# Long-Wavelength VCSELs: Status and Prospects
## 4 Monolithically Grown VCSELs on GaAs Substrates in the 1300-1550 nm Spectral Range  
The large refractive index contrasts of mirrors on GaAs substrates enable a reduction in the number of pairs in DBRs as compared to DBRs on InP substrates. Strained InGaAs, InGaP, InGaAsSb, GaAsSb, GaInNAs, and GaInNAsSb QWs are usually considered active regions for monolithic 1300-nm VCSELs grown on GaAs. Implementation of more long-wavelength VCSELs is possible by using GaInAsNbSb/GaAs QWs. In addition to QWs, quantum dots (QDs) based on InGaAs/AlGaAs were shown to serve well as active regions already two decades ago. InAs QDs and submonolayer QDs enable to control of the emission wavelength more flexibly and potentially reach a higher differential gain and lower threshold currents. The low density of QD arrays makes it necessary to stack rows of QDs to reach the required optical gain [81], which at the same time limited the output power and modulation bandwidth of 1280 nm spectral range devices to 0.3 mW and 2 GHz (the data transfer rate is 2.5 Gbit/s) at 20 \({}^{\circ}\)C, respectively [82]. Submonolayer QDs circumvented this problem, and 20 Gbit/s open eyes were demonstrated between 20 and 85 \({}^{\circ}\)C up to 1250 nm [83]. Using In\({}_{0.42}\)Ga\({}_{0.58}\)As QWs made is possible to demonstrate 10 Gb/s modulation at both 25 and 85 \({}^{\circ}\)C at 1280 nm emission wavelength, which is due to a large gain to cavity detuning. GCD (about 55 nm) (photoluminescence (PL) peak of In\({}_{0.42}\)Ga\({}_{0.58}\)As QWs is about 1220 nm) [84; 85; 86; 87; 88; 89; 90]. However, implementation of effective VCSELs beyond 1300 nm wavelength in the framework of this approach turned out to demand more work. Using InAs\({}_{0.44}\)P\({}_{0.56}\) strain-compensated QWs made it possible to implement multimode lasing at 1300 nm with an output power of about 1.9 mW [91]. Another approach was using GaAs\({}_{0.665}\)Sb\({}_{0.335}\)/GaAs QWs. Operation in continuous-wave (CW) mode was demonstrated at 1230 nm wavelength with 0.7 mA threshold current [92]. Later, the wavelength could be shifted to 1300 nm [93] by using GaAs\({}_{0.63}\)Sb\({}_{0.36}\)/GaAs QWs. CW operation at 70 \({}^{\circ}\)C has been demonstrated. Multiple groups contributed to the development of VCSEL based on diluted nitrides, including industrial research departments (Infeno, Sandia/Cielo, JDSU/Picolight, Alight Technologies, Philips U-L-M Photonics, IQE etc.) [94; 95; 96]. Devices in the 1300 nm spectral range based on GalnNAs QW with 10 Gbit/s operating speed and more than 1.2 mW output power at 20 \({}^{\circ}\)C were demonstrated in Ref. [97]. The use of TJ and microcavity geometry with intra-cavity contacts enabled not just an increase in the modulation bandwidth up to 9.8 GHz at 20 \({}^{\circ}\)C but also an increase in the output power to 4.2 mW [98]. Several groups demonstrated data transfer rates of 10 Gbit/s [94; 97; 98; 99; 100; 101]. However, the growth of GaInNAs QWs is associated with the decomposition of quaternary alloys into microscopic InGaN and InGaAs regions (phase segregation) and the formation of immiscibility regions, which requires more detailed research of the epitaxial growth of GaInNAs QWs. Moreover, the incorporation of N results in an increase in strain and the number of radiation defects. As a result, for long-term laser reliability, the nitrogen concentration in the QW should not exceed 1% for indium concentration larger than 30% [99], which limits the maximum VCSEL wavelength to the 1270-1280 nm range. A partial solution of the phase segregation is associated with the use of Sb, which makes it possible to increase the critical thickness limit associated with the transition to 3 Dgrowth and maintain the high crystalline quality of the growing GaInNAsSb material [102]. Recently, the possibility of implementing devices with a 1300 nm emission wavelength and a maximum small-signal modulation bandwidth of 10.5 GHz has been shown [13; 103]. The structure used was based on GaInNAsSb QWs and doped DBRs based on pairs of AlGaAs/GaAs.