Fig. 1. Schematic diagram for the proposed instantaneous multi-frequency measurement system. TLS, tunable laser source; PC, polarization controller; SUT, signal-under-test; MZM, Mach–Zehnder modulator; EDFA, erbium-doped fiber amplifier; AWG, arbitrary waveform generator; LD, laser diode; DSP, digital signal processor; ADC, analog-to-digital converter; PD, photodetector.

Fig. 2. (a) Waveforms of the photocurrents when tuning the frequency of the SUT from 3 to 18 GHz; (b) the zoom-in view of the zero point; (c) the correlation peak for 7 GHz SUT; (d) the two correlation peaks of a multi-frequency signal comprising 13 and 18 GHz components; (e) measurement results of a fixed 3 GHz component plus a sweeping second frequency.

Fig. 3. Measured results with linear fitting and the measurement errors.