A sampling correlator is described which is useful for characterizing optical pulses with subpicosecond resolution. The correlator exploits the ultrafast nonlinear response of a photoconductor made of low‐temperature‐grown (LTG) gallium arsenide, which is connected to a coplanar‐waveguide line. Instead of using a nonlinear crystal, the LTG‐GaAs correlator uses a nonlinearity associated with the transmission line and photoconductor functioning as a voltage divider. The resulting nonlinearity is used to measure the intensity autocorrelation function. ©1996 American Institute of Physics.