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Spotsize converters (SSCs), consisting of a lateral taper, thin-film core, and ridge, have been accurately designed to develop spotsize-converted laser diodes (SS-LDs) in the 1.3-μm-wavelength region based on the 3-D semivectorial finite-difference beam propagation method (FD-BPM) with nonequidistant discretization. The effects of the structural parameters on the sum of spot-conversion loss and coupling loss between the laser diode eigenmode and single-mode fiber (SMF) eigenmode are clarified. It is shown that a small loss of around 1.5 dB can be realized by introducing nonlinear tapers to these types of SSC's. The calculated results agree well with the measured results.