# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 2 Transmitter for Short-Reach Transmission Systems
### Directly Modulated Laser  
At transmitter side of short-reach transmission systems, another low-cost light source, DML with high output power and small footprint size, is more desirable than other externally modulated components. In recent years, in order to support 100-Gbit/s even beyond 100-Gbit/s transmission systems, the modulation bandwidth of DML is required to increase from commercial 10-GHz to higher bandwidth. Here, an advanced modulation bandwidth of 55-GHz was developed using photon-photon resonance and detuned-loading effects, and it is demonstrated in 112-Gbit/s PAM-4 system [46]. As shown in Figure 2a, the normalized \(S_{21}\) amplitude modulation responses for the bias range from 10-mA to 36.2-mA at 25 \({}^{\circ}\)C were measured, and the 3-dB bandwidth of 55-GHz was obtained at 36.2-mA bias condition. The peak of the normalized \(S_{21}\) amplitude modulation response can be observed around 50-GHz. In addition, a wider frequency modulation bandwidth about 65-GHz was depicted in Figure 2b, which can prove that the DML can work in such high frequency modulation. Thanks to the photon-photon resonance, the bandwidth can be further boosted beyond 100-GHz for a membrane short-cavity DR laser on a SiC substrate [47]. Nowadays, there are a variety of DML-based high-speed data transmission schemes reported in C- or O-band transmission systems. [PERSON] et al. experimentally demonstrated 4 \(\times\) 96-Gbit/s PAM-8 transmission over 15-km SMF by using four O-band DMLs (1269.54-nm, 1290.10-nm, 1309.67-nm, and 1329.12-nm) with the narrowest 3-dB bandwidth of 13.5-GHz [48]. [PERSON] et al. utilized a 16-GHz C-band DML to realize 100-Gbit/s PAM-4 short-reach data transmission over 15-km SMF with only post-equalization [9]. Also based on O-band DML, [PERSON] et al. demonstrated 117-Gbit/s DMT over 40-km and 101-Gbit/s DMT over 80-km transmissions [49]. [PERSON] et al. applied 28-GHz O-band DML to achieve 112-Gbit/s single-carrier, single-polarization PAM-4 short-reach transmission [50]. Furthermore, [PERSON], et al. proposed a simple and cost-effective packaging compensation method to enhance the resonance of the DML module at high frequency so that transmission distance can be increased to 40-km [8]. Recently, beyond 200-Gbit/s, DML-based data transmission schemes also had been widely reported [51; 52; 53; 47]. [PERSON] et al. reported a line rate of 256-Gbit/s PAM-4 transmission over 2-km SMF based on beyond 100-GHz DML laser [47], and this group further demonstrated 325-Gbit/s and 321.24-Gbit/s at back-to-back (BTB) and after 2-km SMF link [51]. [PERSON] et al. designed a short-cavity 54-GHz O-band DBR laser integrated with a short semiconductor optical amplifier (SOA) section, and demonstrated a line rate of 240-Gbit/s PAM-8 transmission over 10-km SMF link [52]. Recently, a fast data transmission of line rate 368.8-Gbit/s over 15-km SMF in the O-band was demonstrated [53]. Here, the recent applications of DML in beyond 100-Gbit/s per lane data transmission are summarized in Table 2. Besides intensity modulation, some groups had been exploring to utilize modulation-dependent frequency chirp of DML to generate phase modulation alone with the intensity for further improving transmission capacity [54; 55]. In conclusion, we believe that DML has great potential to be used in 100-Gbit/s or even higher speed short-reach transmission systems with a few to tens of kilometers transmission distance.