To reduce the coupling loss of a fiber-to-ridge waveguide connection, a planar silica spot-size converter for a wavelength of 1.55 \mum is implemented in the form of a nonperiodic segmented waveguide structure with irregular tapering. A simple single-step lithography process is sufficient for the fabrication of the planar structures. An evolutionary algorithm has been successfully applied for the optimization. The simulated results obtained with a three-dimensional (3-D) finite difference beam propagation method (FD-BPM) program are compared with measurements of implemented couplers, showing very good agreement. A waveguide-to-fiber coupling efficiency improvement exceeding 2 dB per converter is shown. Structures obtained with this approach are very short ({\sim}140\;\mum) and simple to integrate on the same wafer with other planar structures such as phased arrays or ring resonator structures.

[IEEE ]

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