Amplitude modulation to phase distortion conversion in photonic bandpass sampling link

In this paper, we found that the optical pulse amplitude modulation will also convert into the microwave phase modulation during the optics-to-electronics conversion in the bandpass sampling RF photonic link with saturated PD. The converted phase modulation results in the significant 2nd harmonic nonlinearity which still can be observed even if the MZM is quadrature biased. It is well known that the other kind of 2nd harmonic distortion caused by the modulator will generate when the MZM's operational point drifts away from its quadrature bias. We experimentally demonstrate that the two type of nonlinear harmonics show great difference, i.e., the phase difference is π/2. Two comparison experiments are conducted to examine the phase difference.