Broadband double-layer antireflection coatings for semiconductor laser amplifiers

We have investigated a double-layer antireflection (AR) coating for semiconductor lasers, which consists of TiO2 and SiO2 with non-quarter-wave thicknesses. In order to precisely control refractive indices, an electron-beam evaporator with oxygen pressure control was applied. Film thicknesses were accurately measured using the spectroellipsometric technique. Calculations show that the double-layer coating has much enhanced error margins, as well as broader bandwidths, which allows the performance of a double-layer coating even with +/-1.6% of thickness errors in both layers to be comparable to or better than that of an ideal single-layer coating. By applying the double-layer coating design to a 1.55 mu m GalnAs/AlGaInAs semiconductor laser amplifier, a minimum reflectivity of 5.3 x 10(-3) and bandwidths of 140 nm for R < 10(-3) and 31 nm for R < 10(-4) were experimentally obtained.