# Long-Wavelength VCSELs: Status and Prospects
## 6 VCSELs with Hybrid (Metal-Dielectric) Distributed Bragg Reflectors  
To increase the temperature stability of the output characteristics of 1550 nm VCSELs, the research group of the late Prof. [PERSON]. [PERSON] (Walter Schottky Institute, Technical University Munich, Munchen, Germany) proposed a hybrid technology for device fabrication, where a hybrid metal-dielectric mirror is used as a bottom mirror and the BTJ concept is applied [117]. A hybrid metal-dielectric mirror is a dielectric DBR covered by a thick layer of gold (about 50-60 \(\upmu\)m [27; 56]), which acts as a heat sink and provides mechanical stability since the InP substrate is fully etched away. Despite the lower thermal conductivity of dielectrics, the total thermal conductivity of a hybrid metal-dielectric mirror turns out to be significantly higher as compared to InAlGaAs/InAlAs DBRs. Lateral heat dissipation from the active region to the gold heat sink through InP ICCL also contributes to efficient heat removal. A schematic view of a hybrid VCSEL device is shown in Figure 1 (left panel). The use of the BTJ concept enables not only effective current confinement but also optical confinement, similar to oxide-confined current apertures. Moreover, it enables reducing the series resistance and operating voltage, thus reducing the heat generation of the device. The laser structure consists of a cavity with an active region based on 7 InAlGaAs QWs, 6 nm thick, limited below by a bottom hyb  
Figure 1: Schematic cross-section of a LW VCSEL device. (**Left**) is a hybrid short-cavity VCSEL; (**right**) is a WF VCSEL. based on CaF\({}_{2}\)/ZnS and above by a top (output) doped DBR based on 32 n-type InAlGaAs/InAlAs pairs. Carrier injection is carried out via the n\({}^{+}\)/p\({}^{+}\)-InGa(Al)As T) and top n-InP ICCL. The growth was performed by MBE. 1550 nm VCSELs with 5 \(\upmu\)m T) demonstrated single-mode lasing with more than 40 dB side mode suppression ratio, 1.5 mA threshold current, and 2.5 mW maximum output power at room temperature [118]. The series resistance depends on the pump current and varies in the 40-50 Ohm range. Due to the large mismatch between the gain spectrum and the resonance wavelength, a temperature increase leads to the non-linear behavior of the threshold current. As the pump current increases, the modulation bandwidth reaches 10 GHz at 3 mA and saturates at 12 GHz and 10 GHz at 25 \({}^{\circ}\)C and 85 \({}^{\circ}\)C, respectively [119]. Error-free data transfer (BER \(<\) 10\({}^{-12}\)) at 25 Gbit/s for up to 30 km of optical fiber length at 25 \({}^{\circ}\)C [118] and at 12.5 Gbit/s up to 85 \({}^{\circ}\)C [117; 119] was demonstrated. A short cavity with both dielectric mirrors, where carrier injection takes place through intra-cavity contacts layers [120] was recently demonstrated to further increase the data rate of VCSELs based on the hybrid concept. The main difference is the formation of output dielectric mirrors. By replacing the output doped n-type InAlGaAs/InAlAs-based DBR with AlF\({}_{3}\)/ZnS-based dielectric DBR [28] (cf. Figure 1, left), the effective cavity length was reduced, the modulation bandwidth was increased to 18 GHz, and the data rate was increased to 35 Gbit/s at 20 \({}^{\circ}\)C and 25 Gbit/s at 55 \({}^{\circ}\)C [121]. Further BTJ reduction down to 4 \(\upmu\)m enabled us to minimize the mode volume and reach a new record for this laser type, namely, 22 GHz modulation bandwidth and 50 Gbit/s data rate at 20 \({}^{\circ}\)C [28]. Single-mode output power can be increased up to 8 mW and 2.2 mW at T = 20 \({}^{\circ}\)C and 90 \({}^{\circ}\)C, respectively, after optimization of the bottom contact topology to 7 \(\upmu\)m BTJ size [122]. One should note that VCSELs based on the hybrid concept need complicated processing steps and require the use of technology for high-quality dielectric mirror deposition. The research group of Prof. [PERSON] also presented results on 1300 nm VCSELs [30].