# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 2 Transmitter for Short-Reach Transmission Systems
### Electro-Absorption Modulated Laser  
An EML, which monolithically integrates a distributed feedback laser and an electro-absorption modulator on one chip, has been studied and applied in short-reach transmission systems. EML takes the advantages of DFB laser and the excellent modulation property of an external modulator. Therefore, EML can offer higher tolerance to CD and present lower frequency chirp compared with DML. Based on the above advantages, the EML-based systems can achieve higher transmission rate and longer transmission distance than DML-based systems. Meanwhile, EML may present smaller footprint size, lower power consumption, and lower cost than that of MZM. The application of EML had been demonstrated in various beyond 100-Gbit/s verification experiments [68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78]. [PERSON] et al. utilized an 18-GHz O-band EML to realize 100-Gbit/s PAM-4 signal transmission over an 80-km SMF link [68]. In addition, [PERSON] et al. demonstrated an amplifier-less transmission of a single lane 112-Gbit/s PAM-4 signal over 40-km SMF using 25-GHz O-band EML at 7% HD-FEC threshold of 3.8 \(\times\) 10\({}^{-3}\)[70]. [PERSON] et al. designed a novel convolutional neural network-based nonlinear classifier to achieve lower bit error rate (BER) performance 3.5 \(\times\) 10\({}^{-6}\) under the same bitrate and distance [71]. Furthermore, [PERSON] et al. experimentally presented a single-wavelength single-polarization 35-GHz-class commercial EML-based 214-Gbit/s PAM-4 signal transmission [74]. Based on pre-equalization and clipping technique, [PERSON] et al. reported a single-lane C-band EML-based 260-Gbit/s PS-PAM-8 signal transmission over 1-km NZDSF link [75]. In order to achieve higher transmission rate, researchers further explored to enhance the bandwidth of EML. Until now, the EML monolithically integrating a distributed feedback laser and a traveling-wave electro-absorption modulator (DFB-TWEAM) was reported as the maximum bandwidth of 100 GHz [13]. Figure 3a illustrated the DFB-TWEAM and biasing schema. The small signal modulation responses (electrical reflection S\({}_{11}\) and electro-optic S\({}_{21}\)) of DFB-TWEAM were measured as shown in Figure 3b,c. It can be observed that the 3-dB bandwidth was greater than 100-GHz. The applications about the monolithically integrated C-band DFB-TWEAM have been reported in 204-Gbit/s OOK [76], 200-Gbit/s PAM-4 [77], and 200-Gbit/s DMT [78] transmission systems. The recent applications of EML in beyond 100-Gbit/s per lane data transmission have been summarized as shown in Table 4. These results denote that EMLs would have great potential to be used in low-cost and energy-efficient beyond 100-Gbit/s short-to-medium reach transmission systems, \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Tx** & \(\lambda\) & **MF** & **BW** & **Rate** & **Reach** & **Fiber** & **FEC** & **Left.** \\
**Device** & **(nm)** & **MF** & **(GHz)** & **(Gbit/s)** & **(km)** & **Type** & **Limit** & **Ref.** \\ \hline MZM & 1550 & PAM-8 & 30 & 120 & 10 & SMF & \(1\times 10^{-2}\) & [56] \\ DDMZM & 1550 & DMT & 22 & 105 & 80 & SMF & \(4.5\times 10^{-3}\) & [57] \\ MZM & 1545 & PAM-4 & 40 & 168 & 1 & SMF & \(3.8\times 10^{-3}\) & [58] \\ MZM & 1550 & PAM-4 & 33 & 168 & 2 & SMF & \(3.8\times 10^{-3}\) & [59] \\ DDMZM & \(\sim\)1555 & PAM-4 & 30 & 128 & 80 & SMF & \(2.4\times 10^{-2}\) & [60] \\ MZM & 1304.6 & PAM-8 & 30 & 255 & 10 & SMF & \(2.5\times 10^{-4}\) & [61] \\ MZM & 1550 & PAM-4 & 40 & 200 & 40 & SMF + DCF & \(3\times 10^{-4}\) & [62] \\ MZM & C-band & DMT & 30 & 244 & 1 & SMF & \(4.5\times 10^{-3}\) & [63] \\ DDMZM & O-band & PAM-8 & 30 & 240 & 2 & MCF & \(3.8\times 10^{-3}\) & [65] \\ MZM & 1293 & PAM-6 & 28 & 225 & 2 & SMF & \(3.8\times 10^{-3}\) & [66] \\ MZM & 1310.96 & PS-PAM-8 & 60 & 280 & 10 & SMF & \(3.8\times 10^{-3}\) & [67] \\ \hline \end{tabular}
\end{table} 
Table 3: Incomplete summary of MZM applied in beyond 100-Gbit/s per lane data transmission.