# Experimental Demonstration of Terahertz-Wave Signal Generation for 6G Communication Systems
## 2 Simulation Model for THz Generation
These specifications ensured sufficient computational resources for the simulations. As depicted in Figure 2, the initial stage features a single CW laser source operating at 193.1 THz (1552.52 nm) with a linewidth of 100 kHz, alongside a sinusoidal RF signal at 12.5 GHz. Both the CW and RF signals are modulated using the PM, and the resulting output spectrum is presented in Figure 3. In this figure (blue), we observe that the generated OFC lines have a spacing of 12.5 GHz, resulting in a total of 24 lines. Subsequently, these OFC lines are fed into the SOA, where its nonlinear effects are harnessed to produce a greater number of OFC lines, thereby expanding the THz range. As shown in Figure 3 (red), the number of OFC lines obtained after incorporating the SOA rises to 68, approximately three times the amount without the SOA. This increase is attributed to the SOA operating in deep saturation, utilizing its nonlinear FWM-induced carrier density modulation (CDM) properties. Figure 3: Optical spectrum of the generated comb lines: with SOA (red) and without SOA (blue). FWM is a third-order nonlinear optical phenomenon that occurs when multiple optical waves interact in a nonlinear medium, generating new frequency components. It is a parametric process, meaning it does not involve energy exchange with the medium (i.e., no net energy is absorbed by the material). In this paper we utilized the FWM phenomenon to enhance the number of injected carriers into the SOA and establish phase correlation among all the comb lines. In the next stage, we need to select the two required optical frequency comb lines by the WSS, so the output optical spectrum of WSS will include two optical carriers with separation of \((n_{1}-n_{2})\times f_{\text{rf}}\). Where \(f_{\text{rf}}\) is the radio frequency of the RF source, \(n_{1}\) and \(n_{2}\) are the first and second selected carrier, respectively. For example, in our case, we selected \((n_{1}\!=\!4)\), \((n_{2}\!=\!-4)\), and \(f_{\text{rf}}\!=12.5\,\text{GHz}\), so the optical carrier separation will be (8 \(\times\) 12.5 GHz = 100 GHz = 0.1 THz). The first wavelength selected by WSS acts as an optical local oscillator (LO) at \(-4\)th order of OFC lines (at 193.05 THz), and the other is at +4 th order of OFC lines, which is carrying a 4 Gbps QPSK baseband modulation signal, as shown in Figure 4.  
On the receiver side, the two optical carriers are combined and passed through a variable optical attenuator (VOA). This step allows for the precise control of the signal's power level to optimize the performance of the subsequent components. The combined signal is then directed to a UTC-PD, which is a highly efficient photodetector with a responsivity of 1 A/W, indicating that it generates 1 ampere of current for every watt of optical power it receives. Within the UTC-PD, the two optical carriers interfere with each other. This interference results in the generation of a photonic vector THz-wave signal, characterized by a center frequency of 0.1 THz. The process of interference within the UTC-PD is crucial as it converts the optical signals into a terahertz signal, which is essential for high-frequency wireless communication applications. The generated THz signal is modulated using QPSK, a modulation scheme that allows the transmission of two bits per symbol, thereby increasing the data rate. The input optical power to the UTC-PD is maintained at \(-4\) dBm, a level chosen to ensure optimal performance and to avoid any potential issues, such as signal distortion or excessive noise. This entire process is comprehensively illustrated in Figure 5, which offers a detailed visual representation of the signal flow and the interactions among various components within the system. The figure depicts how the optical carriers generated from the laser sources are first combined using a beam combiner, forming a composite optical signal that contains the necessary information for subsequent processing.