# High-Speed Directly Modulated Laser Integrated with SOA
## 3 Device Characterization
However, as \(I_{\text{S}}\) increases, the frequency response undergoes a significant improvement: the dip in the low-frequency region is gradually \"filled,\" and the mid- to high-frequency response is enhanced, resulting in an increased \(-3\) dB bandwidth. For example, in Figure 4d, under a DFB bias current \(I_{\text{M}}=60\) mA, the device achieves a \(-3\) dB bandwidth of 25.8 GHz when \(I_{\text{S}}=30\) mA, which represents a 7.3 GHz enhancement compared to the case without SOA. This demonstrates a substantial improvement in modulation performance enabled by the integrated SOA. The observed bandwidth enhancement originates from the quasi-high-pass filtering effect exhibited by the SOA under saturated conditions. When the modulated optical signal from the DFB section is injected into the SOA, the carrier depletion in the SOA leads to gain compression for low-frequency components, while the gain for high-frequency components remains relatively higher due to the delayed carrier response. As a result, the SOA provides a frequency-dependent dynamic gain response that reshapes the output signal of the DFB.This frequency-selective amplification mechanism effectively suppresses the pronounced low-frequency components in the DFB's original response and simultaneously enhances the high-frequency modulation capability. Consequently, a \"low-frequency suppression-high-frequency preservation\" spectral shaping effect is formed at the output of the integrated device. Since the SOA exhibits stronger gain response at higher frequencies, it compensates for the amplitude roll-off in the high-frequency region of the DFB's frequency response. This compensation shifts the overall \(-3\) dB bandwidth of the  
Figure 4: **(a–d)** Frequency response of the fabricated DML under different \(I_{M}\) and \(I_{s}\): (a) \(I_{M}=40\) mA; (b) \(I_{M}=50\) mA; (c) \(I_{M}=60\) mA; (d) \(I_{M}=80\) mA; (e) frequency response of the DML without SOA at different \(I_{M}\); (f) summary of laser bandwidth. combined DFB-SOA system toward higher frequencies, thereby significantly improving the modulation bandwidth of the device. The bandwidth variation trend is shown in Figure 4f. For a fixed \(I_{M}\), the bandwidth increases with rising \(I_{S}\), attributable to the enhanced amplification from the SOA, which boosts output power and strengthens the quasi-high-pass filter effect, thereby improving frequency response and increasing bandwidth. When \(I_{S}\) is constant, the bandwidth initially increases and then decreases as \(I_{M}\) rises, peaking at \(I_{M}\) = 60 mA. As \(I_{M}\) approaches 65 mA, the laser power saturates, and further increases push the laser out of its linear operating region, resulting in reduced bandwidth. At 25 \({}^{\circ}\)C and \(I_{S}=30\) mA, the spectrum of the DML under varying \(I_{M}\) conditions is shown in Figure 5, featuring a side-mode suppression ratio exceeding 44 dB. Simulation results indicate that the PCG structure effectively mitigates the impact of random phase variations at the rear facet [20], enhancing the mode stability of the laser. These findings are further validated by experimental results. To assess the transmission performance enhancement enabled by the SOA, we conducted 26.5625 Gbps NRZ signal transmission experiments in back-to-back (BTB), 2 km, and 10 km configurations, evaluating the TDEC (Transmitter and Dispersion Eye Closure) and capturing corresponding eye diagrams, as presented in Figure 6. TDEC is a standardized metric used to evaluate the signal quality at the transmitter side. It reflects the eye diagram opening after considering the effects of dispersion, filtering, and other transmission impairments. A lower TDEC value indicates better signal clarity and tolerance at the receiver, directly impacting the bit error rate (BER) performance. Therefore, TDEC serves as an important reference for assessing the overall system transmission quality. Figure 6a demonstrates that, for a given filter bandwidth, the DML with SOA consistently exhibits a lower TDEC than the DML without SOA.