The effect of a saturable absorber in long-wavelength vertical-cavity surface-emitting lasers fabricated by wafer fusion technology

Vertical-cavity surface-emitting lasers emitting at the spectral range of 1.55 $\mu$m based on heterostructures with a buried tunnel junction (BTJ) with a height step of 15 nm are studied. The devices are realized using wafer fusion technique of heterostructures grown by molecular beam epitaxy and demonstrate a single-mode lasing regime at the 8 $\mu$m BTJ-diameter. With a decrease in the BTJ-diameter, a sharp increase in the threshold current, accompanied by an abrupt increase in the output optical power and resonance frequency at the lasing threshold are observed. Stable single-mode lasing is due to the smoothing of the boundary of the overgrown surface relief, which leads to a smooth change in the profile of the effective refractive index in the lateral direction, while maintaining effective current confinement, which makes it possible to significantly reduce the transverse optical confinement factor for high-order modes even at large BTJ-diameter. However, at small BTJ-diameter, it also leads to the formation of a saturable absorber in the non-pumped parts of the active region.