## 4 Enabling Technologies for THz-FSO Communication
### Adaptive Optics  
Adaptive optics (AO) systems are of significant importance in the control of the atmospheric distortions that affect THz-FSO communication. These systems adaptively counteract wavefront errors induced by turbulence to support reliable communication links [64; 85].  
Wavefront correction: wavefront correction guarantees signal quality by adjusting for distortions that occur due to the atmosphere in real-time [61; 85; 86]. The effectiveness of AO systems is often quantified using the Strehl ratio (S):  
\[\mathrm{S}=\mathrm{e}^{-\sigma^{2}} \tag{18}\]  
where S: the Strehl ratio, representing the optical quality. \(\sigma^{2}\): the variance of wavefront errors.  
These Strehl ratios characterize the optical quality, which makes the THz-FSO signal more faithful. Distortions due to turbulence substantially reduce the quality of the transmitted wave over long distances, which makes wavefront correction a key component of AO systems.  
Real-time AO systems employ high-speed actuators and wavefront sensors to control the transmitted beam's phase in real time [65; 87]. They also play an important role in these systems as deformable mirrors, and Shack-Hartmann wavefront sensors are employed for the correction of distortions in real-time [25; 88]. The performance of real-time systems is governed by their response time (T):  
\[\mathrm{T}=\frac{\mathrm{\tau}+\Delta\mathrm{t}}{\mathrm{R}} \tag{19}\]  
where \(\mathrm{\tau}\): the time constant of the actuator. \(\Delta\mathrm{t}\): the sensor delay. R: the control loop bandwidth.  
Optimal AO processes are characterized by low-sensitivity delay, which makes it possible to respond effectively to fluctuations in the surrounding environment. This feature is especially useful in situations characterized by turbulence or dynamism within the organization's operating environment [89; 90].  
Phase-only spatial light modulators: these are utilized in AO systems to provide high-resolution wave-front corrections without altering the amplitude of the passed-through  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Studies** & **Source** & **Advantages** & **Limitations** & **Applications** \\ \hline [12] & QCLs & Compact, high tunability & Cooling requirements & Long-range communication \\ \hline [50] & Photonic Mixers & Broad bandwidth & Phase coherence issues & Spectroscopy, imaging \\ \hline [52] & FELs & Extremely high power & Large size, high cost & Scientific research \\ \hline \hline \end{tabular}
\end{table}
Table 11: Comparison of high-power THz sources.  
signal. These devices can operate with a specific level of phase control to successfully correct the distortions in the atmosphere. The phase modulation (\(\phi\)) introduced by an SLM is described as follows:  
\[\phi=\frac{2\pi\text{d}\Delta\text{n}}{\lambda} \tag{20}\]  
where d: the thickness of the modulator layer. \(\Delta\)n: the refractive index change. \(\lambda\): the wavelength of the light.  
SLMs have high spatial resolution and are known to be suitable for application in correcting phase distortions that exist in a THz-FSO system.  
Table 12 compares adaptive optical technologies with their specific strengths and weaknesses. Wavefront correction increases the quality of the signal but reduces the throughput, and algorithms and real-time AO systems that can adaptably correct signals over large distances also reduce the speed of the hardware. Phase-only SLMs provide accurate phase control on the sub-phase level but cannot perform rapid modulation, making them suitable for the wavefront correction of turbulence-induced spatial phase distortions.