Simultaneous frequency down-conversion, self-interference cancellation, and image rejection for radio-over-fiber systems using a DP-QPSK modulator

Abstract

A photonic approach for simultaneous frequency down-conversion, self-interference cancellation, and image rejection for in-band full-duplex radio-over-fiber systems is proposed based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. The upper dual-parallel Mach–Zehnder modulator (DP-MZM) of the DP-QPSK modulator realizes the radio frequency (RF) self-interference cancellation and generates the upper and lower first-order optical sidebands of the desired RF signal, whereas the lower DP-MZM generates two optical sidebands of the local oscillator. The optical signals from the two DP-MZMs are combined, and the upper and lower optical sidebands are separated by a wavelength division multiplexer (WDM). The two outputs from the WDM are detected at two photodetectors for frequency down-conversion. By combining the two intermediate-frequency (IF) signals at a 90°hybrid coupler, the proposed system can also achieve image rejection. In addition, by changing the bias point of the lower DP-MZM, second-harmonic frequency down-conversion can be implemented. A simulation is performed. A QPSK-modulated RF signal from 5 to 44 GHz is successfully down-converted to 1-GHz IF signal with self-interference and image frequency cancelled, and the EVM of the received IF signal is lower than 15%.