Growth and fabrication of high-performance 980-nm strained InGaAs quantum-well lasers for erbium-doped fiber amplifiers

A 980-nm strained InGaAs quantum-well (QW) laser is the preferred pump source for an Er³⁺-doped fiber amplifier for the next generation of lightwave communication systems because of lower noise, high power conversion efficiency, and low temperature sensitivity. Obtaining long lifetime, narrow far field, high power output in the fundamental transverse mode centered at 980±5 nm, and planarity of the structure while maintaining low threshold current density (J_th) and high differential quantum efficiency (η) are the major challenges. Here, we report our work aimed at optimizing the design, growth, and fabrication of 980-nm lasers to address some of these issues. We demonstrate very low broad-area J_th, of 47 A·cm^-2, operation up to 200°C, and a very low linewidth enhancement factor of 0.54 of these lasers. We have also monolithically integrated 980-nm lasers with 850-nm GaAs QW lasers. To minimize coincorporation of nonradiative recombination impurities like oxygen and displacement of the p/n junction due to Be diffusion during MBE growth, we suggest that the Be doping should be dispensed with on the p-side of the GRIN region and the n-side GRIN region should be doped with Si. The optical properties of InGaAs QW´s are insensitive to the type of the arsenic beam used, As₂ versus As₄. Although strained InGaAs QW lasers grown using As₂ at a constant substrate temperature as low as 570°C have a lower J_th , they also exhibit a 10-25% lower η as compared to the As₄. counterpart in which the AlGaAs cladding layers are grown at ~700°C. To obtain a planar structure and to prevent the fabrication related defects, we have used a novel method in which the laser structure is first grown by MBE, and mesas are formed by in situ melt etching using SiO₂ stripes as a mask followed by regrowth of p^--p-n AlGaAs isolating layers by LPE. Compared to ridge waveguide (RWG) lasers, the buried heterostructure lasers so fabricated have significantly lower threshold current, higher power output; higher temperature operation, lower cavity losses, and kink-free light-current (L-I) characteristics, as expected. A CW power of 150 mW/facet at 986 nm was measured from a 400-μm-long BH laser with 11-μm active stripe width. A minimum threshold current of 2.5 mA was measured for lasers with 3.0-μm active width and 300-400 μm cavity length. The L-I characteristics of 500-, 800-, and 1300-μm-long lasers with 3.0-μm active width were linear up to the currents corresponding to a current density of 10 kA·cm^-2. At higher current densities, a sublinear increase of power with current was observed. Stable fundamental transverse mode operation was obtained up to 100-mW emitted power