## 2 Device Description
Figure 1: (**a**) InP multi-section photonic integrated circuit (PIC) 3D schematic diagram; (**b**) 2D side view of the waveguide structure; and (**c**) 2D two section single laser waveguide structure. Figure 2: (**a**) Continuous-wave (CW) spectrum of the master (biased at 35 mA) and slave (biased at 90 mA) lasers operating in single-mode operation; (**b**) PIC output spectrum after OIL is achieved (bias point 1 in Table 1). A 20 dB optical attenuator is placed at the input of the OSA for both optical spectra. (OSA resolution: 20 pm); (**c**) CW spectrum of the master (biased at 40 mA) and slave (biased at 60 mA) lasers operating in single-mode operation and (**d**) PIC output spectrum after OIL is achieved (bias point 2 in Table 1). A 30 dB optical attenuator is placed at the input of the OSA for both optical spectra, (OSA resolution: 0.04 pm). The independence of the two spectral modes and the fact that each of the distinct modes originates from a different laser can be confirmed by calculating the wavelength/frequency difference between the cavity modes. Figure 3a,b show the optical spectra of the two distinct lasers measured using a 20 pm high-resolution OSA (Yokagawa AQ6370). Figure 3a shows the CW optical spectrum of the slave laser (SG = 80 mA, SR = 40 mA) taken when the master laser is turned off (MG,MR = 0 mA). Within the given OSA resolution, the obtained free spectral range (FSR) between the modes extents between 85 and 100 GHz. A 85 GHz frequency difference corresponds to a cavity length of 550 \(\upmu\)m, which is approximately equal to the length of the slave laser cavity (SG + SR sections) (570 \(\upmu\)m). This is calculated using the following formula: \(\Delta\
u=\text{c}/2\text{nL}\), where \(\Delta\
u\) is the FSR, with n a refractive index of 3.2 and a cavity length of L. Figure 3b shows the CW optical spectrum of the master laser (MG = 80 mA, MR = 8 mA), when the slave laser is biased at transparency (SG = 20 mA, SR = 5 mA). Here, the mode spacing is around 50 GHz, thus corresponding to a cavity length of 937 \(\upmu\)m--approximately equal to the master laser cavity length (MG + MR sections) (927 \(\upmu\)m). These results confirm the possibility of achieving independent single mode emission from each distinct laser. The master laser emission wavelength can then be controlled, by varying its bias and tuned to achieve OIL. The obtained results are summarized in Table 2 below for clarity. The PIC can be operated in an OIL regime by fine-tuning the DC bias applied to the different sections. However, it is important to note that under certain bias points, the PIC portrays operation in a coupled cavity regime. This is mainly because the two lasers share the same optical waveguide, which leads to multi-cavity effects. Furthermore, the presence of finite electrical isolation between the different sections suggests the presence of some current leakage between the device sections. This current leakage, even if small, would mean that variation of the bias applied to one section would affect the other section. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sestion** & **SG** & **SR** & **MG** & **MR** \\ \hline Bias point 1 (mA) & 93.7 & 65.15 & 34.67 & 67.65 \\ Bias point 2 (mA) & 65 & 39 & 54 & 49 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Bias points for injection locking spectra. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bias Condition** & **Frequency Spacing** & **Calculated Cavity Length** & **Actual Cavity Length** \\ \hline SG = 80 mA, SR = 40 mA, & & 85 GHz & 550 \(\upmu\)m & SG + SR = 570 \(\upmu\)m \\ MG,MR = 0 mA & & & \\ SG = 20 mA, SR = 5 mA, & & & \\ MG = 80 mA, MR = 8 mA & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cavity length correpondence calculation. Figure 3: (a) Slave laser CW spectrum (master laser is turned off); (b) Master laser CW spectrum (slave laser is biased at transparency). A 20 dB optical attenuator is placed at the input of the OSA for both optical spectra. and (c) 7-pin high speed butterfly package of the multi-section PIC.