  We characterize the frequency response of channel-interleaved photonic
analog-to-digital converters (CI-PADCs) theoretically and experimentally. The
CI-PADC is composed of a photonic frontend for photonic sampling and an
electronic backend for quantization. The photonic frontend includes a photonic
sampling pulse generator for directly high-speed sampling and an optical
time-division demultiplexer (OTDM) for channel demultiplexing. It is found that
the frequency response of the CI-PADC is influenced by both the photonic
sampling pulses and the OTDM, of which the combined impact can be characterized
through demultiplexed pulse trains. First, the frequency response can be
divided into multiple frequency intervals and the range of the frequency
interval equals the repetition rate of demultiplexed pulse trains. Second, the
analog bandwidth of the CI-PADC is determined by the optical spectral bandwidth
of demultiplexed pulse trains which is broadened in the OTDM. Further, the
effect of the OTDM is essential for enlarging the analog bandwidth of the
CI-PADC employing the photonic sampling pulses with a limited optical spectral
bandwidth.
