Heterodyne-based optical spectrum analysis using data clock sampling

A complex spectrum related to a carrier signal (40) that is modulated (26) with random data is measured as a function of the data clock so that the randomness that is contributed by the modulation of random data can be ignored. In one embodiment, the complex spectrum that is expressed in terms of both spectral amplitude and spectral phase is sampled (76) only at the carrier frequency and at the frequencies that are away from the carrier signal by integral multiples of the clock frequency. This sampling approach reduces the complex spectrum to only those data points that are needed to characterize the average pulse shape of the modulated carrier signal.