Spot size transformer with a periodically segmented waveguide based on InP

We report what is to our knowledge the first optimization of a spot size transformer by use of a periodically segmented waveguide for photonic integrated circuits based on InP. It is shown that a nonlinear variation of the segmentation ratio along the propagation direction allows one to make an adiabatic transition of a short length. A simple method based on the study of a beveled edge transition is presented for the optimization of such a device. Internal losses in the transition region as low as 1.2 dB are obtained between a coupling waveguide and a simple heterostructure waveguide, with a very high transition spot size expansion factor.