## 3 Experimental Results
### Characterization of Single-Gain Lasers  
The characterization of the three lasers used in the optical heterodyne system was performed before the RF signal generation. Figure 5 displays a typical measurement of the fiber-coupled output power of the lasers as a function of the pump current. All three lasers have a threshold current of approximately 11 mA, with maximum output powers of 23 mW for laser 2, 2 mW for laser 3, and 18 mW for laser 4. As shown, even under identical operating conditions, the power levels of each laser differ. These disparities arise from variations in coupling losses between the InP and Si\({}_{3}\)N\({}_{4}\) chips for each laser, as well as minor differences in the reflectivity of their respective SOAs. However, by adjusting the current injected into the gain section and fine-tuning the voltages applied to the heaters within each cavity, output powers exceeding 10 dBm can be achieved for each laser, as demonstrated in [17]. The observed power drop in the three curves is attributed to refractive index changes in the InP PIC waveguide, which occur as the current injected into the gain section increases. These refractive index variations induce a slight shift in the emission wavelength, moving it out of resonance with the Vernier filter created by the two ring resonators [20]. Figure 4: The experimental setup developed for laser characterization and local oscillator (LO) signal generation includes a power meter, used to measure the output power of each laser, as well as optical and electrical spectrum analyzers. The optical spectrum analyzer captures the optical spectrum at the output of the laser module, while the electrical spectrum analyzer records the electrical spectrum after the photodiode. In this work, the single-gain lasers are labeled as L2 (laser 2), L3 (laser 3), and L4 (laser 4). Figure 5: Optical power versus current curve of the three lasers used in the heterodyne system. The lasers exhibit a threshold current of approximately 11 mA, and the power drops indicate mode hops. The measurement was taken at a wavelength of 1550 nm. The curves obtained in this work align with typical behavior observed for these types of lasers. The V-I characteristic curve was not obtained in this study due to the use of a conventional electronics control module, which lacked the capability to monitor the dynamic resistance of the lasers. However, as outlined in the PDKs provided by the foundries, the average power consumption of the lasers is less than 2 W, with approximately 0.5 W attributed to the gain section and 1.4 W to the actuator heaters. While the current setup does not allow for direct extraction of the V-I curve, this limitation can be addressed by implementing a dedicated interposer PCB equipped with separate power supplies to facilitate such measurements. For the generation of the LO signals, laser 2 was used as a fixed reference, and the dual RF signals were generated by combining laser 2 with laser 3 and laser 4, respectively. Independent tuning of the generated signals was achieved by adjusting the wavelengths of lasers 3 and 4. Therefore, obtaining the tuning maps for lasers 3 and 4 is essential to determine the frequency tuning ranges of the generated signals. Figure 6 presents the tuning map of the emission wavelength of laser 3. The tuning of this type of laser is achieved by independently varying the voltage applied to the heaters in the micro-ring resonators. To generate the tuning map, the voltage applied to heater R2 was incrementally increased from 0 V to 12 V at each step. Simultaneously, the voltage on heater R1 was varied within the same range, such that for each voltage setting on R1 (ranging from 0 V to 12 V), the voltage on R2 was adjusted from 0 V to 12 V. This approach allowed for independent control over the tuning of both micro-ring resonators, and the voltage on each micro-ring was adjusted in 0.2 V steps. By tuning both micro-rings, a wavelength tuning range of approximately 80 nm was observed in laser 3.