Free-Space Optical Communication Link with Liquid Crystal Beam-Steering

Published in IEEE Photonics Technology Letters with Guanxiong Zhang as first author. Fast optical wireless links need narrow beams and narrow receiver fields of view, which only cover a wide area if they can be steered. This paper steers both ends of the link with liquid crystals and no moving parts.

Abstract

In this letter, we present a non-mechanical 2D beam-steering system suitable for optical wireless communication. Steering is achieved using polarization gratings combined with nematic liquid crystal cells operating as voltage-controlled polarization shifters. These steering elements are combined with a holographic diffuser matched to the discrete steering angles of the polarization grating, enabling continuous angular coverage. Beam-steering is also used at the receiver, allowing a large collection area receiver with a relatively narrow field of view to be used. The approach presented here could, in principle, be applied to a broad range of wavelengths (including visible light and near-infrared wavelengths). Furthermore, the technique does not inherently limit the transmission data rate. Besides, it improves the link margin and offers the potential for a bidirectional steered link using the same beam-steering elements. Details of the approach are set out in the letter, followed by experimental results from a 50 Mbit/s optical link operating over one meter. Future directions are then discussed.

Key Findings

Each steering stage is a nematic liquid crystal cell, acting as a voltage-controlled waveplate, followed by a polarization grating that sends circularly polarized light to ±10° depending on its handedness. Two stages, one per axis, give four steering positions. A holographic diffuser after the gratings spreads each position into a 10° (FWHM) beam, giving continuous coverage and helping with eye safety.

The receiver uses the same kind of stack to bring incoming light back onto its optical axis, so it can have a large collection area (20 mm diameter) with a narrow field of view. With $N \times N$ steering positions at each end, the link margin could grow by up to $N^4$ : $N^2$ from concentrating the transmitter’s power and $N^2$ from the larger receiver area.

The optics place no limit on data rate. Steering speed is set by the liquid crystal response time, 170 ms here, which polymer stabilisation could cut to a few milliseconds. The transmitter stack’s 3.07 dB insertion loss is mostly Fresnel reflection, which index matching would reduce.

A 520 nm link carried 50 Mbit/s OOK at a BER below $10^{-3}$ over more than 1 m point to point. Splitting the power between two receivers cost 3 dB and shortened the range to 80 cm.