# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 3 Results and Discussion
### Static Characteristics
It should be noted that short-cavity VCSELs with a single mesa and 3\(\lambda\)-long cavity (2.1 \(\upmu\)m effective cavity length) demonstrate the threshold current and voltage are about 1 mA and 1 V. The rollover current is about 14 mA, which limits the maximum output power to \(\sim\)3 mW for a 5 \(\upmu\)m BTJ diameter [21]. The maximum WPE is about 20%. The change to a double-mesa design allows one to increase the output power (to about 4.5 mW) and WPE (to 28%) for the 5 \(\upmu\)m BTJ diameter [28; 56] by optimization mesa capacitance, thermal management, and output mirror losses (change from CaF\({}_{2}\)/ZnS to AlF\({}_{3}\)/ZnS pairs). The \(I_{\mathrm{roll}}\) is \(\sim\)17 mA. Figure 4a, right, shows the lasing spectra at 20 \({}^{\circ}\)C. A Yokogawa AQ6370C-10 optical spectrum analyzer was used to measure the VCSELs emission spectra. The side mode suppression ratio (SMSR) exceeds 45 dB across the whole range of drive currents. As the temperature rises to 70 \({}^{\circ}\)C, the maximum output power decreases to 2.1 mW, the threshold current increases to 6 mA, and the WPE is 9%. The maximum temperature where lasing occurred is 95 \({}^{\circ}\)C. Above 50 \({}^{\circ}\)C, an abrupt increase in the output power (so called LI kinks) appears as the drive current increases close to the lasing threshold. This effect is apparently associated with a saturable absorber effect in the unpumped parts of the active region (depending on the TJ etching depth), which previously was observed for 1550 nm WF VCSELs based on thin InGaAs QWs and an In(Al)GaAs TJ [39] and studied in detail for 1300 nm WF VCSELs based on an InAlGaAs superlattice and a composite In(Al)GaAs TJ [57]. The CW LIV characteristics for 1550 nm WF VCSELs with \(S\)-type device topology measured at various heatsink temperatures are shown in Figure 4b. The differential efficiency exceeds 0.27 W/A. The threshold current and voltage are 2.1 mA and 1.1 V. The largest WPE is about 11%. This decrease in differential efficiency restricts the maximum output power to 3.4 mW. The rollover current is 17 mA. The lower output power as compared to the \(L\)-type device topology can be explained by an increase in thermal resistance caused by the decrease in the second mesa size (increased self-heating). The reduction in the differential efficiency might be caused by an increase in light scattering in the top DBR, caused by the decrease in the first mesa size (excess in-plane optical losses). The lasing spectra measured at 20 \({}^{\circ}\)C demonstrate again single-mode lasing with an SMSR better than 45 dB for drive currents of 4-16 mA (cf. Figure 4b, right panel). At 70 \({}^{\circ}\)C, the threshold current increases to 3.9 mA. As a result, the maximum output power and WPE values are limited to 1 mW and 6%, respectively. The maximum operating temperature of these devices is about 95 \({}^{\circ}\)C, in contrast to the \(L\)-type device topology. One can see some wiggles in the \(LI\) curves for both device types (cf. Figure 4, left panel). Previously, a similar \(LI\) behavior was reported for 1300 nm WF VCSELs [58] as well as for 1570 nm WF VCSELs [59], but the reasons for this behavior were not discussed. It looks like mode hopping, but as was mentioned before, devices demonstrate an SM lasing in the current range studied. Moreover, polarization switching was also not observed. We suppose that this effect is related to the contribution of emission that has gone toward the substrate and is back-reflected at the lower semiconductor-air interface (in the case of using bilaterally polished GaAs substrates and/or after thinning the substrate), which is partially returned to the microcavity. A change in the external temperature or internal temperature (with increasing current) leads to a variation in the emission wavelength and the optical length of the external cavity that appears between the lower DBR and the substrate-air interface and, as a consequence, to a change in the output power.