A very short planar silica spot-size converter using a nonperiodic segmented waveguide

To reduce the coupling loss of a fiber-to-ridge waveguide connection, a planar silica spot-size converter for a wavelength of 1.55 mu m 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 (similar to 140 mu m) and simple to integrate on the same wafer with other planar structures such as phased arrays or ring resonator structures.