## 4 Enabling Technologies for THz-FSO Communication
### High-Power THz Sources  
Moreover, the production of high-power THz signals is crucial, especially for enhancing the data rate and for the effective realization of THz-FSO systems in long-distance communication. Significantly improving the performance of these systems' QCLs, photonic mixers and FELs have recently undergone several improvements [41; 66; 73]. Quantum cascade lasers (QCLs): QCLs are among the smallest and most efficient sources of THz radiation, and they employ broadband electron transitions between multiple quantum wells to produce high-power coherent THz waves [13; 73; 78]. Both designs offer the potential for adjusting emission frequencies with a high degree of accuracy and are versatile. The output power (P\({}_{\mathrm{out}}\)) of QCLs is influenced by their quantum efficiency (\(\eta\)) and operating conditions: \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Studies** & **Device** & **Functionality** & **Advantages** & **Challenges** \\ \hline
[23] & Quasi-optic modules & Wireless links & Ultra-broadband & Signal alignment \\ \hline
[28] & MZI-MRR circuits & Pulse processing & Programmability & Fabrication precision \\ \hline
[31] & Cascaded MIMO equalizers & Data integration & Enhanced bandwidth & Energy demand \\ \hline
[35] & Integrated antennas & Beamforming & Compact size & Thermal stability \\ \hline
[39] & Phase modulators & Signal modulation & High precision & Design complexity \\ \hline
[16] & Photonic waveguides & Light propagation & Low dispersion & Material losses \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Integrated photonic devices for THz-FSO communication. \[\mathrm{P_{out}}=\eta\cdot\frac{\mathrm{VI}}{\lambda} \tag{17}\]  
where \(\mathrm{P_{out}}\): the output power. \(\eta\): the quantum efficiency. V: the voltage. I: the current. \(\lambda\): the wavelength. THz QCLs have excellent features that lend themselves to incorporation in compact THz systems, especially in long-range communication systems. However, they are not very efficient in generating large amounts of power due to requiring expensive cooling mechanisms to be embedded in them. Constant research is being undertaken to explore the use of other lighter and high-strength materials, and to find better thermal management systems. Quantum cascade lasers (QCLs) are among the most promising THz light sources due to their ability to operate in the mid-infrared and THz spectra with high power efficiency. However, unlike edge-emitting semiconductor lasers that operate in the near-infrared range, the direct modulation of QCLs using a pumping current remains a significant challenge due to carrier lifetime limitations and thermal effects. Traditional near-infrared edge-emitting lasers can achieve high-speed direct modulation up to several GHz by manipulating carrier injection dynamics and photon lifetimes [79]. In contrast, THz QCLs experience longer carrier lifetimes and slower electron transitions, restricting their modulation bandwidth to much lower values. This limitation suggests that external THz modulators or hybrid photonic-electronic modulation techniques are necessary to optimize THz signal encoding for high-speed wireless communication. THz photonic mixers are widely used for frequency conversion and the generation of high-purity THz signals, and typically rely on optical heterodyning or nonlinear difference-frequency generation (DFG) techniques. While this review discusses various frequency mixing approaches, it previously overlooked the novel integration of DFG-based THz sources within QCLs. Recent advancements in DFG-QCLs demonstrate that these devices can achieve high spectral purity, tunability in the 1-6 THz range, and room-temperature operation, making them suitable for high-speed THz wireless networks [80]. Unlike conventional THz sources, which require external nonlinear crystals, DFG-QCLs incorporate nonlinear gain media within the laser cavity, enabling the fabrication of compact, monolithic THz emitters with superior performance.