## 3 Experimental Results and Discussion
### Large-Signal Modulation Performance  
The large-signal modulation performance of the IMC laser was investigated via a data transmission experiment. As shown in Figure 9, non-return-to-zero (NRZ) signals with a 2\({}^{31}\)-1 pseudorandom binary sequence were generated from a pulse pattern generator (SHF-12125B). Then, the electrical NRZ signals were amplified to approximately 1.2 V peak-to-peak by a broadband electrical amplifier. The data signals were combined with a DC injection current through a broadband bias tee and applied to the DFB1 section using a GSG probe. The IMC laser was under MIL with DFB1 biased at 136 mA, while I\({}_{\text{SOA}}\) and I\({}_{\text{DFB2}}\) were adjusted to obtain a flat frequency response. The eye diagram was observed by a sampling oscilloscope (Keysight 86100D) using an optical module (Keysight 86116C) as a photodetector. As shown in Figure 10, the BTB eye diagram of the solitary DFB laser was very poor at 25 Gb/s and even closed at 40 Gb/s due to the small modulation bandwidth of 13.1 GHz. However, the eye diagram's quality was significantly improved when the IMC laser was biased into the MIL state, in which clear open eyes could be observed at both 25 and 40 Gb/s. The dispersion of the 25-km SMF was about -15 ps/nm at an emission wavelength of 1306 nm. The eye diagrams maintained their shape after the 25-km transmission, which partially benefited from the SMF dispersion. In comparison with the quality of the electrical signal, the measurement was primarily limited by the test system. However, in Figure 10, the dynamic extinction ratio (ER) of the eye diagram was 1.1 dB due to the limitation of the MIL range with the change of DFB1's current, which influenced the working condition of the IMC laser (Figure 6). The IMC laser could maintain the MIL state when the I\({}_{\text{DFB1}}\) was within the limited range. When the DFB1 section was applied to a higher modulation voltage, the IMC laser may have left the MIL state, inducing the degradation of the modulation response,  
Figure 8: (a) Lasing spectra and (b) corresponding small-signal modulation response of the IMC laser with different I\({}_{\text{SOA}}\) values. Here, I\({}_{\text{DFB1}}\) and I\({}_{\text{DFB2}}\) were fixed at 136 and 40 mA, respectively. Figure 9: Experimental setup for the eye diagram. PPG: pulse pattern generator; BEA: broadband electrical amplifier; ISO: isolator; OC: optical coupler; OSA: optical spectrum analyzer; OSC: oscilloscope; TEC: thermoelectric cooler. or entered a nonlinear dynamic state. This resulted in a non-flat frequency response with a significant gap, causing the deterioration of the eye diagram. To further improve the large-signal modulation performance, it is desirable to adopt complex-coupled gratings [36], more reliable electrical isolation, and detuned gratings [26]. The single-mode stability of the DFB section could be improved by adopting the complex-coupled gratings [36], which are helpful to ensure the IMC laser is working in a more stable locking state with a large tuning range of \(\mathrm{I_{DFB1}}\). In addition, the electrical crosstalk from each adjacent section can be reduced by a more reliable electrical isolation, which allows each section to work more independently and prevents changing the current of the SOA section when DFB1 is under modulation. This allows a more stable injection locking state for the IMC laser. Moreover, the detuned gratings of the IMC lasers can be used to optimize the PPR frequency and working conditions [26], which may facilitate better bias condition of each section to allow a larger locking range of the IMC laser as DFB1 is modulated. Moreover, the chirp performance of the IMC laser was investigated by measuring the optical spectra under large signal modulation. For comparison, we tested the optical spectra with a 1.5 nm span under 10 Gb/s modulation for a dynamic ER of 3.2 dB. The modulation rate was selected according to the limited modulation bandwidth of the solitary DFB laser.