# Long-Wavelength VCSELs: Status and Prospects
## 7 VCSELs Fabricated by Wafer Fusion
To compensate for the low efficiency of InP-based DBRs, an approach was proposed using the fusion of AlGaAs/GaAs-based DBRs with InP-based active regions. This approach is very promising, enabling selective AlAs oxidation to provide current and optical confinement [73]. However, engineering difficulties related to the emergence of mechanical defects and cracks resulted at the beginning in no progress: the threshold current of the devices amounted to a few milliamperes, and the output power did not exceed 10 \(\upmu\)W.  
Double fusion technology enabled selective oxidation of Al\({}_{0.98}\)Ga\({}_{0.02}\)As layers in the top DBR to form current confinement. The InP-based active region was grown by MOVPE [138]. A large characteristic temperature T\({}_{0}\) of 132 K was reported up to 35 \({}^{\circ}\)C. Doped DBRs enable increasing the output power and reducing optical losses [139]. The small-signal modulation frequency of such VCSELs amounted to 4.7 GHz for a 7 \(\upmu\)m mesa diameter. Later, one of the contacts was moved inside the cavity to the active region based on InP/InGaAsP consisting of six QWs. Such a VCSEL demonstrated operation at room temperature and also at higher temperatures [140; 141]. The maximum working temperature was 105 \({}^{\circ}\)C. The output power at 20 \({}^{\circ}\)C amounted to 0.65 mW, and at 80 \({}^{\circ}\)C, 0.22 mW. Further, the oxide-confined current aperture was replaced by a TJ, while the fusion process of the active region with DBRs was done by the interface containing an unfilled TJ with the formation of air gaps [142]. The first results in the development of WF VCSELs with BTJ were presented in ref. [143] by a group from the University of California, Santa Barbara, USA. Further results were presented by a group from the Ecole Polytechnique Federale de Lausanne, Switzerland [144; 145]. The structures were grown with MOVPE. For 6 \(\upmu\)m BTJ size, an output power of more than 1 mW and a modulation bandwidth of more than 6 GHz up to a temperature of 100 \({}^{\circ}\)C with error-free data transfer at 10 Gbit/s across 10 km optical fiber were reported [145]. In 2010, microelectromechanical systems (MEMS)-tunable HCG 1300 nm VCSELs were presented [35]. The capability of meeting 40 G BASE-LR4 requirements was demonstrated [146]. No degradation at 10 mA at 90 \({}^{\circ}\)C for 2000 h was observed. Degradation tests based on the Telcordia GR-468-CORE standard confirmed the reliability of 1300 nm VCSELs [147], which advantageously distinguishes the results of this research group from all the other results obtained for 1300-1550 nm spectral range VCSELs and makes the approach they proposed a priority for developing an industrial technology for the production of these devices. In addition, transverse mode discrimination by intra-cavity patterning is demonstrated [148; 149]. Further, extra InGaAsP/InP thin layers were added for better fusion, and a cavity adjustment technique was applied [150]. As a result, industrially compatible 10-Gbit/s full coarse wavelength division multiplexing (CWDM) wavelength-set VCSELs for CWDM systems were realized. A record single-mode power (6.8 mW) at room temperature was realized by adjusting the reflectivity of the top of the DBR [151]. Controlling the photon cavity lifetime by shallow etching or layer-pair removal of DBR was realized [152]. As a result, the maximum modulation bandwidth is increased to 8.6 GHz. In 2016, 25 Gbit/s data transfer was demonstrated by adjusting the strain in the QWs and the cavity photon lifetime [153]. The maximum modulation bandwidth was about 11.5 GHz for eighteen-pair top DBR devices. In 2017, 38 Gbit/s with four-level pulse-amplitude modulation (PAM4) was shown [154]. A spectral efficiency of 3.8 bit/s/Hz was achieved using PAM4. Traditionally, InAlGaAs QWs are used to form 1300 nm WF VCSELs grown by MOVPE [146; 150; 151; 152; 153] due to their larger conduction band offset as compared to Ga\({}_{\text{In-X}}\)P\({}_{\text{Y}}\)As\({}_{\text{1-Y}}\) QWs [155].