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Characterization of refraction at a waveguide step for fabrication of mode index lenses

Refraction of guided TE modes at thickness discontinuities in an Air/Si3N4/SiO2 planar waveguide was measured by collecting light scattered into the far field from different modes. The stepped waveguides guides studied in this work are the basic refractive interfaces from which we have constructed mode index lenses. Experimental measurements of the refraction at straight step discontinuities are compared with theoretical calculations of refraction in waveguides. The coupling between incident and undesirable transmitted guided modes was found to be lower than that predicted from calculation for an ideal step. A short taper at the step may be responsible for this improvement. The maximum angle of incidence between the incoming guided mode and the normal to the interface with acceptable transmittance was found to be 50 degrees (i.e. the angle measured in the plane of the waveguide). This result is being used in other work on mode index lens design.