# Quick Fabrication VCSELs for Characterisation of Epitaxial Material
## 3 Results and Discussion
### Output Power  
Another important characteristic of VCSEL performance is the emitted optical power. In Figure 5a we compare the light-current characteristics of 6 \(\upmu\)m aperture standard and QF devices. The standard VCSEL has a slope efficiency of 0.89 \(\pm\) 0.04 W/A and produces a peak output power of 12.0 \(\pm\) 0.36 mW, which is \(\sim\)3 mW greater than that of the oxidised QF VCSEL, which has a slope efficiency of 0.60 \(\pm\) 0.03 W/A. The uncertainty in the power measurement is taken from the 3% uncertainty of the integrating sphere at this wavelength range and this, with the resolution of the current source, combines to give the uncertainty in the extracted slope efficiency. We observe the current at which the light output peaks to be greater for the QF VCSEL and we find this to be generally true for the QF devices. This is likely due to the leakage current of the QF VCSELs which, even when comparing equivalent active areas, results in a larger pumped area relative to the standard devices and hence results in a higher current at thermal rollover.  
In Figure 5b we compare the light-current characteristics of 30 \(\upmu\)m mesa lapped and unlapped unoxidised QF VCSELs. The slope efficiency for the unoxidised devices are similar with values of 0.37 and 0.39 \(\pm\) 0.02 W/A for the lapped and unlapped device, respectively. However, we see that the peak output power is greatest for the lapped device at 11.0 \(\pm\) 0.33 mW and this is \(\sim\)4 mW higher than that of the unlapped device. Additionally, the current at which thermal rollover occurs is higher for the lapped device and given that the bridge leakage current of these device is equivalent, we find that the improved heat-sinking properties facilitates higher power output. Additionally, we observe features of instability in the L-I characteristics, occurring around thermal rollover for the lapped device but at much lower currents for the unlapped device. This is likely due to heating-induced mode instability, which we refer to in more detail in the later section on wavelength current tuning.  
Further, in Figure 6, we compare the mean peak output power as a function of device size for each of the device types. We see a separation between the oxidised and unoxidised devices, with the oxidised devices producing much higher output powers.  
In general, the power output of the unoxidised devices is low (<10 mW) and, in comparison to the clear increase with aperture size for the oxidised devices, this remains approximately constant for the full range of device sizes. This may be due to high threshold currents of the unoxidised devices driving internal heating, thus limiting the power output and greater non-radiative recombination at exposed sidewalls.  
Although we find that the output characteristics of the QF devices suffer with the rapid fabrication process relative to the standard VCSELs, we still obtain moderate optical powers that would be sufficient for the use of these devices in manufacturing. The mechanisms that result in the observed difference in performance between the QF and standard devices are not immediately observable, hence the predictability of standard device performance from measurements of these QF structures alone is difficult. However, analysis of changes in relative performance across a single wafer, or between wafers with modified epitaxial structures, would still provide valuable information for the purpose of material characterisation in manufacturing.  
Figure 5: Light–current characteristics for (**a**) 6 μm aperture standard and QF VCSEL; (**b**) 30 μm mesa lapped and unlapped unoxidised QF VCSEL.  
Figure 6: Comparison of peak output power as a function of device size.