Electro-optic sampling is an optoelectronic technique of optical sampling, which exploits the linear electro-optic effect. When ultrashort optical Pulses are used in the electro-optic probe, there is only a short time interval in which the electric field at the probe can influence the light. This effect can then be measured without requiring a very FAST photodetector. A full waveform of a periodic signal can then be obtained by slowly varying the arrival time of the probe pulse, i.e., by sequential sampling of a repetitive signal. For each setting of the relative time delay, the signal obtained can be averaged over many pulses, SO that Noise is averaged out and a very high Sensitivity is achieved. The principle is the SAME as for an electronic sampling oscilloscope, where however the electronic sampling gate is replaced with the electro-optic probe.