Investigation Of Optical Waveguides Fabricated By Ion Implantation And Thermal Diffusion

Integrated optics and optical communication technology is a rapidly developing comprehensive technical domain.As one of the fundamental component of the integrated optics circuits,the optical waveguide structure,which confines low emissive wave in the medium of micron magnitude,displays great importance in the field of modern optical communication.Utilizing integrated optics technology to fabricate waveguides based on the functional requirement and coupling it with fiber or fiber array is the research focus in the field of optical devices.In order to obtain waveguide,large amount of experiments and evaluations about the materials and fabrication methods were implemented since 1970s-the underway stage of both mixed and single-chip integrated circuit.High polymer synthesized material waveguide, glass waveguide and LiNbO₃ waveguide were proved to have superior performance. At present,integrated circuit is still not a mature product.The theoretical discussion surpasses the fabrication technology.Integrated circuit has various advantages such as high power density,small volume.Micromachining technology during the fabrication procedure is demanded since device scale is at the same order of magnitude with wavelength.In order to make such micro scale devices,advanced thin film formation methods and exquisite picture processing technology are necessary.Scientists and engineers have been trying to find ways to fabricate "high-quality" optical waveguide.Several methods such as ion implantation,ion substitution,thin film deposition and femto-second etching are commonly used at present.Ion implantation can be classified into two main categories named light-ion implantation and heavy-ion implantation.Waveguides formed in optical crystals through these two strategies are significantly different.Nowadays,the light ions,mainly He and H,with low energies are often implanted into the optical materials to form waveguide structures with depth of several microns beneath the surface of the sample.This structure is named barrier-type waveguide.A low refractive index optical "barrier" is formed at the end of the track where most of the displacement damage occurs.Light propagates between the optical barrier and the surface of the sample.One of the drawbacks of light ion implantation is that,the waveguide formation often needs quite high doses up to 10¹⁶ions/cm².Even so,the waveguide is still hard to fabricate in some optical materials,while heavier ion needs relatively lower doses.The mechanism of the heavy ion implantation differs according to spices of materials.It is generally considered that,during the procedure of implantation,ions implanted will induce lattice disturbance in the near-surface and the refractive index changes consequently.This type of waveguides relies on the enhanced refractive index layer and mostly be sensitive to the polarization direction of the light.Planar waveguide confines optical field one-dimensionly,which limits its application in waveguide devices,while channel waveguides restrict optical wave in two directions.Most optical active and passive devices such as optical coupler, modulator,optical switch and waveguide laser are based on channel waveguide structure.Nowadays,the exploration of fabrication methods of channel waveguide has become the research emphasis.The attempt of fabricating channel waveguide structure is of great importance in both application of waveguide and the combination between nuclear technology and optoelectronics.Thermal diffusion is also a waveguide fabrication method of forming waveguide layer by means of treating substrate with deposited layer of metal or other materials on surface in high temperature apparatus.The refractive index in the diffused waveguide layer is slightly higher than the substrate and the ordinary refractive index and extraordinary refractive index changes similarly.The modification amplitude of refractive index and the depth of the waveguide layer depend on the thickness of the metal film adhered to the surface of the substrate.In the thermal diffused layer,the concentration distribution of the diffusive material shows smooth curve.Waveguide thickness depends on diffusion time and temperature,while change of the surface refractive index lies on the thickness of the metal film.The refractive index profile along vertical direction of the diffused waveguide layer presents Gaussian distribution. If ion implantation is implemented on the surface of the diffused layer,the refractive index profile and polarization characteristic may be partly modified,and this may provide an approach of improving the property of the waveguide.Another merit of the thermal diffusion method is that,2-D waveguide can be obtained by make metal graphic mask using photolithography technique before thermal diffusion.In this thesis we mainly focus on three aspects:(1)Characteristic of planar optical waveguides that fabricated by means of ion implantation.We also tested the stability of waveguides by operate annealing experiment.(2)The fabrication method of optical channel waveguide.We succeeded in fabricating channel waveguide in KTA crystal by photographic masking and following direct H⁺ ion implantation at 500keV.Near filed optical intensity profiles were investigated.(3)We attempt to launch ion implantation on the surface of Ti:LiNbO₃ waveguide,study the characteristic of waveguide made by both these two method.More recently KTiOAsO₄(KTA),with a space group of Pna2 and a point group of mm2,has been widely utilized because of its outstanding feature.It has a higher figure of merit for SHG;larger electro-optic conversion factor high laser damage threshold.Large nonlinear coefficient is combined with broad angular and temperature bandwidth.Additional advantages of the arsenate are lower dielectric constants and higher melting point than KTP.In present work,we fabricated planar waveguide by implanting 500 keV H⁺ of high dose and investigated the dark mode characteristic of the waveguide.According to the refractive index profile reconstructed by using RCM code,we can draw that, positive change of refractive index is considered to be responsible for the waveguide formation.SRIM2006 was used to simulate the procedure of ion implantation. Thermal stability of the waveguide was studied by implementing annealing treatment.We also successfully fabricated optical channel waveguide in KTA crystal by using photographic masking and following direct H⁺ ion implantation at 500keV with dose of 6×10¹⁶ions/cm².Quasi-TM and TE guided modes are both observed by end-face coupling method.We also used MeV Cu ion implantation to form planar waveguides in KTA crystal.Dark modes are measured by prism coupling method at 632.8nm as well as telecommunication wavelength of 1539nm.RCM method was used to reconstruct the refractive index profile of the waveguide.We investigated the performance of the waveguide under annealing treatments of different parameters.After the annealing treatment,the effective refractive index of dark modes increased significantly which means the refractive index of waveguide recovered to some extent.LiNbO₃(LN)crystal is a promising nonlinear optical crystal which has series of electronic-optic properties,also easily to fabricate low loss thin film waveguide. Metal thermal diffusion method is an effective way to fabricate LN waveguide. Researchers found that Titanium is the most suitable metal resource for diffusion.The Ti:LiNbO₃ waveguide shows the best transforming properties and stable refractive index profile.The electro-optical characteristic of the diffused layer can also be primely reserved.We studied the dark mode characteristic of Ti:LiNbO₃ optical waveguides. Titanium films of different thickness(20-100nm)were deposited by electron beam evaporation on LiNbO₃ crystal surface.Diversiform diffusion parameters were used in order to form waveguide with high quality.500keV He ions at dose of 3×10¹⁶ions/cm² were then implanted to the surface of the Ti:LiNbO₃ waveguides.We tested the dark mode and near field optical intensity profile both before and after the ion implantation for comparison and analyse.