# High-Speed Directly Modulated Laser Integrated with SOA
## 3 Device Characterization  
Figure 2a shows the power-current (P-I) characteristics of the DML with and without an integrated SOA. The pink curve serves as the baseline for the DML without SOA, while the other curves illustrate the performance with SOA under varying SOA currents \(I_{S}\). The slope, indicative of modulation efficiency, is determined by the modulation current \(I_{M}\). Power saturation becomes evident as \(I_{M}\) approaches 70 mA, resulting in a reduced slope efficiency and limiting further optical power scaling. Without SOA, the optical power remains below 20 mW; however, with SOA, it increases to 28 mW at \(I_{M}=60\) mA and \(I_{S}=30\) mA, and scales further with increasing \(I_{S}\). Figure 2b depicts the relationship between SOA gain and both \(I_{M}\) and \(I_{S}\). Notably, at \(I_{S}=1\) mA, the gain plateaus for \(I_{M}>40\) mA, indicating SOA gain saturation and limiting amplification capacity. In this saturation regime, the increase in carrier density, driven by higher \(I_{S}\), becomes proportional to the increase in photons involved in stimulated emission. Consequently, for a constant \(I_{M}\), the optical power increase in the linear region is directly proportional to the increase in \(I_{S}\). The SOA consumes approximately 36.84 mW under a driving current of 30 mA and a voltage of 1.228 V. In this device, the SOA section is relatively short (150 \(\upmu\)m) and exhibits strong optical confinement, resulting in a low saturation power (\(P_{\mathrm{Inst}}\)), which makes it prone to entering the saturation regime. As shown in Figure 3, under a fixed DC bias applied to the DFB section, the SOA gain initially increases rapidly with increasing injection current above threshold, but then saturates and even slightly decreases. This behavior reflects a typical static gain compression effect, indicating that the SOA can easily reach gain saturation even under relatively low input optical power. This static behavior lays the foundation for the frequency-dependent dynamic gain response under modulation. As further confirmed by the small-signal frequency response shown in Figure 3, the gain compression is more pronounced at low modulation frequencies, while the gain remains higher at high frequencies. This results in a typical quasi-high-pass filtering characteristic exhibited by the SOA. Figure 1: (**a**) Schematic diagram of the proposed DML. (**b**) Top-view optical microscope image of the fabricated laser chip. To understand the SOA's impact on the DML's dynamic performance, we measured the small-signal \(S_{21}\) response of both the SOA region alone and the integrated DML, as shown in Figure 3 and Figure 4, respectively. The measured \(S_{21}\) response of the SOA region (Figure 3) exhibits characteristics consistent with a standalone SOA [15], confirming the quasi-high-pass filtering effect introduced by the SOA integration. Figure 3: (**a–d**) Frequency response of the SOA under different \(I_{M}\) and \(I_{s}\). Figure 2: (**a**) Measured P–I characteristics in continuous wave mode of the DML with and without SOA. (**b**) Power gain of the DML with SOA measured under different currents \(I_{S}\). For comparison, DMLs without the SOA region but with identical DFB parameters, were fabricated. Figure 4e shows the frequency response of the DFB laser without SOA integration. A noticeable dip appears in the low-frequency region (2-12 GHz), and the overall curve is not flat. The \(-3\) dB bandwidth is approximately 18.5 GHz, indicating a performance limitation caused by factors such as modulation efficiency and carrier recombination rate. Figure 4a-d present the frequency responses of the DFB-SOA integrated laser under different DFB bias currents. In each subfigure, multiple curves correspond to various SOA bias currents (\(I_{\text{S}}\)), including the case where the SOA is inactive (\(I_{\text{S}}=0\)) and active (\(I_{\text{S}}>0\)). It can be observed that for all DFB bias conditions, the curves at \(I_{\text{S}}=0\) exhibit a similar low-frequency dip as seen in Figure 4e.