## 3 Static Characterization
### Linewidth and RIN Measurements  
With the ever-growing demand for bandwidth, future short-reach networks may need to employ coherent modulation schemes that combine both amplitude and phase modulation to increase the amount of data being transferred through an optical network [21]. In such networks employing higher-order modulation formats, optical transmitter parameters such as the linewidth and RIN play an important role in determining the overall system performance. Hence, the linewidth and RIN are characterized to determine the suitability of the proposed PIC to be used in next-generation coherent short-reach networks. Characterization results show that applying injection from the master laser into the slave laser improves important parameters such as linewidth and RIN, as long as the device is operating at single-mode regime. Many operating OIL bias points listed in Table 3 could be chosen as highlighted earlier. However, for the sake of clarity, only two of these optimum operating points' (bias points 1, 2 listed in Table 3) linewidth results are highlighted in this paper. Figure 4: Contour plots of varying SG, SR bias currents and observing the following parameters: (**a**) slave peak power of the main mode; (**b**) slave SMSR of the output modes of the PIC; (**c**) slave emission wavelength of the main mode; and varying MG, MR bias currents and observing the following parameters: (**d**) master peak power of the main mode; (**e**) master SMSR of the output modes of the PIC and (**f**) master emission wavelength of the main mode. The delayed self-heterodyne (DSH) technique [22], with a resolution of 50 kHz, is used to measure the linewidth of the device at different operating points. The linewidth measurements results are presented in Figure 5a,b where the PIC is biased at two different optimum operating points \"bias points 1, 2\" as indicated in Table 3 below for Figure 5a,b, respectively. The blue trace shows the master laser linewidth on its own, the red trace shows the slave laser linewidth on its own, and the green trace shows the linewidth of the slave laser injection-locked by the master laser. From Figure 5a, it is evident that the free-running slave laser has a large linewidth of 8 MHz, which can be ascribed to its short cavity. The long cavity master laser at bias current of 112 mA has a much lower linewidth of 2 MHz. The optical linewidth of the slave laser can be significantly reduced by employing OIL from the lower linewidth master laser. OIL is achieved at bias point 1 listed in Table 3. As OIL allows for the transfer of the phase noise characteristics from the master to the slave laser [23], the linewidth of the latter is reduced to 2 MHz, as shown in Figure 5a. Similar behaviour is shown in Figure 5b, where again the lower linewidth of the master laser (2.7 MHz) was imposed into the slave laser through OIL. This results in decreasing the slave laser large linewidth from 5 MHz to 2.7 MHz (the device is biased at \"bias point 2\" listed in Table 3). It is important to note that this PIC is a proof of concept device and the potential of further improvement exists. We anticipate that there will \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Section** & **SG** & **SR** & **MG** & **MR** \\ \hline Bias point 1 (mA) & 45 & 58 & 112 & 74 \\ Bias point 2 (mA) & 100.16 & 69.7 & 45.3 & 87.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Bias points for RIN measurements. Figure 5: (**a**) Electrical spectra of the measured linewidth of master laser (ML), slave laser (SL) and injection-locked slave laser (IL SL) the master laser for bias point 1 in Table 3; (**b**) same for bias point 2 in Table 3; (**c**) RIN values for bias points 1,2 in Table 4 with (red, orange) and without injection (blue, green) and (**d**) RIN measurement setup diagram.