## 3 Experimental Results
### Dual LO Generation  
To experimentally validate the optical heterodyning system, signals were generated at 29 GHz and 41 GHz. These specific frequencies were selected due to their direct relevance to the THORMUX project. Table 1 shows the different operating conditions under which the three lasers were used to generate the signals of interest. Figure 8: Measurement of the (**a**) optical linewidth [23] and (**b**) phase noise of the reference laser used in the optical heterodyne system. Laser 2 exhibits an optical linewidth of 30 kHz, measured using a delayed self-heterodyne setup. Additionally, the phase noise analysis revealed values below \(-110\) dBc/Hz at a 1 MHz offset. Figure 7: The wavelength tuning map for laser L4 was obtained by varying the voltage applied to the micro-ring resonators in the external cavity. The laser exhibited a tuning range higher than 70 nm. During these measurements, the SOA current was kept constant at 100 mA. The optical and electrical spectra, measured under these operating conditions, are presented in Figure 9a,b, respectively. In the optical spectrum, each laser exhibits a side-mode suppression ratio (SMSR) exceeding 50 dB, indicating a high degree of spectral purity and stable single-mode operation. This ensures minimal interference from unwanted side modes, which is critical for high-precision applications. Furthermore, as shown in Figure 9b, the two-tone generation at 29 GHz (Ka-band) and 41 GHz (V-band) was successfully achieved. Additionally, a 70 GHz signal is generated due to the beating between laser 3 and laser 4. However, this signal lies outside the bandwidth of the ESA and is therefore not displayed. To evaluate the independent tunability of the generated LO signals, the wavelength of laser 3 was gradually increased, thereby increasing its separation from laser 2. This process allowed the Ka-band LO signal to be tuned to three distinct frequencies within the Ka-band: 29 GHz, 31 GHz, and 32 GHz, as shown in Figure 10a. Notably, the tuning of the Ka-band signal did not significantly impact the V-band LO signal, demonstrating the system's capability to maintain stability and independence across multiple frequency bands. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Laser** & **IsoA [mA]** & **V\({}_{\text{RI}}\) [V]** & **V\({}_{\text{R2}}\) [V]** & **V\({}_{\text{HI}}\) [V]** & **V\({}_{\text{H3}}\) [V]** \\ \hline Laser 2 & 103 & 6.6 & 8 & 3.6 & 5.3 \\ Laser 3 & 152 & 4.2 & 13.1 & 7.21 & 7 \\ Laser 4 & 134 & 12.6 & 13.6 & 2.1 & 10.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Operating conditions of the lasers in the heterodyne system for generating signals at 29 GHz (Ka-band) and 41 GHz (V-band). Figure 9: **(a)** Optical spectrum showing the three optical sources used in the heterodyne system. Laser 2 serves as the reference, while tuning lasers 3 and 4 enables the generation of local oscillator signals in the Ka- and V-bands. **(b)** Electrical spectrum of the dual local oscillator signals, demonstrating generation of carriers at 29 GHz and 41 GHz [23]. In addition, the electrical linewidth of the generated signals was measured, showing that all signals have linewidths below 100 kHz. Specifically, the measured linewidths were 45 kHz, 72 kHz, and 48 kHz at 29 GHz, 31 GHz, and 32 GHz, respectively. Figure 10b shows the electrical linewidth measurement for the highest frequency signal generated in the Ka-band. These results underscore the high precision and stability of the generated signals. Furthermore, since the electrical linewidth reflects the combined optical linewidths of the lasers involved in signal generation, these measurements highlight the potential for significantly reducing optical linewidth through careful optimization of the laser biasing. By precisely adjusting both the gain and heater sections, the optical linewidths can be reduced to less than half of those obtained when only the gain sections are biased. This illustrates that fine-tuning the laser biasing conditions can lead to highly stable, narrow-linewidth signals.