We report measurements of intensity as a function of both time and frequency for frequency modulation and switching of a tunable semiconductor laser. Because of the uncertainty principle limitations, the measured time‐frequency signal can have a complex structure and does not show the simple‐minded picture of a laser spectrum whose center frequency varies in time. The observations are explained by a theory of the time‐dependent spectral measurements, well known in the field of speech analysis. We discuss implications for channel switching speed and channel interference in switched, frequency‐multiplexed optical networks.