We propose a low-speed photonic sampling for independent high-frequency characterization of a Mach–Zehnder modulator (MZM) and a photodetector (PD) in an optical link. A low-speed mode-locked laser diode (MLLD) provides an ultra-wideband optical stimulus with scalable frequency range, working as the photonic sampling source of the link. The uneven spectrum lines of the MLLD are firstly characterized with symmetric modulation within the interesting frequency range. Then, the electro-optic modulated signals are down-converted to the first Nyquist frequency range, yielding the self-referenced extraction of modulation depth and half-wave voltage of the MZM without correcting the responsivity fluctuation of the PD in the link. Finally, the frequency responsivity of the PD is self-referenced measured under null modulation of the MZM. As frequency responses of the MZM and the PD can be independently obtained, our method allows self-referenced high-frequency measurement for a high-speed optical link. In the proof-of-concept experiment, a 96.9 MS/s MLLD is used for measuring a MZM and a PD within the frequency range up to 50 GHz. The consistency between our method and the conventional method verifies that the ultra-wideband and self-referenced high-frequency characterization of high-speed MZMs and PDs.