| In recent years, with the development of optical communication, optical information processing and optical computation, plus the improvement of material science and manufacturing technology, the ability to integrate optical, photoelectric and electronic component in a single structure or substrate make OEIC(opto-electronic integrated circuit) and IOC(integrated optical circuit) with single or multiply functions come true. It is estimated by 2020, the speed of development of OEIC and IOC will be equivalent to that of micro electronic technique in 1970s. Multi-functional optical components and opto-electronic integrated components will be serialized and the speed of integrated optical signal processing will be up to 1 GHz. Optical waveguide, as elementary unit of integrated optics, has a significant effect on manufacture of various kinds of components. Optical waveguide is to confine light in a small area to enhance optical density and better use the nonlinear property of crystal. Generally methods to fabricate waveguide include ion implantation, ion exchange, metal diffusion, epitaxial growth and so on.Ion implantation can be divided into light ion implantation and heavy ion implantation. Mechanism of both implantations is different. Light ions such as He and H will 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. The problems faced by light ion implantation are high implantation dose needed to form waveguide, simply about 1016 ions/cm2, and high costs. Heavy ions with high energy and low dose mainly cause crystal lattice disturbance in the range of ion implantation, change the property of birefringence of crystal; enhance the refractive index of surface of crystal material affected by ion implant.Ion exchange is a method which is take place under certain condition, used to substitute ions in substrate for the ones in mixed solution and end up with waveguide with refractive index changed.KTP crystal is an excellent nonlinear optical material, with high nonlinear optical coefficient, high optical damage threshold, large linear electro-optical coefficient, and high thermal stability, etc. These advantages make KTP widely used in a variety of frequency doubling and optical parametric oscillation. In addition, KTP crystal can be used as an integrated optical device which is benefit to achieve the miniaturization of optical devices.This thesis is mainly concentrate on planar waveguide and channel waveguide formed by Rb+/Ba2+-K+ exchange in KTP crystal and modulate the mismatch between the diffusion depth of Rb+ and Ba2+. The main contents include:(1) Proper molar concentration of RbNO3 and Ba(NO3)2 and exchange temperature is significant to form waveguide. According to many research papers, KTP is easily subject to smashed into pieces when exchange temperature is higher and the molar concentration of Ba2+ is bigger. For this reason, we research the ion exchange under different temperature and different molar concentration. In our experiment, we choose two kinds of RbNO3 and Ba(NO3)2 which is 92%:8% and 97%:3%, respectively. The exchange temperature is selected at 380℃å'Œ340℃. Results show that KTP is more likely to end up with cleavage when the molar concentration of RbNO3 and Ba(NO3)2 is 92%:8% and the exchange temperature is 380℃.(2) The diffusion depth is deduced according to the research results combined with iWKB(inverse Wentzel-Kramers-Brillouin) method and RBS(Rutherford Back Scattering). The fitting results indicate that the mismatch existed in the diffusion depth of Rb+ and Ba2+ in KTP crystal and the diffusion depth of Ba2+ is less than 1/e of that of Rb+. And that is the main reason which cause that PPKTP present a number of drawbacks such as domain inversion depth is shallow, domain inversion area mismatch with index enhanced area, waveguide formed is generally multimode and so on. on the basis of John Bielein's theory, ion exchange only with Rb+ could cause domain inversion and the doping of Ba2+ or other divalent ions is the key reason. Ion implantation is supposed an effective method to solve this problem. So in our experiments a barrier is formed by He+ implantation to confine the further diffusion of Rb+ and Ba2+. Single mode waveguide is achieved and the exchange depth is contained in 1μm.(3) To achieve QPM(Quasi Phase Matching) needs the periodical modulation with ferroelectric domain material. Thus we try to form periodical channel waveguide in KTP crystal. To our knowledge, methods to obtain domain inversion include electric polarization and ion exchange. Due to high conductivity of KTP crystal, it is not to achieve domain inversion. Ion exchange, as an alternative method has a drawback which can induce mismatch between domain inversion depth and waveguide depth. Thus a Cr coating patterned with periodical open strips is deposited on the KTP surface. An open area without Cr coating can undertake ion exchange while other area cannot. The near field distribution of channel waveguide is measured by end face coupling. |
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