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Review
. 2025 Mar 27;25(7):2109.
doi: 10.3390/s25072109.

Beyond Fiber: Toward Terahertz Bandwidth in Free-Space Optical Communication

Rahat Ullah  1   2   3 , Sibghat Ullah  4 , Jianxin Ren  1   2   3 , Hathal Salamah Alwageed  5 , Yaya Mao  1   2   3 , Zhipeng Qi  1   2   3 , Feng Wang  1   2   3 , Suhail Ayoub Khan  6   7 , Umar Farooq  8
Affiliations
Review

Beyond Fiber: Toward Terahertz Bandwidth in Free-Space Optical Communication

Rahat Ullah et al. Sensors (Basel). .

Abstract

The rapid advancement of terahertz (THz) communication systems has positioned this technology as a key enabler for next-generation telecommunication networks, including 6G, secure communications, and hybrid wireless-optical systems. This review comprehensively analyzes THz communication, emphasizing its integration with free-space optical (FSO) systems to overcome conventional bandwidth limitations. While THz-FSO technology promises ultra-high data rates, it is significantly affected by atmospheric absorption, particularly absorption beyond 500 GHz, where the attenuation exceeds 100 dB/km, which severely limits its transmission range. However, the presence of a lower-loss transmission window at 680 GHz provides an opportunity for optimized THz-FSO communication. This paper explores recent developments in high-power THz sources, such as quantum cascade lasers, photonic mixers, and free-electron lasers, which facilitate the attainment of ultra-high data rates. Additionally, adaptive optics, machine learning-based beam alignment, and low-loss materials are examined as potential solutions to mitigating signal degradation due to atmospheric absorption. The integration of THz-FSO systems with optical and radio frequency (RF) technologies is assessed within the framework of software-defined networking (SDN) and multi-band adaptive communication, enhancing their reliability and range. Furthermore, this review discusses emerging applications such as self-driving systems in 6G networks, ultra-low latency communication, holographic telepresence, and inter-satellite links. Future research directions include the use of artificial intelligence for network optimization, creating energy-efficient system designs, and quantum encryption to obtain secure THz communications. Despite the severe constraints imposed by atmospheric attenuation, the technology's power efficiency, and the materials that are used, THz-FSO technology is promising for the field of ultra-fast and secure next-generation networks. Addressing these limitations through hybrid optical-THz architectures, AI-driven adaptation, and advanced waveguides will be critical for the full realization of THz-FSO communication in modern telecommunication infrastructures.

Keywords: FSO communication systems; THz communication systems; THz-bandwidth; integrated THz-FSO systems; software-defined networking.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Toward terahertz bandwidth in free-space optical communication.
Figure 2
Figure 2
Data rates and stability comparison.
Figure 3
Figure 3
Bandwidth comparison of communication technologies.
Figure 4
Figure 4
THz communication advancement spectrum.
Figure 5
Figure 5
Atmospheric absorption.
Figure 6
Figure 6
Enabling technologies for THz-FSO communication.
Figure 7
Figure 7
Synergy with hybrid communication systems.
See this image and copyright information in PMC

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