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Low-loss chip to-fiber coupling can be achieved by on-chip spot-size transformers. Two simple versions using laterally tapered InP-InGaAsP waveguides at the 1.55 μm wavelength have been fabricated: one consisting of two differently tapered layers, the other with only one taper. The influence of the taper shape and of fabrication tolerances on their performance is compared both experimentally and theoretically. The optimum insertion loss for a single transformer without antireflection coating, including Fresnel losses, amounts to 1.1 dB.