Vertical-cavity surface-emitting lasers emitting at the spectral range of 1.55 µm 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 μm 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.
The results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering plates of high-q Bragg reflectors and the active region on the basis of thin strained InGaAs/InAlGaAs quantum wells have been presented. It has been shown that the proposed design of the laser provides a record low level of internal optical losses (less than 6.5 cm^–1) and high efficiency of current injection (more than 90%) at room temperature, which allows the realization of submilliampere threshold currents. As the temperature rises to 85°C, the current injection efficiency drops to 70% due to the thermal emission of charge carriers from the active region, accompanied by an increase in internal optical losses to 9.1 cm^–1 because of an increase in absorption on free carriers and/or intersubband absorption in the valence band.
An active region design based on the InGaAs/InGaAlAs superlattice for laser diodes of 1535-1565 nm spectral range was proposed and experimentally realized. It has been shown that the use of active region design based on superlattice allows increasing the modal gain at equal values of the pump current density in comparison with a common used active-region design based on a set of InGaAs quantum wells.
The design of the n++-InGaAs/р++-InGaAs/р++-InAlGaAs tunnel junction (TJ) for 1.55 μm range vertical-cavity surface-emitting lasers (VCSELs), developed by wafer fusion technique of InAlGaAsP/InP optical cavity with AlGaAs/GaAs distributed Bragg reflectors is proposed and realized. The presence of oxidation-resistant InGaAs layers allows the use of molecular-beam epitaxy at all stages of the heterostructure fabrication, including for regrowth of the TJ surface relief. In the case of using the n++-InGaAs/р++-InGaAs/р++-InAlGaAs TJ, a noticeable increase in the internal optical losses compared to the n++/р++-InAlGaAs TJ design was not obtained. The increase in internal optical loss in lasers can be avoided due to Burshtein-Moss effect in n++-InGaAs layers and thickness minimization of р++-InGaAs layer. As a result, the characteristics of fabricated lasers are comparable with characteristics of VCSELs with n++/p++-InAlGaAs TJ with a similar level of mirror losses.
An elastically balanced heterostructure of a 4.6-μm wavelength range quantum-cascade laser was grown by using molecular beam epitaxy. Heterostruсture was based on a heteropair of In0.67Ga0.33As / In0.36Al0.64As solid alloys and indium phosphide layers, which served as waveguide claddings. A high homogeneity of the layers composition and thicknesses in cascades over the wafer was proved by using X-ray diffraction for the grown heterostructure. Lasers with four cleaved facets operate at room temperature at a wavelength near 4.6 μm with a low threshold current density of 1.1 kA / cm2
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