## 3 Static Characterization
### Linewidth and RIN Measurements
\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) & 97.9 & 50.98 & 98 & 92 \\ Bias point 3 (mA) & 99.93 & 99.64 & 56.73 & 54.83 \\ Bias point 4 (mA) & 93.7 & 65.18 & 34.67 & 68.96 \\ Bias point 5(mA) & 100.18 & 69.7 & 45.35 & 85.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Bias points for linewidth measurements. be a second generation of this device, which could be expected to give improved performance and linewidth results. However, the reduced linewidth as a result of the injection can still enable the device to be used with higher order modulation formats at high baud rates [24]. RIN is another important parameter of an optical transmitter, quantifying the intensity fluctuations of the laser output power [25]. High RIN values (\(>\)\(-\)120 dB/Hz) can be a limiting factor for the usage of a laser in DCNs, especially ones employing multi-level intensity modulation schemes such as PAM 4/8 [26]. Moreover, it can also have a negative effect on other modulation schemes such as orthogonal frequency division multiplexing (OFDM) and discrete multi-tone transmission (DMT) [27] that may be used to directly modulate the laser transmitter. The RIN characterization of the proposed four-section device has been carried out using an experimental set-up depicted in Figure 5d [28]. The optical emission of the DC biased device is detected by a 20 GHz high-speed photodetector (Thorlabs DXM20 AF). The output of the photodiode is then connected to a bias tee (through the RF + DC terminal) to separate the DC and the AC components of the generated photocurrent. The DC port of the bias tee is connected to a multimeter, while the AC output is connected to an electrical amplifier, followed by a 6 GHz electrical spectrum analyser (ESA) (R&S FSL6). The latter is used to capture the AC component, resulting from optical power fluctuations of the laser. The RIN is computed based on both the AC component obtained from the ESA and the DC component obtained from the multimeter, using the equation below:  
\[\text{RIN}\ =\ \frac{\Delta P_{E}}{P_{EO}}, \tag{1}\]  
where \(P_{EO}\) is the average electrical power (DC component), while \(\Delta P_{E}\) is the overall noise of the laser (AC component). The latter consists of three different components: the noise of the laser itself (\(N_{L}\)), the thermal (\(N_{th}\)), and shot (\(N_{q}\)) noise. Figure 5c shows the obtained RIN values for bias points \"1 and 2\" listed in Table 4 below with and without injection. The obtained averaged value of RIN (DC to 6 GHz) of \"bias point 1\" in Table 4 without injection (blue trace) (both MG and MR biases are set to zero) is \(-\)143.4 dB/Hz. Turning the master laser sections on (MG = 112 mA, MR = 74 mA) (red trace) decreases the average RIN to \(-\)146.1 dB/Hz. The reduction of RIN value for the injected case can be attributed to the reduction of cavity gain by the injected signal. This reduction also causes the carrier density to deplete, resulting in a decrease of the overall spontaneous emission rate, which in turn reduces the overall RIN of the PIC [29]. A similar reduction in the RIN value is shown also for \"bias point 2\" listed in Table 4 in the same figure. The RIN value obtained without injection (green trace) is \(-\)151.6 dB/Hz. This value decreases to \(-\)154.3 dB/Hz by turning the master laser on (MG = 45 mA, MR = 87 mA) and applying optical injection (orange trace). The obtained results demonstrate that the integrated device has a relatively low RIN value qualifying it as a potential candidate to be used in direct intensity modulated short-reach communication networks [30]. Having concluded the static characterization of the PIC, we further investigate the dynamic performance of the PIC to determine its usefulness as a directly modulated transmitter for DCNs.