## 2 Device Description  
The schematic of the multi-section PIC is shown in Figure 1a. The device is fabricated using a standard 1550 nm laser material (InP). Five strained ((Al\({}_{0.23}\)Ga)\({}_{0.3}\)In\({}_{0.7}\)As) quantum wells of 5 nm thickness form the active region. This active region is placed on an n-doped InP substrate. There are in total 22 slots with a depth of 1.395 \(\upmu\)m, as shown in Figure 1b, which represents the side view of the device. These slots are used to control the longitudinal spectrum. Standard electron beam lithography is used in the fabrication of the slots to provide precise control of the slots' dimensions (width, depth) and location [16]. The total length of the device is \(\sim\)1.5 mm. The PIC consists of two regrowth-free Fabry-Perot (FP) lasers, integrated into a master-slave configuration. Each laser consists of two electrically isolated sections, with different cavity lengths named gain and reflector sections, as shown in Figure 1c. The gain section of each laser is a FP structure that generates and amplifies the light inside the cavity. This gain section has an anti-reflection (AR)-coated facet. Each of the multiple reflector sections of the laser serves as a grating. The multiple uniformly distributed slots in the reflector section act as a filter that enhances various FP mode(s), but suppresses other modes of the FP resonance. Hence, it restricts the FP laser to a single mode of the allowed FP cavity modes. This results in achieving a single longitudinal mode emission [17]. These distributed slots can be considered as an active distributed Bragg grating [18]. The left facet at the end of the slots region (reflector section) is high-reflection (HR)-coated. Both reflector sections of the two lasers are etched into the surface ridge waveguide, close to the active region, to provide internal reflections and enable a high SMSR (\(\sim\)50 dB) [16]. Furthermore, they provide improved control of the emission wavelength and the injection power inside the PIC. Both sections of the laser (gain and reflector) share the same active layer and InP substrate. Besides consisting of two main sections, which are the master reflector (MR) and the master gain (MG), the master laser shares the slave reflector (SR) with the slave laser, which enables a reduction of the overall length of the device, providing better confinement and control of the total temperature of the PIC. The length of each section is optimized for its specific functions which enable the optimization of the PIC as a device for employment in short reach networks. The master gain is designed to be the longest of all sections (712 \(\upmu\)m), in order to achieve a low linewidth emission. The slave gain section, on the other hand, is much shorter (355 \(\upmu\)m), to ensure a high modulation bandwidth. The two reflector sections have an equal length of 215 \(\upmu\)m. The isolation between different sections is achieved by separating them with a 2 \(\upmu\)m wide etched trench, whose impedance is measured to be 2.29 k\(\Omega\). As the PIC consists of two distinct lasers, two independent spectral modes, corresponding to each laser, can be generated by the device. A continuous-wave (CW) spectrum of the master (biased at 35 mA) and slave (biased at 90 mA), operating in single-mode regime, is shown in Figure 2a (OSA resolution: 20 pm). Biasing the device at \"bias point 1\" listed in Table 1 results in achieving the injection locking spectrum shown in Figure 2b. To further confirm the independence of the two lasers, the two independent spectral modes are shown again in Figure 2\(c\), where the OSA is set to a finer resolution of 0.04 pm. The master laser and slave lasers are biased at 40 and 60 mA, respectively. OIL is achieved again at \"bias point 2\" listed in Table 1 and its spectrum is shown in Figure 2d. It is important to note that OIL could be achieved at several other bias points, but we only highlight the two listed points for brevity.