Single-mode, single-lobe operation of surface-emitting, second-order distributed feedback lasers

Abstract

A thin p-cladding InGaAs/InGaP/GaAs laser structure (λ=0.977 μm) with a second-order Au/air grating (70% duty cycle) and asymmetrically coated (30%, 5% reflectivity) cleaved facets emits in a diffraction-limited (0.11°) single lobe in a direction virtually normal to the chip surface. The near-field pattern corresponds to (grating) phase shifts of 10° and 40° at the low- and high-reflectivity cleaved-mirror facets. An analysis of 30%/0% coated, 500-μm-long devices shows that single-lobe surface emission occurs for a wide variation in grating phase with respect to the high reflectivity mirror, ΔφHR: 10°–80°. For 99%/0% coated devices, single-lobe emission occurs with relatively uniform near-field intensity profile and external differential quantum efficiency ηd around 20% for ΔφHR values close to π/4 (i.e., in the 45°–65° range). Single-lobe emission normal to the chip surface (i.e., symmetric-mode lasing) can then be obtained from devices without cleaved mirrors by introducing a phase shift close to π/2 in the center of the second-order grating. For the structure used, a phase shift of 90°–130° in the center of 1-mm-long gratings is found to provide single-lobe surface emission with substantially uniform near-field profile, and ηd values as high as 22%. Increasing the grating-section length increases ηd (e.g., 35% for 1.5-mm-long gratings) at some price in near-field uniformity.