Temporal resolution limits of time-to-frequency transformations.
Time-to-frequency converters are devices that transfer the intensity of a light pulse to its spectrum. The two architectures of these converters are studied: a dispersive line followed by a phase modulator and a single time lens operating in the spectral Fraunhofer regime. These two configurations are shown not to be equivalent in general: the first one provides an incoherent time-to-frequency mapping, whereas the second depends on the degree of coherence of the pulse. In this case, the recorded spectrum is the intensity of a partially coherent residually dispersed pulse, and the spectral Fraunhofer condition is the requirement of negligible residual dispersion. Converters operated outside the spectral Fraunhofer limit can achieve a subpicosecond resolution with moderate time-lens phase factors. Their use for pulse characterization is briefly analyzed.