# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 3 Results and Discussion  
### Static Characteristics  
The CW light output power-current-voltage (LIV) characteristics were measured using a low-voltage Keithley 2400-LV sourcemeter and a calibrated photodiode Thorlabs SC122C. Figure 4a depicts the CW LIV characteristics for 1550 nm WF VCSELs with \(L\)-type device topology measured at various heatsink temperatures. At 20 \({}^{\circ}\)C, the lasers show a threshold current \(I_{\text{th}}\) less than 2 mA, with a slope efficiency (SE) of more than 0.45 \(W\)/A (equivalent to \(\sim\)36%). The threshold voltage is 1.07 V. The maximum wall plug efficiency (WPE) is \(\sim\)20%. The thermal rollover current \(I_{\text{roll}}\) is \(\sim\)19 mA, which limits the maximum output power of \(\sim\)4.8 mW. Increasing the BTJ size allows to increase the saturation current and the maximum output power, but at the same time the mode volume rises, which deteriorates the VCSELs modulation performance. In addition, the BTJ size is affected by the etching depth \(\Delta L_{\text{SR}}\) of the TJ: an increase in the \(\Delta L_{\text{SR}}\) value limits the maximal BTJ diameter for maintaining single-mode lasing. Thus, an increase in the BTJ size (up to  
Figure 3: The 1550 nm InGaAs QW-based WF VCSEL: (**a**) STEM/TEM cross-sectional images; (**b**) schematic cross-section of device. Figure 3b is adapted, with permission, from Ref. [39]. 2023, IEEE. 7-8 \(\upmu\)m) in combination with the planarization effect during MOVPE overgrowth (to reduce the index guiding for higher-order transversal modes) made it possible to increase the SM output power of WF VCSELs to more than 6 mW [33; 35]. An additional increase in the \(I_{\text{roll}}\) value can be achieved by increasing the thermal conductivity of the device. This depends not only on the size of the BTJ in our VSCELs but also the thermal conductivity of the bottom DBR and the bottom IC layer. The thermal heat management and planarization in the MOVPE overgrowth enabled the demonstration of 1550 nm range short-cavity VCSELs with an output power of 6.7 mW at a BTJ size of 5.5 \(\upmu\)m [55]. An increase in the bottom AlGaAs/GaAs DBR thermal conductivity can be realized by increasing the Al content in the bottom DBR. Influences of AlGaAs heterointerface roughness on the VCSEL performance should be studied additionally. It should be noted that a further increase in output power requires the use of special methods for the creation of mode-selective mirror losses in order to provide SM emission for even larger BTJ diameters [32]. However, an increase in the BTJ size leads to an abrupt increase in the mode volume and a decrease in the modulation bandwidth. The threshold current of \(L\)-type devices is comparable to that for MBE-grown wafer-fused 1550 nm VCSELs based on thin InGaAs QWs and n\({}^{+}\)/p\({}^{+}\)-In(Al)GaAs BTJs with a similar optical cavity design (\(I_{\text{th}}\) = 1.4 mA, SE = 0.51 W/A for an 8 \(\upmu\)m BTJ diameter [35]), indicating a relatively low level of excess optical losses caused by optical absorption in the composite In(Al)GaAs BTJ. Note that WF VCSELs based on thick InAlGaAs QWs and an n\({}^{+}\)/p\({}^{+}\)-In(Al)GaAs BTJ fabricated by fusing MOVPE-grown wafers are noticeably inferior in terms of the threshold current for comparable mirror losses (\(I_{\text{th}}\) = 4.3 mA, SE = 0.44 W/A for 1490 nm WF VCSELs with a 7 \(\upmu\)m BTJ diameter [34]; \(I_{\text{th}}\) = 4.1 mA, SE = 0.52 W/A for 1560 nm WF VCSELs with a 7 \(\upmu\)m BTJ diameter; and \(I_{\text{th}}\) = 4.7 mA, SE = 0.46 W/A for 1580 nm WF VCSELs with a 7 \(\upmu\)m BTJ diameter [33]). Figure 4: CW LIV characteristics of 1550 nm InGaAs QW-based WF VCSELs with 6 \(\upmu\)m BTJ diameter measured in CW regime at 20–80 \({}^{\circ}\)C: (**a**) \(L\)-type device topology; (**b**) \(S\)-type device topology. The right-hand side shows lasing spectra at 20 \({}^{\circ}\)C and at various drive currents. Figure 4b is reprinted, with permission, from Ref. [39]. 2023, IEEE.