Citation: Raj, A.A.B.; Krishnan, P.; Darusalam, U.; Kaddoum, G.; Ghassemlooy, Z.; Abadi, M.M.; Majumdar, A.K.; Ijaz, M. A Review–Unguided Optical Communications: Developments, Technology Evolution, and Challenges. Electronics 2023, 12, 1922. https://doi.org/10.3390/ electronics12081922 Academic Editors: Chien-Hung Yeh and Giovanni Crupi Received: 6 February 2023 Revised: 19 March 2023 Accepted: 29 March 2023 Published: 19 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). electronics Review A Review–Unguided Optical Communications: Developments, Technology Evolution, and Challenges A. Arockia Bazil Raj 1, * , Prabu Krishnan 2 , Ucuk Darusalam 3,4 , Georges Kaddoum 5,6 , Zabih Ghassemlooy 7 , Mojtaba Mansour Abadi 7 , Arun K. Majumdar 8 and Muhammad Ijaz 9, * 1 Free Space Optical Communication Laboratory, Defence Institute of Advanced Technology, Pune 411025, India 2 Electronics and Communication Engineering, National Institute of Technology Karnataka, Mangalore 575025, India 3 Department of Informatics, Faculty of Information and Communications Technology, Universitas Nasional, Jakarta 12520, Indonesia 4 Department of E-Learning Center, Universitas Siber Asia, Jakarta 12550, Indonesia 5 École de Technologie Supérieure (ÉTS), Université du Québec, Montréal, QC H3C 1K3, Canada 6 Cyber Security Systems and Applied AI Research Center, Lebanese American University, Byblos P.O. Box 36, Lebanon 7 Optical Communications Research Group, Northumbria University, Newcastle NE1 8ST, UK 8 Department of Physics, Colorado State University, Pueblo, CO 81001, USA 9 Department of Engineering, Faculty of Science and Engineering, Manchester Metropolitan University, Manchester M1 5GD, UK * Correspondence: brazilraj.a@diat.ac.in (A.A.B.R.); m.ijaz@mmu.ac.uk (M.I.) Abstract: This review paper discusses the complete evolution of free-space optical (FSO) commu- nication, also known as unguided optical communication (UOC) technologies, all the way back to ancient man’s fire to today’s machine-learning-supported UOC systems. The principles, significance, and developments that have happened over the past several decades, as well as installation method- ologies, technological limitations, and today’s challenges of UOCs are presented. All the subsets of UOC: FSO communication, underwater optical wireless communication (UOWC), and visible light communication (VLC), with their technology/system developments, potential applications, and limitations are reviewed. The state-of-the-art developments/achievements in (i) FSO channel effects and their mitigation techniques; (ii) radio-over-FSO techniques; (iii) wavelength division multiplexing and sub-carrier multiplexing techniques; (iv) FSO for worldwide interoperability for microwave access applications; (v) space optical satellite communication (SOSC); (vi) UWOC; (vii) photoacoustic communication (PAC); (viii) light-fidelity; (ix) VLC; (x) vehicular VLC (V 2 LC); and (xi) optical camera communication are reviewed. In addition, the current developments on emerging technologies such as artificial intelligence (to improve the performance of UOC systems), energy harvesting (for the effective utilization of UOC channels), and near-future communication network scenarios (mandatory for secured broadband digital links) are covered. Finally, in brief, to achieve the full potential of UOC systems, challenges that require immediate research attention are summarized. Keywords: FSO communication; environmental effects; radio-over-FSO; visible light/space optical communications; artificial intelligence in FSO systems; energy harvesting from FSO channel; Internet of Things (IoT); optical camera communications; underwater optical communications; photoacoustic communications; UOC research challenges 1. Introduction to Unguided Optical Communication (UOC) 1.1. Revolution of UOC This review paper covers the evaluation of today’s free-space optical (FSO) communica- tion, also known as unguided optical communication (UOC), and their techniques/systems, right from prehistoric fire-based signal transmission. The technology developments in the Electronics 2023, 12, 1922. https://doi.org/10.3390/electronics12081922 https://www.mdpi.com/journal/electronics