## 4 Enabling Technologies for THz-FSO Communication
### Integrated Photonic Devices  
Incorporating photonic devices within THz-FSO systems offers great potential by advancing their overall communication in terms of their data rate, latency, and energy consumption. These devices play significant functions in data flow, real-time computation, and system extensibility [2; 10; 42].  
Quasi-optic modules: quasi-optic modules for the 60-180 GHz range have been widely applied to terahertz frequency-selective optics systems. These modules employ Avan-gel photonic integration that increases the signal bandwidth, thus leading to higher data rates [76]. Their quasi-optic structures improve the system's beam pointing and allow efficient wireless-to-optical interconversion. The evolution of new versatile quasi-optic receivers has also made their deployment compatible with future-generation communi  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Studies** & **Material** & **Applications** & **Advantages** & **Limitations** \\ \hline
[43] & Graphene & THz modulators & High mobility, tunable properties & Fabrication scalability \\ \hline
[74] & 2D materials & Photodetectors & Ultra-fast response & Integration challenges \\ \hline
[70] & Metamaterials & Beam shaping & High precision & High cost \\ \hline
[41] & Nanoparticles & Loss reduction in photonic fibers & Reduced dispersion & Limited bandwidth \\ \hline
[75] & Metamaterials & Polarization control & Enhanced stability & Complex design \\ \hline
[22] & ITO/SeO\({}_{2}\) & Optical modulators & Enhanced link performance & Modulation instability \\ \hline \hline \end{tabular}
\end{table}
Table 9: Advanced materials for THz-FSO communication.  
cation systems. However, perfectly matching the alignment across particularly differing environmental conditions is proving to be rather difficult.  
On-chip photonic circuits: programmable pulse processing functions have arisen from on-chip photonic circuits that consist of cascaded Mach-Zehnder interferometers (MZIs) and micro-ring resonators (MRRs) [13; 77]. MZIs employ phase modulation to achieve dynamic signal control, with phase shifts governed by  
\[\Delta\phi=\frac{2\pi\Delta\mathrm{L}}{\lambda} \tag{16}\]  
where \(\Delta\phi\): the phase difference. \(\Delta\mathrm{L}\): the path length difference. \(\lambda\): the wavelength.  
These circuits are critical to realizing THz frequency data processing in real-time. Their use enables flexibility in how the system responds to signals, although they require accurate fabrication and subsequent tuning to the appropriate parameters. There are discussions of using advanced manufacturing technologies, including lithography, to optimize the reproducibility of these devices.  
Cascaded MIMO equalizers: Cascaded MIMO equalizers, especially, are used to perform data integration for fiber-THz wireless systems. These devices allow straightforward switching between the optical mode and THz mode, making the computational bandwidth very high and exhibiting low latency [76]. Since MIMO equalizers help to minimize different distortions that resulting from the multipath, they trigger signal characteristics and allow for the reliability of a given system. However, their deployment demands enhanced energy utilization, which is a hurdle in resource-poor settings.  
In Table 10, we defined the functional aspects of and developments regarding integrated photonic devices in THz-FSO systems. From the quasi-optic modules to superior waveguides, all these devices serve to improve data throughput rates and signal readability. However, issues like the accuracy of fabrication, energy issues, the loss of material, etc., call for more study and development.