An interferometric technique measures the time-dependent electric field of a periodic or a non-periodic (data-encoded) optical signal under test using samples of its interference with a reference source of short optical pulses. The reference signal is a sequence of optical pulses at a repetition rate different from that of the signal under test. The difference in repetition rates of the two signals performs a scanning of the relative delay between the two signals, i.e. each pulse from the reference signal will overlap with the signal under test at a different time. The real and imaginary part of each of the plurality of interference between the two signals are then measured to determine samples of the electric field of the optical signal under test at each of those times. When needed, various types of averaging are performed on the samples of the electric field. If the signal under test is a data-encoded source, averaging is performed on groups of samples corresponding to the same symbol state of the data-encoded source.