## 3 Experimental Results and Discussion
### Small-Signal Modulation Response
Here, \(\mathrm{I_{DFB2}}\) and \(\mathrm{I_{SOA}}\) were fixed at 40 mA and 5 mA, respectively. Figure 6: (**a**) Lasing spectra and (**b**) corresponding small-signal modulation response of the solitary laser (brown line) and the IMC laser with different \(\mathrm{I_{DFB1}}\) values. Here, \(\mathrm{I_{DFB2}}\) and \(\mathrm{I_{SOA}}\) of the IMC laser were fixed at 40 and 5 mA, respectively. spectrum indicated that the sidelobe was gradually moving farther from the main mode and that the intensity increased as I\({}_{\text{DFB2}}\) increased. As a result, the magnitude and frequency of the corresponding PPR peak increased (Figure 7b). The trend seemed to be opposite that of the simulation results. In the experiment, the lasing wavelength of DFB1 biased at a higher current was on the long-wavelength side of the lasing mode of DFB2 due to the heating effect. As I\({}_{\text{DFB2}}\) increased, the wavelength of DFB2 moved toward a longer wavelength; hence, the detuning frequency increased from negative to positive, as depicted in the inset of Figure 7b. Therefore, the SMSR of the lasing mode decreased, resulting in reducing the damping of the PPR peak [13; 24]. When I\({}_{\text{DFB2}}\) was above 50 mA, the IMC laser was closer to the locking range boundary, causing a narrow and sharp PPR peak. The optical spectrum is like the black line in Figure 5, which indicates that the IMC laser has entered an unlocking state [33]. It is also called high-frequency pulsations [26] or a mode beating state [31]. Although the modulation response between the CPR and PPR peaks dropped below \(-3\) dB, it could still serve well for narrow-band applications [20] or optoelectronic oscillators [35]. Though the simulation results tuning I\({}_{\text{DFB2}}\) appeared to differ from the experiment, the frequency response's dependence on the detuning frequency was consistent. Moreover, the detuning frequency's change with varying currents could be practical for adjusting the working condition of the IMC laser. Additionally, the simulation could be modified by calibrating the VPI model in an upgraded version of VPI, considering the heating effect. As shown in Figure 8, the IMC laser's modulation response was extremely sensitive to the SOA's injection current. When I\({}_{\text{SOA}}\) increased, the coupling strength between DFB1 and DFB2 increased. Moreover, the PPR frequency could be increased by increasing the coupling strength in the MIL scheme [24]. Consequently, the mode separation between the main mode and sidelobe increased from 0.135 to 0.22 nm by increasing the SOA current, which corresponded to the PPR frequency increase from 23.4 to 38.4 GHz (Figure 8b). When I\({}_{\text{SOA}}\) varied to 7 mA, the PPR peak was slightly damped, primarily due to the higher coupling strength [15; 24]. Then, when I\({}_{\text{SOA}}\) was over 7 mA, the IMC laser worked close to the locking range boundary routing to the unlocked state. This was dominated by the change of the coupling phase because the working condition was more sensitive to it, as mentioned previously. As a result, the SMSR of the IMC laser degraded, and the PPR peak became higher and sharper again, inducing an unfilled gap between the CPR and PPR peaks. Thus, there is a trade-off between a higher PPR frequency and flat modulation response. For broadband communication, a flat modulation response curve could be obtained by adjusting the bias current. Here, a maximum flat modulation bandwidth of 38.7 GHz was obtained as I\({}_{\text{SOA}}\) = 7 mA. Figure 7: (**a**) Lasing spectra and (**b**) corresponding small-signal modulation response of the IMC laser with different I\({}_{\text{DFB2}}\) values. Here, I\({}_{\text{DFB1}}\) and I\({}_{\text{SOA}}\) were fixed at 136 and 5 mA, respectively. Inset: schematic diagram of the detuning frequency’s variation as I\({}_{\text{DFB2}}\) varied.