## 3 Experimental Results and Discussion  
### Small-Signal Modulation Response  
The fabricated IMC laser chip was mounted onto an AlN submount upon a Cu heat sink, enabling the microwave signal to feed into DFB1 through a ground-signal-ground (GSG) probe. The stage temperature was stabilized at 20 \({}^{\circ}\)C by a thermoelectric cooler for detailed device characterization, and each section was driven by a precise current source. The laser from the DFB1 section's facet was coupled into a lensed SMF and tested by an optical spectrum analyzer (Advantest Q8384) with 0.01-nm resolution. Meanwhile, the small-signal modulation responses were measured with a vector network analyzer (VNA, HP 8510C). The working condition of the IMC laser with varying I\({}_{\textrm{DFB1}}\) values was investigated, while I\({}_{\textrm{DFB2}}\) and I\({}_{\textrm{SOA}}\) were fixed at 40 and 5 mA, respectively. Figure 5 shows the optical spectra of the IMC laser in the unlocked and MIL states, respectively. As shown in the red curve in the figure, the lasing mode of the IMC laser with I\({}_{\textrm{DFB1}}\) = 131 mA was locked in single-mode and entered the MIL state. Additionally, a small sidelobe appeared (Figure 5) due to the red-shifted cavity mode caused by the interaction between the two lasers under the MIL state [24]. Upon increasing I\({}_{\textrm{DFB1}}\), the IMC laser went through a series of nonlinear dynamic states [33]. When the DFB1 section was biased at 142 mA, the IMC laser operated under the unlocked state with an SMSR below 10 dB, as shown by the dark curve in Figure 5.  
Figure 4: (**a**) Schematic and (**b**) optical photograph of the IMC laser. SEM of (**c**) the top view of the grating and (**d**) cross-section of the mesa. For comparison, the solitary DFB laser was cleaved from the same wafer as the IMC laser and had the same structure as the DFB1 section of the IMC laser. The lasing spectra of the solitary DFB (brown line) and the IMC laser (red line), biased at the same current of 130 mA, are shown in Figure 6a. Both devices can lase in single mode with an SMSR over 47 dB. The detailed lasing spectrum under the MIL range is displayed in Figure 6a, which exhibits a stable single-mode with an SMSR over 32 dB in the whole range from 125 to 139 mA. This indicates that the tuning range was nearly four times larger than in our previous work [29]. The small-signal response of the solitary DFB laser (brown line) was measured, as depicted in Figure 6b. The modulation bandwidth was only 13.1 GHz when the laser was biased at 130 mA. Compared with the response of the solitary DFB laser, it can be seen that the CPR peak of the IMC laser was slightly suppressed and that the modulation bandwidth was extended by the PPR peak due to MIL (Figure 6b) [15]. More importantly, a PPR peak at a higher frequency corresponding to the mode spacing between the main mode and sidelobe was formed to further enhance the modulation bandwidth of the IMC laser, which was caused by MIL. For \(\mathrm{I_{DFB1}}=139\) mA, the mode spacing was about 0.17 nm (Figure 6a), corresponding to the PPR frequency of around 29.2 GHz. As \(\mathrm{I_{DFB1}}\) increased, the sidelobe's intensity increased, resulting in higher overshooting of the PPR peak in the frequency response (Figure 6b). This increase of \(\mathrm{I_{DFB1}}\) led to the increase of the photon number in the whole cavity, resulting in the enhancement of the first-order pole frequency [11], thus confirming the numerical simulation results above. Moreover, an enhanced 3-dB bandwidth of 34 GHz was obtained when \(\mathrm{I_{DFB1}}\) ranged from 134 to 139 mA. Then, the influence of \(\mathrm{I_{DFB2}}\) on the modulation response of the IMC laser was investigated, wherein the \(\mathrm{I_{DFB1}}\) and \(\mathrm{I_{SOA}}\) were fixed at 136 and 5 mA, respectively. As shown in Figure 7a, the optical  
Figure 5: The optical spectra of the IMC laser with \(\mathrm{I_{DFB1}}=131\) mA (red) under the MIL state and \(\mathrm{I_{DFB1}}=142\) mA (dark) under the unlocked state.