# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 2 Transmitter for Short-Reach Transmission Systems
### Vertical-Cavity Surface-Emitting Laser  
Multimode (MM) VCSEL together with multimode fiber (MMF) have been widely used in short-reach optical interconnects, especially in transmission distances less than 300 m [24; 29]. For MM-VCSEL, a record 3-dB modulation bandwidth of 30-GHz had been reported with low threshold current and high energy efficiency [30]. As shown in Figure 1a, the MM-VCSEL started to work when the voltage exceeded 1.5-V and the threshold current reached 0.25-mA. The optical spectrum at a bias current of 2-mA was inserted in Figure 1a. The small-signal modulation responses at different bias currents (e.g., 1-mA, 1.3-mA, 2-mA, and 4.1-mA) were depicted in Figure 1b. The measured modulation response at a bias current of 4.1-mA can reach the highest bandwidth 30-GHz ever reported. Owing to the small oxide aperture of the MM-VCSEL, the resonance frequency \(f_{r}\) increased rapidly with the square root of current \(I_{b}\)-\(I_{th}\) at a rate of 17.5-GHz/mA\({}^{1/2}\), reaching a maximum value of 27-GHz, as shown in the illustration of Figure 1b. Similarly, 3-dB modulation bandwidth \(f_{3 MB}\) increased rapidly with the square root of current \(I_{b}\)-\(I_{th}\) at a rate of 20.6-GHz/mA\({}^{1/2}\), reaching a value of 25 GHz already at 1.8-mA. In here, \(D\)-factor was defined as a rate factor consisting of resonance frequency \(f_{r}\) and square root of current \(I_{b}\)-\(I_{th}\). MCEF was the modulation current efficiency factor, which was a rate factor consisting of 3-dB bandwidth \(f_{3 MB}\) and square root of current \(I_{b}\)-\(I_{th}\). At present, various beyond 100-Gbit/s MM-VCSEL-based short-reach transmission solutions have been reported [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. [PERSON] et al. demonstrated 107.5-Gbit/s MultiCAP short-reach transmission over 10-m OM4 MMF [31]. [PERSON] et al. reported on the transmission of 108-Gbit/s PAM-8 signal over 25-m OM3 MMF [32]. [PERSON] et al. utilized an unpackaged MM-VCSEL to realize beyond 168-Gbit/s PAM-4 data transmission over 50-m OM5 [35]. Subsequently, [PERSON] et al. experimentally demonstrated 112-Gbit/s duobinary PAM-4 short-reach optical interconnects with a 200-m MMF link [36], and then the transmission rate was further increased to 200-Gbit/s [37]. For these schemes, mode dispersion resulting in severe inter-symbol interference (ISI) was considered to be the main limiting factor in terms of high-speed transmission rate and long-haul transmission distance. Here, one effective solution was to apply single-mode (SM) VCSEL to reduce the number of transverse modes. Compared to the MM-VCSEL-based systems, the SM-VCSEL combining MMF and single-mode fiber (SMF) can improve system performance and support more than a few kilometers transmission reach [39; 40; 41; 42; 43; 44; 45]. In addition, since the light direction of VCSEL laser output is perpendicular to the substrate, it is particularly convenient for array integration (i.e. multiple VCSEL lasers can be arranged in parallel in the direction perpendicular to the substrate). Here, the recent applications of SM-/MM-VCSEL in beyond 100-Gbit/s per lane data transmission are summarized in Table 1.  
Figure 1: (a) Light-current-voltage characteristics (Inset: optical spectrum at a bias current of 2-mA); (b) small-signal modulation responses at bias currents of 1-mA, 1.3-mA, 2-mA, and 4.1-mA (Inset: resonance frequency \(f_{r}\) and 3-dB bandwidth \(f_{3 MB}\) of VCSEL plotted against square root of bias current \(I_{b}\) above threshold \(I_{th}\)); \(I_{b}\): bias current; \(I_{th}\): threshold current; \(D\)-factor: a rate factor consisting of resonance frequency \(f_{r}\) and square root of current \(I_{b}\)-\(I_{th}\); MCEF: the modulation current efficiency factor; (From [30], © 2015 IEEE).