Thermally stable high numerical aperture integrated waveguides and couplers for the 3 μm wavelength range

The femtosecond laser direct write technique was used to fabricate mid-infrared compatible waveguide couplers into Suprasil 3001, a fused silica glass with an OH content of as low as ≤1 ppm. Smooth positive step-index change multi-scan waveguides were produced with a high index contrast of 1 × 10^-2, measured directly using quadriwave lateral shearing interferometry. Waveguides were annealed at 400 °C for 15 h and found to be highly stable, with only <5% reduction in positive index change. Brillouin microscopy and cathodoluminescence are introduced as novel tools that complement Raman mapping and electron microscopy for the investigation of the laser-induced structural changes within the glass matrix, and it was found that although a uniform step index profile is observed across the entire guiding region, different physical mechanisms underpin the index change in the upper and lower sections of the waveguide cross-section, respectively. Waveguides were optimized for mode-matching with optical fibers for the 3.2 μm wavelength range, and evanescent four-port directional couplers with coupling ratios ranging from 5:95 to 50:50 were designed and fabricated. This demonstration opens the door to the development of fully integrated and temperature-stable hybrid chip/fiber systems for the important mid-infrared spectral range.