The response of ultrafast photodetectors at 1.55mum wavelength has been measured by electro-optic sampling using adiabatically compressed soliton pulses. 750fs-duration pulses with no wings were used to both pump and probe the waveguide pin photodiode. The measured bandwidth of the photodiode is more than 110 GHz, which is the highest value for a detector operating at 1.55 mum.