# Long-Wavelength VCSELs: Status and Prospects
## 5 Monolithically Grown VCSELs on InP Substrates for the 1300-1550 nm Spectral Range  
Monolithic VCSELs in the 1300-1550 nm spectral range fabricated on InP substrates are currently produced by the South Korean company RayCan [111] and the US company Bandwidth 10 [112]. RayCan developed 1550 nm VCSELs based on InAlGaAs/InP in a single epitaxial process with carrier injection via n-type intra-cavity contact layers (ICCL) and n-InP/p\({}^{+}\)-InAlAs/n-InP TJs [78]. The structures consist of a bottom (output) undoped DBR based on 28 InAlGaAs/InAlAs pairs, a bottom 2A ICCL n-type layer, a 0.5\(\lambda\) cavity with an InAlGaAs QW-based active region, placed in the antinode of the optical wave, an n-InP/p\({}^{+}\)-InAlAs/n-InP TJ, a top 2\(\lambda\) n-type ICCL, and a top undoped DBR based on 38 InAlGaAs/InAlAs pairs. Epitaxial structures are grown by MOVPE. For the formation of current and optical confinement, an air-gap aperture is used, obtained by selective lateral etching of optical cavity layers between ICCLs. To maximize the reflectivity and thermal conductivity of the top mirror, a bottom-emitted vertical cavity structure (emission output downwards via an InP substrate) is combined with a hybrid metal-semiconductor mirror in the form of a top DBR covered by a gold layer through a phase-matching layer. This approach is also used by [PERSON]'s group (Electronics and Telecommunications Research Institute, Republic of Korea). A cross-section of such a VCSEL is presented in refs. [78; 113]. These VCSELs demonstrate single-mode lasing with more than 40 dB side mode suppression, 2.5 mA threshold current, and 1.6 mW maximum output power at room temperature. The laser's series resistance amounted to 150 Ohm. A temperature increase to 70 \({}^{\circ}\)C leads to a threshold current increase to 4.2 mA, a decrease of the slope efficiency, and output power to 0.5 mW. As a result, the \(f_{\text{3 dB}}\) cut-off frequency does not exceed 4-5 GHz [114]. Error-free (bit error rate, BER \(<10^{-12}\)) data rates of 2.5 Gbit/s for up to 30 km optical fiber length were demonstrated [78]. In ref. [115], results of degradation studies of monolithic 1550 nm spectral range VCSELs produced by the RayCan are presented, using high-temperature tests; extrapolation of the results shows VCSEL life times of \(2\times 10^{7}\) h. The development of prototype monolithic VCSELs for the 1550 nm spectral range up to 10 Gbit/s was recently announced [116]. For single-mode VCSELs, the threshold current is 2 to 3 mA, and the output power is 1 mW. For multi-mode VCSELs, the threshold current is 3 to 4 mA, and the output power is about 2 mW. A research group from the University of California, Berkeley, and the US company Bandwidth 10 [112] also use a monolithic approach to make 1550 nm spectral range VCSELs. HCGs are used for the top output mirror [34; 70; 71]. The epitaxial structures consist of a bottom DBR with 40-55 pairs, an active region based on GaAlInAs QWs, a p-type cladding layer, TJ, a sacrificial layer, a layer to provide the HCG, and a top contact layer. The current aperture is provided by proton implantation. The aperture diameter depends on the design and varies in the range of 8 to 20 \(\mu\)m. HCGs are developed by electron-beam lithography and anisotropic vertical etching techniques. In CW mode, such VCSELs demonstrated output powers of 2.4 mW at 15 \({}^{\circ}\)C and 0.5 mW at 85 \({}^{\circ}\)C. HCGs provide the opportunity to implement single-mode laser operation with large diameters of the aperture, up to 12 \(\mu\)m. The maximum frequency of small-signal modulation is 7.8 GHz at a 2.5 threshold current. If a microelectromechanical actuator is added to the HCG, one can modify the wavelength in the 1540-1565 nm range while keeping the single-mode operation mode. Bandwidth 10 is currently implemented in industrial production of VCSELs at 1300 nm and 1550 nm in TO-cans and TOSA-packaged versions, demonstrating rates of 10 Gbit/s at 1 mW output power into an optic fiber.