# Experimental Demonstration of Terahertz-Wave Signal Generation for 6G Communication Systems
## 3 Experimental Setup for THZ Signal Generation
The output of the PM is fed into two types of SOAs: the Kamelian SOA (with an input power of 3.47 dBm and an output power of 14 dBm) and the Amonics SOA (with an input power of 3.47 dBm and an output power of 14.4 dBm). Both SOAs yielded similar results, showing no significant change in the number of generated OFC lines, as illustrated in Figure 11a,b. When compared to the simulation results presented in Figure 3, the SOAs used in the lab have specific parameters that cannot be modified, including dimensions, optical confinement factors, insertion losses, and the index of gain couplers. The only adjustable parameter is the injection current for the SOAs, which is set at 0.25 A for the VPI SOA, 0.225 A for the Kamelian SOA, and 0.4 A for the Amonics SOA. The output of the OFC is first amplified using Amonics AEDFA and then directed to the WSS to isolate two primary carriers spaced 0.1 THz apart. One carrier operates at baseband without modulation, exhibiting a power level of \(-4.1\) dBm, while the second carrier, with a power level of \(-5.1\) dBm, is modulated using an IQ modulator. The modulated carrier supports data rates of 4 Gbps for QPSK, and is subsequently amplified with an EDFA. The input and output signals of the WSS are illustrated in Figure 12.  
Subsequently, the two carriers, one modulated and the other unmodulated, are combined using a 50:50 optical coupler and directed into a photodiode (PD) with an input power of \(-6.4\) dBm. This photodiode has a responsivity of 0.6 A/W allowing us to obtain the electrical 0.1 THz signal. However, due to hardware limitations, the analyzer is unable to display the electrical signal in the 0.1 THz range. Specifically, since the oscilloscope used has  
Figure 11: Optical frequency comb lines after (**a**) Kamelian SOA and (**b**) Amonics SOA. Figure 12: Optical spectrum of WSS input and output. a 32 GHz bandwidth and cannot directly measure the 0.1 THz signals, we employed a heterodyne down-conversion technique. This involved mixing the high-frequency THz signal with a local oscillator (LO) signal to produce an intermediate frequency (IF) within the measurement bandwidth of the oscilloscope. The resulting lower-frequency signal could then be accurately captured and analyzed. Although we can assess the performance of the generated THz signal, as will be elaborated in the following sections. The electrical signal is then fed into a low-noise amplifier, which is necessary because the signal transmitted over a 0.1 THz RF link experiences significant power loss. The amplified signal is transmitted over a 1-m wireless channel using two horn antennas. The THz signal is subsequently captured by a high-bandwidth digital storage oscilloscope, which features a bandwidth of 32 GHz and a sampling rate of 80-GSa/sec. The received signals undergo analysis using vector signal analyzer software VSA 21.2. Before demodulation, several digital signal processing (DSP) steps are performed, including signal down-conversion, carrier and clock recovery, baseband filtering with a root-raised cosine (RRC) filter with a roll-off factor of 0.35, and adaptive equalization to compensate for linear distortions. The EVM analysis was conducted across three different scenarios to evaluate the system's performance under various transmission conditions. The first scenario involved a BtB configuration, serving as a baseline measurement where the signal is processed without any fiber transmission. Results from this scenario are presented in Figure 13, providing a reference for the system's optimal performance. It is observed that the EVM remains within acceptable limits up to a certain power level, with the forward error correction (FEC) threshold for QPSK modulation, typically around 37% rms as reported in [65; 66], being reached at approximately \(-13.5\) dBm. The constellation diagram accompanying this figure illustrates the quality of the received signal at this threshold, showing well-defined constellation points with minimal distortion, which indicates reliable demodulation and decoding.