## 3 Static Characterization
### DC Characterization of the PIC  
Here, we start by applying different combinations of DC bias to the four different sections of the PIC using a multi-channel current controller (LDC-3900). The light emitted by the SG section is recorded using an OSA with a spectral resolution of 20 pm. Several important parameters can be extracted from the optical spectrum, such as the main mode peak power and emission wavelength, as well as the SMSR. Both the current controller and the OSA are driven by an automated Python script, which varies the bias of a specific section at each sweep. Contour plots are generated for both the slave and master lasers. Figure 4a-c show the main mode peak power, SMSR, and main mode emission wavelength of the slave laser, as a function of the SG and SR bias currents, respectively. For this test, the master laser is turned off (MG and MR biased at 0 mA). Figure 4d-f show the same parameters for the master laser, as a function of MG and MR bias currents. Here, the slave laser is biased just above transparency (SG = 10 mA, SR = 5 mA) to minimize any interaction between master and slave lasers. The SG and MG sections biases are varied from 0 to 150 mA, while the SR and MR section biases are varied from 0 to 100 mA as per the safe operating conditions of the PIC. From Figure 4a, it can be observed that the SG has an average threshold of around 32 mA, when the SR is biased over 20 mA. This threshold value decreases with the increase of the SR bias. In addition, it can be noted that lasing from the SR section can be observed when the SG is at least biased at 30 mA and the SR is biased at least 65 mA. From Figure 4a,b, it can be seen that for single-mode operation (high SMSR > 40 dB), the peak power of the slave reaches a maximum of 9 dBm. The SMSR contour plot in Figure 4b allows one to identify the bias currents that result in a high SMSR (dark red regions) as well as the currents at which the device shows multi-mode behaviour (blue and cyan regions). The latter has been confirmed by observing a bi-modal operation of the slave laser on the OSA. A high SMSR of around 55-60 dB can be achieved for several bias points. Figure 4c shows the emission wavelength of the slave laser main mode. From the plot, it can be observed that by varying the currents, the wavelength of the slave laser can be tuned gradually over \(\sim\)2 nm, before a mode hop occurs (marked by an abrupt change in colour). The multi-coloured region on the left of the plot corresponds to the noise in the OSA, since for those currents the laser is biased below the threshold. The gradual variation in the emission wavelength (between the mode hops) is caused by temperature changes due to the Joule effect [19] occurring with the increase in the bias of the SG and SR sections. When the temperature increases, the central emission wavelength of the laser experiences a redshift [20]. This dependence is more obvious for the SG section than for the SR, due to lack of a grating in the SG section. Master laser characterization shows that the MG has an average threshold of \(\sim\)45 mA (around 13 mA higher than SG threshold), when the MR is biased higher than 20 mA, as depicted in Figure 4d. The higher threshold value can be attributed to the fact that the master laser has a longer cavity length. Again, it can be seen that the MR loses only when the MG is biased above 40 mA, as illustrated in the same figure. The master laser SMSR contour plot shown in Figure 4e manifests a comparable behavior with that of the slave laser. By careful control of the biases applied to the two sections of the master laser, a SMSR as high as 50 dB can be achieved. Figure 4f shows the capability of the master laser wavelength to be gradually tuned over \(\sim\)1.5 nm, through variation of the bias currents. Mode hopping only takes place for a few points of bias currents where we experience low SMSR.