The new polarization integrated-optic scheme based on weak-guided waveguide in glass is proposed - the passive 90-degree convertor of linear polarization. The converter has the evoulution type of it working principle. Converter consists of four-layer waveguide, which includes ion-exchange waveguide in glass with high-index cladding film, having tapered thickness or (in another variant of converter) tapered width. The working principle of converter has been explained with mode analysis techniques. The calculations showed that in the small range of thickness (or width) of nano-scaled cladding film the second and third waveguide modes have nearly equal effective indices and hybrid structure of fields, and out of this range the same modes have the field structure of modes TM0 and TE1. The smooth (adiabatic) increase of waveguide cladding film thickness (or width) along it longitudional geometric axis means that such waveguide is a polarization converter, which transforms mode TM0 to mode TE1.
Keywords: Integrated optics, polarization converter, optical waveguide, guided-wave mode
Physico-mathematical modeling of multi-step processes of waveguide fabrication in glass K-8, based on the combined, solution of two-dimensional nonlinear diffusion equation, electrostatic equation and wave equation has been made. The diffusion equation was solved by application of implicit finite-difference two-layer Crank-Nicolson scheme with appropriate boundary conditions for each step of technology. The general case of three-step technological process is considered, including thermal ion exchange ”silver-sodium” in salt melt mixture AgNO3:NaNO3, electric field-assisted waveguide burial in salt melt NaNO3 and final step of glass substrate annealing. The technological conditions for the fabrication of single-mode waveguides, which have the size of mode field suitable for efficient butt coupling of waveguides with optical fibers at 1.55 micrometer wavelength, have been determined. The calculated mode size of waveguides is 10.6*9.5 um; the coupling efficiency with fiber is above 99%. Also the modeling of partially buried waveguide has been performed, where the field-assisted burial was made with the deposited mask covering the half-surface of ion-exchanged waveguide. Such waveguide has asymmetric distribution of refractive index and hybrid guided modes, and it may be the base of passive waveguide polarization TE-TM converters.
Keywords: integrated optics, optical waveguides, ion exchange in glass, electrically stimulated migration of ions, the nonlinear diffusion equation, waveguide mode