Frequency-octupled millimeter-wave ROF system without bit walk-off effect by polarization modulators

Abstract

A novel, to the best of our knowledge, frequency-octupled millimeter-wave (MMW) radio over fiber (ROF) system without bit walk-off effect, based on two polarization modulators (PoIM) connected in parallel, is proposed. At the central station, the downlink data signal is split into two beams; one beam modulates the radio frequency (RF) carrier through an electrical phase modulator (PM), the other beam is amplified by an electrical amplifier (EA), and the two beams are combined into one, which is used as the RF driving signal of the PoIM. By adjusting the modulation index of the PoIMs, the phase shift of the phase shifter, and the polarization angle of the polarizer (Pol), the main components of the output signal from the PoIMs are ±4th-order sidebands. By adjusting the phase modulation index of the PM and the amplification of the EA, the downlink data signal is modulated only on the +4th-order sideband. At the base station, a part of the −4th-order sidebands are reflected by a fiber Bragg grating (FBG) and used as the optical carrier for uplink for carrier reuse; the ±4th-order sidebands passing the FBG are injected into the photodetector (PD) for photoelectrical conversion, and the frequency-octupled MMW with the downlink data signal is generated. The main innovation of the scheme proposed in this paper is that the data of the uplink and the downlink is modulated on ±4th-order sidebands separately, which eliminates the periodic fading effect and bit walk-off effect due to fiber dispersion and greatly improves the fiber transmission distance of the system, and also avoids the problems of DC bias drift and extinction ratio limitation of the Mach–Zehnder modulator.