Title: Frequency-oriented sub-sampling by photonic Fourier transform and I/Q demodulation,
Abstract: Sub-sampling can acquire directly a passband within a broad radio frequency
(RF) range, avoiding down-conversion and low-phase-noise tunable local
oscillation (LO). However, sub-sampling suffers from band folding and
self-image interference. In this paper we propose a frequency-oriented
sub-sampling to solve the two problems. With ultrashort optical pulse and a
pair of chromatic dispersions, the broadband RF signal is firstly short-time
Fourier-transformed to a spectrum-spread pulse. Then a time slot, corresponding
to the target spectrum slice, is coherently optical-sampled with
in-phase/quadrature (I/Q) demodulation. We demonstrate the novel bandpass
sampling by a numerical example, which shows the desired uneven intensity
response, i.e. pre-filtering. We show in theory that appropriate time-stretch
capacity from dispersion can result in pre-filtering bandwidth less than
sampling rate. Image rejection due to I/Q sampling is also analyzed. A
proof-of-concept experiment, which is based on a time-lens sampling source and
chirped fiber Bragg gratings (CFBGs), shows the center-frequency-tunable
pre-filtered sub-sampling with bandwidth of 6 GHz around, as well as imaging
rejection larger than 26 dB. Our technique may benefit future broadband RF
receivers for frequency-agile Radar or channelization.