Fabrication of highly reflective gratings in 1.5 μm semiconductor lasers using focused ion beam-based etching

This paper reports on fabrication of semiconductor/air gratings in 1.5 μm double-section semiconductor lasers to achieve a high reflectivity in order to compensate low round-trip gain. Fabrication of the gratings with varying thicknesses and with thicknesses down to 160 nm is carried out at the gain section of the double-section diode laser using focused ion beam etching (FIBE) and inductively coupled plasma (ICP) techniques. Theoretical results of reflectivity are given for 1.5 μm AlGaInAs/InP semiconductor lasers by adding wavelength dependence of the refractive index into the calculations. We also compare our reflectivity results with that of a commercial simulation program and show a good agreement between them. Our results demonstrate that the gratings fabricated consist of only six air/semiconductor layer pairs and achieve theoretical reflectivity higher than 99%. Due to a high index contrast of the both layers, nl = 1, nh ∼ 3.5, a reflectivity bandwidth of >230 nm is obtained in 1.5 μm semiconductor lasers. Finally, lasing operation from AlGaInAs/InP semiconductor lasers with highly reflective grating section is achieved with a low threshold current of ∼8 mA, which is almost three times lower than devices without semiconductor/air gratings.