# High-Speed Directly Modulated Laser Integrated with SOA
## 2 Device Structure and Fabrication  
The structure of the proposed DML is shown in Figure 1 and primarily consists of the distributed feedback (DFB) modulation region and the SOA region, each with a length of 150 \(\upmu\)m. The rear facet near the modulation region has a high-reflectivity coating (>95%), while the output facet near the SOA features a 0.5% reflective anti-reflective coating. A 10 \(\upmu\)m-wide, 200 nm-deep isolation region is etched between the modulation and SOA regions to ensure electrical isolation. The laser utilizes an identical active layer (IAL) structure [18; 20; 21] with a partially corrugated grating design [18]. A grating with a coupling coefficient \(\kappa\) of approximately 7788 m\({}^{-1}\) is etched in the modulation region, positioned solely on the SOA side, leaving an unetched waveguide section between the high-reflectivity facet and the grating. This design effectively reduces the impact of random phase variations, enhancing mode stability.  
To enhance the structural integrity and reproducibility of the device, the fabrication process and key parameters of the DFB-SOA integrated laser are further elaborated. The device is based on an InP substrate with an InGaAsP multiple quantum well (MQW) active region, designed for 1310 nm emission. The fabrication begins with epitaxial growth of the active and grating confinement layers, followed by grating etching and subsequent growth of the cladding and Ohmic contact layers. A uniform active layer is used across the entire wafer, with the DFB and SOA sections completed in a single epitaxial step without regrowth. This enables a monolithic integration with continuous structure and functional separation. A 2.4 \(\upmu\)m-wide ridge waveguide is formed in both sections via dry etching.  
\begin{table}
\begin{tabular}{c l c c c c c} \hline \hline
**Reference** & **Structure** & \multicolumn{2}{c}{**Bandwidth**} & **Power** & **SMSR** & **SOA Bias** & **SOA Function** \\ \hline This work & DFB + SOA & 25.8 & 28 & \(>\)44 & Saturation & Frequency response shaping, power enhancement \\
[12] & two-\(\kappa\) DBR + SOA & 65 & 14.5 & – & Linear & Power enhancement \\
[19] & two DFB + central SOA & 38.7 & – & \(>\)47 & Saturation & Enhanced PPR locking, frequency response shaping \\ \hline \hline \end{tabular}
\end{table}
Table 1: Performance comparison of representative SOA-integrated directly modulated lasers (DMLs).  
In the DFB section, partial periodic gratings are defined using electron beam lithography and shallow etching (approximately 50 nm), yielding a grating coupling coefficient of \(\kappa=7788\,\mathrm{m}^{-1}\) to balance feedback and modulation bandwidth. No grating is etched in the SOA section, which is used solely for optical amplification. Electrical isolation between the DFB and SOA sections is achieved by a 10 \(\upmu\)m-wide, 200 nm-deep trench fabricated via wet etching. This trench effectively blocks carrier diffusion and enables independent control of current injection. Ti/Pt/Au trilayer electrodes are deposited and patterned via a lift-off process to separately bias the DFB and SOA regions. A SiO\({}_{2}\) passivation layer is deposited for surface protection, and metal pads are defined for subsequent electrical testing. Figure 1b shows the top-view microscope image of the fabricated laser chip.