Optical Beamforming System Based on Polarization Manipulation With Amplitude–Phase Coupling Suppression

Traditional phased arrays based on phase shifters suffer severely from the beam-squint problem when operating at the wideband mode. Optical beamforming is one feasible solution for broadband beam steering, of which the performance, however, is affected by the amplitude–phase coupling. To solve this problem, we propose an optical beamforming system with amplitude–phase coupling suppression based on polarization manipulation. A polarization controller is used to control the transmission of carrier light along the fast or slow axis in the polarization-maintaining fiber. As there is a fixed transmission time difference between the two orthogonal modes, the phase adjustment can be achieved with low amplitude variation because of the low polarization-dependent loss. In addition, the intensity of the signal can be controlled using a polarizer with negligible phase variation, realizing amplitude control with low phase coupling. Experimental results show that the amplitude–phase coupling coefficient is about $5.8\times 10^{-3}$ ps/dB, while the phase–amplitude coupling is less than $7.5\times 10^{-3}$ dB/ps. The performance of the optical beamforming is analyzed by numerical simulation and proof-of-concept experiment, showing that the beam pointing deflection is less than 0.1°, while the main-lobe-to-sidelobe suppression ratio deteriorates less than 1 dB when the beam scans from 0° to 45°.

I. Introduction

Phased arrays traditionally use phase shifters to control the beam direction, which shows great advantages in terms of fast beam scanning and high reconfigurability [1], [2]. With the demand for target recognition, a large bandwidth is required for a phased array to achieve high resolution. However, as the bandwidth increases, the beam-squint problem would present [3], [4], [5], [6]. To address this issue, beamforming based on optical true-time delays (TTDs) is considered an effective solution [7], [8], [9], [10].