffusion of defects and the repair of the lattice damage, which will affect the ion exchange greatly.(3) The atmosphere usually acts as a part of waveguide to confine the propagation of light. In this case, the propagation of light and its field pattern in the waveguide will be directly affected by the surface situation of the sample. For buried waveguide, no loss from surface is introduced and usually it has relatively symmetrical refractive distribution, which makes them attractive to researchers. We fabricate the buried waveguide on KTP crystal by combining the Cs-K ion exchange and Si ion implantation. The temperature of ion exchange is 430℃and the time is 60 min. The energy and the dose of Si⁺ ion implantation are 550keV and 1×10¹⁵ions/cm² respectively. The end fire method is used to measure the near field pattern of the buried waveguide.(4) The channel waveguide is formed by ion exchange in molten CsNO₃. Cr film is applied to the z face of KTP sample by sputtering and patterned using standard photolithography techniques. During the ion exchange process, the Cr will be dissolved in molten salt because of the high temperature. The results of the energy dispersion X ray fluorescence measurement show that Cr can be detected in the region that covered with Cr film after ion exchange. That is to say, chromium can be doped into KTP in ionic form through exchange. The dark mode spectra are measured by prism coupling method and the refractive index profiles are reconstructed by iWKB method. The refractive index increase also in the region covered by Cr film, but it only exists at a very thin layer.The channel structure of the waveguide is observed by microscope and scanning electron microscope after ion exchange. The end fire method is used to measure the near field pattern of the channel waveguide at 1550 nm and 633 nm. The etching results of an ion exchange KTP channel waveguide show a different etch rate between the region covered by Cr film and that without Cr film. The fact of selective etching indicates that the doping of Cr ions may result in the change of the ferroelectric domain of crystal. The results provide some important information about fabricating periodically domain inverse KTP crystal by ion exchange.