Room temperature photopumped 1.5-μm quantum well surface-emitting lasers with InGaAs/P/InP distributed Bragg reflectors

Abstract

Vertical cavity surface-emitting lasers (VCSEL) are attractive for various uses. For high bit rate (≥10 Gbit/s) optical data transmission over several km silica fiber, 1.3–1.55-μm emission is required in view of fiber dispersion. At present, room temperature cw operation on VCSEL has been achieved only in GaAs-based structures with emission wavelengths in the range of 0.77–0.98 μm. The key element of these lasers is the epitaxially grown distributed Bragg reflectors (DBR), which allow high reflectivity and thus low threshold current operation. In this paper we report room temperature photopumped performance of 1.5-μm InGaAs quantum well VCSEL with InGaAsP/InP DBR. The wafer was grown by one-step chemical beam epitaxy and, from the substrate, consisted of a 40-pair n-type InP/InGa-AsP (λE g = 1.42 μm), 8 InGaAs(40 Å)/InGaAsP undoped quantum wells are a p-type 0.4-μm InP cap. Reflectivity spectra of similarly grown DBR structures (without quantum well actives) exhibited a flat-topped stop band 80 nm wide with >99% reflectively that agreed with numerical calculation. After the epitaxial growth the wafer was e-beam coated with four pairs of quarter wavelength Si/SiO2 to form the top mirror. Excitation pulses of 1.06 μm 250 ns at 1-kHz rate were focused from the front side onto a 25–30-μm FWHM spot. The 1.5-μm lasing through the transparent substrate was clearly manifested in (1) a sharp hockey-stick shaped output vs input characteristic, (2) almost linearly polarized output above threshold, (3) greatly reduced linewidth at threshold , and (4) high differential quantum efficiency. Electric injection of the VCSEL here can be achieved through proper regrowth steps to form a capped mesa buried heterostructure, similar to CMBH edge-emitting lasers. The p-metallic electrode can serve as the top mirror, whose reflectivity can be enhanced (>99.5%) with an addition of once dielectric layer.