Broadband Double-Layer Antireflection Coatings for Semiconductor Laser Amplifiers

We have investigated a double-layer antireflection (AR) coating for semiconductor lasers, which consists of TiO2and SiO2with 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 µm GaInAs/AlGaInAs semiconductor laser amplifier, a minimum reflectivity of 5.3×10-3and bandwidths of 140 nm for R<10-3and 31 nm for R<10-4were experimentally obtained.