  Using proper beam energy chirp and the undulator detuning effect, we propose
a modified optical replica synthesizer scheme to characterize the temporal
structure of a relativistic electron bunch, which predicts a 100-fs temporal
resolution in numerical simulation. The proof of principle experiment
demonstrates a peak current of 9 A and a slice energy spread of about 0.5 keV
for the uncompressed electron beam of the Shanghai Deep UV Free Electron Laser
Facility.
