Direct-laser-written integrated mid-IR directional couplers in a BGG glass

The development of coherent sources and other optical components for the mid-infrared has been hampered by the lack of sturdy materials that can withstand high power radiation or exposition to harsh environment. BGG glasses are robust materials transmitting over the 2.5-5 μm region. We report here the direct femtosecond laser fabrication of efficient directional couplers integrated in a BGG glass chip. The photonic components are characterized from 2.1 to 4.2 μm and compared to similar structures inscribed in silica glass samples. At 2.85 μm, a 99% relative cross transmission is reported in BGG glass. The experimental measurements are in good agreement with the coupled mode theory for wavelengths up to 3.5 μm.