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

A thin p-cladding InGaAs/InGaP/GaAs laser structure (lambda=0.977 mu m) wit h a second-order Au/air grating (70% duty cycle) and asymmetrically coated (30%, 5% reflectivity) cleaved facets emits in a diffraction-limited (0.11 degrees) single lobe in a direction virtually normal to the chip surface. T he near-field pattern corresponds to (grating) phase shifts of 10 degrees a nd 40 degrees at the low- and high-reflectivity cleaved-mirror facets. An a nalysis of 30%/0% coated, 500-mu m-long devices shows that single-lobe surf ace emission occurs for a wide variation in grating phase with respect to t he high reflectivity mirror, Delta phi(HR): 10 degrees-80 degrees. For 99%/ 0% coated devices, single-lobe emission occurs with relatively uniform near -field intensity profile and external differential quantum efficiency eta(d ) around 20% for Delta phi(HR) values close to pi/4 (i.e., in the 45 degree s-65 degrees range). Single-lobe emission normal to the chip surface (i.e., symmetric-mode lasing) can then be obtained from devices without cleaved m irrors by introducing a phase shift close to pi/2 in the center of the seco nd-order grating. For the structure used, a phase shift of 90 degrees-130 d egrees in the center of 1-mm-long gratings is found to provide single-lobe surface emission with substantially uniform near-field profile, and eta(d) values as high as 22%. Increasing the grating-section length increases eta( d) (e.g., 35% for 1.5-mm-long gratings) at some price in near-field uniform ity. (C) 1999 American Institute of Physics. [S0003-6951(99)00233-8].