# Experimental Demonstration of Terahertz-Wave Signal Generation for 6G Communication Systems
## 3 Experimental Setup for THZ Signal Generation  
The experimental setup for the photonic THz generation technique is depicted in Figure 9. The initial stage focuses on the generation of an OFC, with a SOA as the primary component to produce additional OFC lines. A low-noise single-frequency continuous wave (CW) laser diode operates at a wavelength of 1552.52 nm (193.1 THz) with an output power of 16 dBm and a linewidth of 100 kHz. This laser is connected to a phase modulator (PM) from Eospace, which is driven by a 12.5 GHz RF signal generated by a vector signal generator (VSG) with 11.3 dBm output power. The output spectrum of the optical signal before and after modulation are illustrated in Figures 10a and 10b, respectively. Figure 10: Output spectrum (**a**) before PM and (**b**) after PM. Figure 9: Experimental photonic setup for THz signal generation. The non-ideal behaviors of SOA, particularly ASE noise and gain recovery dynamics, significantly influence system performance. The ASE noise generated within the SOA is a critical factor, with the noise figure (NF) typically ranging between 5-7 dB, representing the excess noise added by the amplifier. This elevated ASE noise directly contributes to increased phase noise and amplitude fluctuations in the optical signal, which deteriorate the signal-to-noise ratio. Consequently, these effects impair phase coherence and cause uneven flatness across the comb spectrum, leading to higher bit error rates (BER) and increased error vector magnitude (EVM). Additionally, the carrier lifetime (\(\tau\)c) and gain recovery time (\(\tau\)g) are vital parameters that determine the dynamic response of the SOA. Usually spanning hundreds of picoseconds to nanoseconds, the carrier lifetime influences how quickly the gain can respond to input power fluctuations. The gain recovery time indicates how long it takes for the gain to stabilize after saturation; shorter recovery times enable better handling of high-speed modulation but may induce gain fluctuations. For example, a carrier lifetime of around 300 ps and a gain recovery time near 1 ns can lead to gain saturation effects during high modulation rates, causing amplitude and phase distortions. These gain dynamics introduce amplitude modulation (AM) to phase modulation (PM) conversion, resulting in phase noise and reduced phase coherence among the comb lines. Moreover, gain fluctuations can cause spectral non-uniformity, affecting the flatness of the comb spectrum [63]. The combined effects of ASE noise and gain dynamics also impact the uniformity and coherence of the comb lines. Variations in ASE noise and gain behavior can cause amplitude deviations exceeding 1-2 dB across the spectrum, compromising line flatness. Simultaneously, gain fluctuations and ASE-induced phase noise introduce phase jitter, which reduces mutual coherence between lines. This can be characterized via phase noise spectral density or linewidth measurements, illustrating the degradation in phase coherence. These impairments translate into increased BER, especially in higher-order modulation schemes, where phase and amplitude noise elevate the likelihood of symbol errors often resulting in BER degradations of \(10^{-3}\) or more. Similarly, EVM, which quantifies the deviation of the received signal from the ideal constellation, increases with these instabilities, with values exceeding 10-15% indicating significant impairment due to SOA non-idealities [64]. In summary, the ASE noise characterized by the noise figure and the gain recovery dynamics governed by carrier lifetime and recovery times significantly shape the spectral and temporal qualities of optical combs. A thorough quantitative understanding of these factors enables system designers to optimize SOA biasing, implement noise filtering, and apply advanced signal processing techniques to mitigate their adverse effects, thereby improving comb flatness, phase coherence, BER, and EVM in practical systems.