## 2 Hybrid Multi-Laser Module  
Figure 1 shows the hybrid multi-laser module, which consists of five integrated InP/Si\({}_{3}\)N\({}_{4}\) hybrid lasers: a dual-gain laser (laser 1, intended for optical frequency comb generation) and four single-gain lasers (lasers 2 through 5, intended for optical heterodyning). The lasers are mounted on a common thermally stabilized base, with thermal stabilization provided by a Peltier, a temperature controller (TEC), and an NTC thermistor. The core of the hybrid multi-laser module consists of a 1.6 cm \(\times\) 2.74 cm TriPlex(r) PIC from LioniX (Enschede, The Netherlands), six InP gain sections developed by Fraunhofer HHI (Berlin, Germany), and two fiber arrays, all mounted on a copper submonut. The module has a final footprint of 74 mm \(\times\) 122.2 mm and was operated at a constant temperature of 21 \({}^{\circ}\)C during all measurements. All relevant electronics are mounted on an \"n\"-shaped base plate. Dual LO generation is demonstrated in this work, using three out of the four single-gain lasers. The layout of the four single-gain lasers is presented in Figure 2. The hybrid laser was created through butt coupling an InP quantum well (QW) gain chip and a passive Si\({}_{3}\)N\({}_{4}\) chip (highlighted by a dashed red box). The Si\({}_{3}\)N\({}_{4}\) PIC includes two coupled micro-ring resonator mirrors with slightly different Free Spectral Ranges (FSRs), 208 GHz for micro-ring resonator 1 (top) and 215 GHz for micro-ring resonator 2 (bottom). This configuration enables single-frequency operation through the Vernier effect [21], since the difference in FSRs allows only one wavelength to be aligned in both micro-resonators at a time, suppressing other modes. This selection scheme results in single-frequency operation because the overlapping resonant wavelengths create a unique feedback condition that stabilizes the laser at a single, well-defined frequency. Thus, the Vernier effect enables selective resonance by tuning only a single frequency within the external cavity. As shown in Figure 2, the cavity includes five phase thermo-optic actuators (heaters): one (H1) to determine the frequency positions of the cavity modes, two (H2 and H3) to control the light levels entering and exiting the laser cavity, and two (R1 and R2) to control the resonance of the micro-ring resonators and tune the laser wavelength. On the other hand, the InP gain PIC consists of a semiconductor optical amplifier (SOA) with a highly reflective coating on the left side and an anti-reflective coating on the right side, allowing the emission of light into the Si\({}_{3}\)N\({}_{4}\) chip. In addition to the four single gain lasers, the PIC includes a dual-gain laser, with its layout shown in Figure 3. As illustrated, the primary difference between the single-gain and dual-gain lasers lies in the addition of a second gain section in the latter. Furthermore, a second thermo-optic actuator, identified as H4, has been incorporated into the Si\({}_{3}\)N\({}_{4}\) PIC to control the phase of the light. The actuators utilize the thermo-optic effect, where changes in temperature induce variations in the refractive index of the waveguide material. While the physical length of the waveguide remains unchanged, these  
Figure 1: An image of the hybrid multi-laser subsystem, consisting of the laser module and control electronics. On the **right**, a detailed view of the TriPlex® photonic integrated circuit is shown, which includes the six InP chips and the Si\({}_{3}\)N\({}_{4}\) chip. The image also identifies two fiber arrays: FA1, containing 28 fibers, and FA2, with 24 fibers. Figure 2: Layout of the InP/Si\({}_{3}\)N\({}_{4}\) hybrid integrated single-gain lasers. The laser cavity is formed by an InP chip (gain section) and a Si\({}_{3}\)N\({}_{4}\) chip (dashed red box) as the external cavity. Single-mode operation is achieved using a Vernier filter consisting of two Si\({}_{3}\)N\({}_{4}\) micro-ring resonators.