Novel Ring Resonator-Based Integrated Photonic Beamformer for Broadband Phased Array Receive Antennas—Part I: Design and Performance Analysis

A novel optical beamformer concept is introduced that can be used for seamless control of the reception angle in broadband wireless receivers employing a large phased array antenna (PAA). The core of this beamformer is an optical beamforming network (OBFN), using ring resonator-based broadband delays, and coherent optical combining. The electro-optical conversion is performed by means of single-sideband suppressed carrier modulation, employing a common laser, Mach-Zehnder modulators, and a common optical sideband filter after the OBFN. The unmodulated laser signal is then re-injected in order to perform balanced coherent optical detection, for the opto-electrical conversion. This scheme minimizes the requirements on the complexity of the OBFN, and has potential for compact realization by means of full integration on chip. The impact of the optical beamformer concept on the performance of the full receiver system is analyzed, by modeling the combination of the PAA and the beamformer as an equivalent two-port RF system. The results are illustrated by a numerical example of a PAA receiver for satellite TV reception, showing that - when properly designed - the beamformer hardly affects the sensitivity of the receiver.

I. Introduction

Photonic beamforming in phased array antennas (PAAs) is an interesting example of applying optical technology in wireless transmission systems. A PAA consists of an array of multiple antenna elements (AEs), corresponding transmission and/or reception units, and a beamformer, enabling direction-sensistive transmission and/or reception of electromagnetic waves [1]. In receive mode each individual AE signal consists of a time-delayed version of some desired signal, possible time-delayed versions of undesired signals (from different directions), and noise (sky noise and antenna noise). The values of these time delays are different for each AE, and depend on the geometrical distribution of the AEs and the direction(s) of the incoming wave front(s). The beamformer therefore consists of a delay-and-combine network that equalizes the delay values of the signal terms that correspond to the desired received signal, such that the desired signal terms add up in phase and are reinforced, whereas the undesired signal terms do not add up in phase and are hence suppressed. (Although some people would use the terms beamformer and beamforming network exclusively for PAAs in transmission mode, it is widely accepted to use them for receive mode as well, so we also do that in this paper.) In many applications it is desirable that the time delays are tunable, in order to be able to alter the reception angle of the PAA. When the amplitudes of the AE signals are also controlled, the shape of the beam pattern can be altered as well, for example to minimize sidelobes. PAAs offer several advantages when compared to mechanically steered antennas, such as agile beam steering, relatively low maintenance costs, reduced drag when applied in for instance vehicles or aircraft, and the possibiliy of supporting multiple antenna beams [1].