Impact of transverse optical confinment on performance of 1.55 $\mu$m vertical-cavity surface-emitting lasers with a buried tunnel junction

The impact of transverse optical confinement on the static and spectral characteristics of 1.55 $\mu$m vertical-cavity surface-emitting lasers (WF-VCSEL) with a buried tunnel junction (BTJ) $n^{++}$-InGaAs/$p^{++}$-InGaAs, implemented using molecular-beam epitaxy and wafer fusion. For lasers with a tunnel junction (TJ) etching depth of $\sim$15 nm, it was found that the single-mode lasing occurs up to 8 $\mu$m BTJ mesa size due to a relatively weak lateral optical confinement, while the effect of a saturable absorber (SA) appears when the BTJ mesa size is less than 7 $\mu$m. Enhancing lateral optical confinement by increasing the BTJ etching depth up to $\sim$20 nm leads to suppression of the SA effect at the BTJ mesa size of 5–6 $\mu$m, but at the same time limits the maximum single-mode optical power. According to the results of the analysis, an increase in the spectral mismatch between the maximum of the gain spectrum of the active region and the resonance wavelength of the WF-VCSEL up to $\sim$35–50 nm will make it possible to suppress the undesirable SA effect in a wide range of the BTJ mesa sizes maintaining the single-mode lasing.