## 1 Introduction  
Facing the predicted 396 exabytes of global data traffic per month by 2025, there is a manifest need for communication systems that support high-bandwidth and ultra-latency communication. The health of Ericsson is well documented by its latest report, especially as it relates to growth in the following areas of technology: 5G, autonomous vehicles, the IoT, and high-definition video streaming [1]. Currently, communication systems, such as fiber-optic systems, have been effective. Still, these systems have some constraints because they require stationary systems and may not be helpful under certain circumstances [2]. Free-space optical (FSO) communication, also referred to as wireless optical communications, has increasingly attracted interest as a method for high-speed wireless communication that does not require wires or fiber optic systems [3; 4]. FSO systems work in the visible and near-IR ranges and can thus transmit at speeds from Gbps to Tbps when conditions are favorable.  
Nevertheless, due to the direct dependence of this method on the atmospheric conditions, the system can be easily disturbed by the presence of fog, rain, or other turbulences, which negatively impact the system's performance [5; 6]. Traditional FSO systems have certain drawbacks, which have led to the development of a suggested terahertz (THz) band range of 0.1-10 THz. The THz band has over 10 times the bandwidth of 100 GHz millimeter-wave (mmWave) communication and provides data rates that exceed 1 Tbps, providing connectivity between microwave and optical frequencies [7; 8]. Other advantages of shorter wavelengths in the THz spectrum include their compactness and power efficiency, which are typical requirements for portable or space-demanding technologies in various applications [3; 9]. However, multiple hurdles must be overcome before THz communication technology can be fully adopted. Signal losses over 15 dB/km occur due to atmospheric attenuation, which is mainly caused by water vapor absorption bands [4]. Another essential feature is the line-of-sight alignment, because even a tiny deviation concerning this parameter can lead to considerable disruptions in data transmission. The low output power of THz sources and the absence of high-sensitivity and inexpensive THz detectors make it difficult to implement the THz-FSO system [10; 11]. Recent developments in photonic technologies like QCLs and metamaterial-based gadgets have enhanced the characteristics of THz systems [12; 13]. Hence, adaptive optics and machine learning algorithms for achieving alignment are also being implemented to counteract the influence of turbulence in maintaining a reliable communication connection. Such advancements point to the chances of incorporating the THz frequency within FSO systems to improve a new generation of dense networks [11].  
Figure 1 shows different aspects of toward terahertz bandwidth in free-space optical communication. The uses of THz-FSO communication are as follows: inter-satellite communication, high-speed backhaul for 5G communication networks, and military communication [6; 14]. THz-FSO links benefit from a fairly narrow beam width, making those used for critical applications much safer since interception is unlikely. Furthermore, THz-FSO systems, due to their low latency and high bandwidth, are applicable to upcoming technologies such as 6G wireless networks and car-to-car data transfer [15; 16]. This review article presents THz-based FSO communication alongside theoretical fundamentals, relevant enabling technologies, and the issues that are confronted in this promising field. This paper aims to establish how THz-FSO communication can be the solution to fulfillingthe requirements of future networks by combining and synthesizing the most relevant discoveries from the literature [6; 14; 15; 16].