# Experimental Demonstration of Terahertz-Wave Signal Generation for 6G Communication Systems
## 2 Simulation Model for THz Generation  
Figure 2 illustrates the core principle behind the terahertz (THz) wave generation technique employed in our system, which involves three main stages: the transmitter, the wavelength selection switch (WSS), and IQ modulation, ultimately leading to the receiver. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Generation** & **Modulation** & **Freq Separation** & **Generated** & **Method of** & **Remarks** & **Reference** \\
**Technique** & **Format** & **(GHz)** & **Frequency (THz)** & **Conduct** & **Remarks** & **Reference** \\ \hline Tunable optoelectronic oscillator (OEO) & QPSK/16 QAM & 17.33 & 0.101 and 0.242 & Experimental & The generated THz signals depend on the OEO stability, which makes the implementation critical. & [57] \\ \hline Optical frequency comb using two-phase modulators & QPSK/16 QAM & 25 & 0.4 & Simulation & This paper utilized two cascaded phase modulators, leading to expensive implementation. & [58] \\ \hline Single-section chirped multiple InAs/InP\({}^{\prime}\) quantum-dot (QD) mode-locked laser (MLL) & – & – & 1–2.9 & Experimental & The programmable optical filter can work only in the C-band, so the longer wavelength modes cannot be extracted. & [59] \\ \hline Optical feedback mode-locked laser diode & 64-QAM & – & 0.042-0.377 & Experimental & Focusing on reducing the optical linewidth to use higher-order modulation. & [60] \\ \hline Integrated dual-distributed feedback (DFB) laser & 16 QAM-OFDM & 9.951 & 0.408 & Experimental & This paper used a monolithically integrated (DFB) laser chip attached to a (UTC-PD) with a THz antenna & [61] \\ \hline OFCG using phase modulator with SOA and UTC-PD & QPSK & 12.5 & 0.1 & Simulation/ experimental & This work presents a simple and efficient method for generating THz signals. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Closely Related Works in THz Generation. Figure 2: Simulation model for THz generation based on optical frequency comb by using single PM and SOA. At the heart of the transmitter is the generation of multiple OFC lines, which serve as the optical carriers for THz generation. This process begins with a continuous wave (CW) laser, which provides a stable optical carrier. To produce a broad spectrum of frequencies, a radio frequency (RF) source is used to modulate the phase of the laser output via a phase modulator (PM). This phase modulation results in the creation of multiple sidebands, forming a comb-like spectrum of OFC lines. In our proposed model, these initial OFC lines are further amplified using a single semiconductor optical amplifier (SOA). The purpose of the SOA is twofold: to increase the power of the generated OFC lines and to exploit the nonlinear properties of the SOA to generate additional lines through nonlinear interactions. Specifically, when a few OFC lines are launched into the SOA, nonlinear phenomena such as four-wave mixing occur, which effectively generate new frequency components, thereby increasing the total number of optical carriers available for subsequent processing. Following the OFC generation, the WSS is employed to select specific wavelengths or sets of wavelengths from the comb spectrum. This selection process is critical for controlling the spectral components that will participate in the THz wave generation, ensuring that the desired spectral lines are transmitted to the next stage with appropriate power levels. The selected optical signals are then modulated using in-phase and quadrature (IQ) modulators, which encode the information onto the optical carriers in a phase-coherent manner. These modulated signals are subsequently transmitted over the optical fiber and, after suitable processing, detected at the receiver end to recover the transmitted information and generate the THz signal. The simulation was conducted using the virtual photonic integration (VPI) tool and MATLAB R2023a running on a computer Windows 11 64-bit equipped with Intel Core i7-12700K, 3.6 GHz, and memory specifications of 32 GB RAM.