Straight directional couplers via scan-engineered index control for dense photonic integration.

A novel, to the best of our knowledge, design for straight directional couplers and interferometers was demonstrated in glass, fabricated using femtosecond laser direct writing and operating at telecommunication wavelengths (~1550 nm). The devices consisted of parallel waveguides with a spacing of 15 µm, where the coupling strength was controlled by scan-engineered refractive index modulation along the length of the waveguide. Using this approach, we realized a 50:50 directional coupler formed by two identical waveguides with a footprint as small as 40 µm × 15 µm × 1.5 mm, as well as a Mach-Zehnder interferometer with unbalanced arms. A waveguide array with 15 µm spacing was also demonstrated, highlighting the potential for compact, high-density, and three-dimensional photonic integration.