# Long-Wavelength VCSELs: Status and Prospects
## 7 VCSELs Fabricated by Wafer Fusion
Very low internal optical losses (about 3.2 cm\({}^{-1}\)) and (about 5.5 cm\({}^{-1}\)) were estimated for devices with a 6 \(\upmu\)m BTJ diameter at 20 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, respectively, being lower than those for 1300-nm MOVPE- WF-VCSELs and comparable to record low values for 850-nm oxide-confined VCSELs. NRZ transmission at 25 Gbit/s was demonstrated across a 5 km single-mode fiber at 20 \({}^{\circ}\)C using a 5-tap feed-forward equalizer (cf. Figure 3 [67]). The first results on WF VCSELs with BTJ at 1550 nm were presented in 2011 by using MOVPE [167]. A large output power of 5.5 mW at 20 \({}^{\circ}\)C was realized. Single-mode lasing (more than 1 mW) for a wide temperature range (up to 85 \({}^{\circ}\)C) was presented. In 2014, control of the photon cavity lifetime was realized [168]. As a result, a small-signal bandwidth of 9 GHz was reached for the case of an 80 nm deep-etched top DBR. However, only modest modulation values of 10 Gbit/s were achieved [168]. Figure 2: LIV characteristics were measured at various temperatures for MBE–grown WF VCSELs with a 5 \(\upmu\)m BTJ diameter (**left panel**). The spectra for MBE–grown WF VCSEL (**right panel**) with 5 \(\upmu\)m BTJ diameter at 20 \({}^{\circ}\)C and 80 \({}^{\circ}\)C. Reprinted/adapted with permission from Ref. [10]. 2022, IEEE. We used MBE to fabricate WF VCSELs with BTJ in the 1550 nm spectral range. A modulation bandwidth comparable to the results for WF VCSELs grown by MOVPE of 7 GHz and 9 GHz for devices with unetched and etched top DBR [169; 170] was demonstrated. Then, thin tensile-strained InGaAs QWs were used to increase both the differential gain of the active region and reduce nonradiative recombination [171]. These VCSELs demonstrated both lower threshold currents and a larger single-mode output power at smaller bias currents compared to the MOVPE-grown VCSELs with the same top DBR reflectivity (cf. Figure 4[169]). This is because of the larger slope efficiency of the devices based on thin QWs (0.51 W/A). For a 25.78-Gbit/s bit rate, a BER of 9E-9 was reported (cf. Figure 5[169]). Further optimization of photon lifetime in the cavity made it possible to increase the modulation bandwidth of such lasers to 10-11 GHz [172]. Optimization of the structure design and the VCSEL device topology (cf. Figure 1, right panel) made it possible to increase the modulation bandwidth up to 13 GHz and to demonstrate 37 Gbit/s non-return-to-zero data transmission under back-to-back conditions at room temperature (cf. Figure 6[4]). Here, the number of QWs was increased to 10, aiming to increase the total modal gain. The barrier thickness was reduced to 7 nm. As a result, the total strain in the InGaAs QW-based active region was increased to 0.36%, which is larger than the previous value of 0.27% [169]. In contrast to MOVPE-grown WF VCSELs [168], no strain compensation in barriers was used. It should be pointed out that both highly-doped contact layers and precision opening of contact layers occur during wet etching. A composite n\({}^{++}\)/P\({}^{++}\) In(Al)GaAs BTJ was used instead of n\({}^{++}\)/P\({}^{++}\) InAlGaAs TJ reduced the serial resistance. To decrease the parasitic capacitance, the doping level of the n-InP overgrowth layer and the device mesa size were both decreased. Figure 4: LIV characteristics of MBE-grown WF VCSEL with an 8 \(\upmu\)m BTJ diameter (**left panel**). A spectrum for an unetched top DBR MBE-grown VCSEL measured at a bias current of I = 12 mA and 20 \({}^{\circ}\)C (**right panel**). Reprinted/adapted with permission from Ref. [169]. 2017, IEEE. Figure 3: Eye diagram of the 1300–nm SL–based MBE-grown WF VCSEL with 5–\(\upmu\)m BTJ diameter at (**a**) 25 Gbit/s: B2B and (**b**) more than 5–km SMF measurements using 5–tap FFE. The drive current is 14 mA with a 0.5 V peak–to–peak voltage. Reprinted/adapted with permission from Ref. [67]. 2022, SPIE.