We present the first demonstration of an optical sampling system, using the optical Kerr effect in a chip-scale device, enabling combined capability for femtosecond resolution and broadband signal wavelength tunability. A temporal resolution <500 fs is achieved using four-wave mixing in a 7-cm-short chalcogenide planar waveguide. The use of a short length, dispersion-shifted waveguide with ultrahigh nonlinearity (104 W-1??km-1) enables high-resolution optical sampling without the detrimental effect of chromatic dispersion on the temporal distortion of the signal and sampling pulses, as well as their phase mismatch. Using the device, we successfully monitor a 640-Gb/s optical time-division multiplexing (OTDM) datastream, showcasing its potential for integrated chip-based monitoring of signals at bitrates approaching and beyond Tb/s. We discuss fundamental limitations and potential improvements.