Frequency-oriented sub-sampling by photonic Fourier transform and I/Q demodulation

TL;DR

This paper introduces a novel frequency-oriented sub-sampling method using photonic Fourier transform and I/Q demodulation to improve RF signal acquisition by reducing interference and enabling tunable, pre-filtered sampling.

Contribution

It proposes a new sub-sampling technique that employs ultrashort optical pulses, chromatic dispersion, and I/Q demodulation to achieve pre-filtered, tunable RF sampling with high image rejection.

Findings

Demonstrated 6 GHz bandwidth sub-sampling around a tunable center frequency.

Achieved image rejection larger than 26 dB.

Theoretically showed pre-filtering bandwidth can be less than sampling rate.

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.