Origin of clock-frequency components in NRZ-formatted optical signals

We show that a conventional optical nonreturn-to-zero modulator based on a Mach-Zehnder interferometer always produces optical amplitude modulation at the clock frequency of the modulating signal, which in turn can generate a large clock-frequency component (CFC) in the received electrical signal when the optical signal has experienced excessive group velocity dispersion (GVD). We calculate the amplitude of these CFCs as a function of GVD for various optical bit sequences and pulse shapes.