Amplitude Modulation to Phase Modulation Conversion in Photonic Bandpass Sampling Link

The photonic bandpass sampling has been proved to be potential in multi-carrier communication, frequency-agile coherent radar, compressive sensing, etc. It supports the direct down-conversion of multi-carrier signals using a low-speed analog-to-digital converter (ADC). We observed the distortion resulted from amplitude-modulation to phase-modulation (AM-PM) conversion caused by the saturated photo-detector in the photonic bandpass sampling link. The nonlinearity caused by the AM-PM conversion is theoretically analyzed, which is the 2nd harmonic distortion and has a phase difference of 90 degrees with the fundamental signal. Due to the AM-PM conversion, the 2nd harmonic distortion occurs even the Mach-Zehnder modulator (MZM) is quadrature biased in the photonic bandpass sampling link. Two comparative experiments were conducted to test the nonlinearity. When the MZM works at different bias points, the phase of the 2nd harmonic distortion will be different. In addition, we also compared the differences in 2nd harmonic distortion between standard continuous wave photonic links and photonic bandpass sampling links.