Spectral phase-locking in ultrafast all-optical Mach-Zehnder-type semiconductor wavelength converters

We studied the spectrum-domain mechanism of the delayed-interference signal-wavelength converter (DISC) analytically and found that the role of the passive Mach-Zehnder interferometer in this converter differs completely from that of conventional spectrum filters: it converts the semiconductor-optical-amplifier-induced cross-phase-modulated spectrum phase from out-of-phase to in-phase (constant phase). This converter therefore efficiently generates ultrashort nearly transform-limited pulses having a broad phase-locked spectrum. These conclusions are consistent with the output pulse shapes and spectra measured in experiments in which the wavelength of 2-ps 42-GHz pulses was converted from 1560 nm to 1553 nm.