Frequency response measurement of high-speed electro-optic phase modulators via a single scan based on low-speed...

An approach to measuring frequency response of high-speed electro-optic phase modulators via a single scan is proposed and experimentally demonstrated. This method employs low-speed photonic sampling to transfer the response information at any high frequency to a fixed low-frequency duplicate in the first Nyquist frequency range by setting the microwave frequency sweeping step equal to the repetition rate of the mode-locked laser. Through low-frequency detection and analysis in the electrical domain, the relative frequency response can be directly calculated from the relative intensity between the low-frequency duplicate and the direct current component without calibration of the photodetector response. In the experiment, the relative frequency responses (i.e., the S21 curve) of two 20 Gb/s LiNbO₃-based electro-optic phase modulators and two 40 Gb/s samples are measured with a resolution of 96.9 MHz by using the proposed method, where the measurement results fit in with those obtained by using the conventional optical spectrum analysis method. The frequency measurement resolution can be further improved by using a mode-locked laser with a lower repetition rate.