Selectively buried ion-exchanged waveguides for photonics applications

Abstract

Selectively buried ion-exchanged waveguides in glass are investigated theoretically and experimentally for use in low-loss coupling to other optical media. Selectively buried waveguides (SBWGs) are integrated optic structures in which light makes a transition from a buried waveguide with a maximum refractive index that lies 5-20 μm beneath the glass surface to a surface section with a maximum refractive index that lies at or near the surface. The buried sections provide low propagation losses and convenient coupling to optical fibers. Surface sections allow interaction between the guided mode and a superstrate material; in these sections the glass and superstrate form a composite waveguide in which the optical field propagates in both materials as a single optical mode. Adiabatic transition regions connect buried and surface sections. SBWGs are modelled by use of the finite difference method. The refractive index profile is first computed. The mode profiles and effective indices of the modes that the waveguide supports are then solved. The beam propagation method is applied to determine how the mode changes as it propagates through the SBWG. The transition from a buried to a surface waveguide is modelled, and it is found that the transition is adiabatic and low-loss. The surface section is modelled with a polymer superstrate, and the confinement factor in the polymer is computed. Ag⁺/K⁺ ion exchange is used to fabricate SBWGs, and a thorough experimental investigation of their properties is conducted. Refractive index profiles in buried, surface, and transition regions are measured by use of the refracted near-field method, and it is demonstrated that the maximum refractive index lies approximately 20 μm beneath the surface in buried regions and approximately 3 μm beneath the surface in surface regions. A novel method of simultaneously measuring the mode profile and depth of an ion-exchanged waveguide is presented and applied to SBWGs. Losses in these devices are measured, and the magnitudes of various losses are estimated.