-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

- ---
title: "Free-Space Optical Communication Link with Liquid Crystal Beam-Steering"
date: 2023-11-15T10:00:00+00:00
draft: false
category: paper
excerpt: "IEEE Photonics Technology Letters, first-authored by Guanxiong Zhang: liquid crystal cells and polarization gratings steer an optical link at both ends with no moving parts, carrying 50 Mbit/s over 1 m."
- ---

::publication[13]

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 ~wave: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.
-----BEGIN PGP SIGNATURE-----
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=CSh5
-----END PGP SIGNATURE-----