Low-Distortion Optical Null-Steering Beamformer for Radio-Over-Fiber OFDM Systems

In this paper, we report a novel optical beamformer based on fiber Bragg grating technology capable of adjusting not only main beam direction, but also side-lobe level ratio (SLR) and null positions in the array radiation pattern. This beamformer is compatible with dense wavelength-division multiplexing (DWDM) configurations, and consists of two chains of cascaded gratings that can independently control both the phase and the amplitude of the signals, feeding each array element for 2pi and 10 dB, respectively. We characterized the effect of this beamformer on signal quality in an experimental radio-over-fiber (ROF) link by externally modulating and transmitting an IEEE 802.11a compliant signal over the beamformer. The measured error vector magnitude of the received signal confirmed that there was no additive distortion or noise, and consequently, we achieved error-free transmission for the highest possible bit rate. Finally, we demonstrated the capability of this beamformer in beam-/null-steering and SLR reductions by measuring the radiation pattern of the array antenna of an optimized DWDM-ROF link. The measurements are compared to simulations for several examples of radiation patterns.