Low-confinement silicon nitride waveguides manufactured via direct glass bonding

Reducing the fabrication cost of photonic integrated circuits while maintaining low optical losses and technological simplicity is essential for their wider implementation. In conventional manufacturing methods, the dielectric cladding thickness around waveguides is usually limited to $\sim20~\mu$m, which complicates suppression of radiative losses and parasitic scattering in low-confinement geometries. In this paper, we propose and experimentally demonstrate an alternative technology for forming low-confinement waveguides in Borofloat 33 glass by thermal fusion bonding of two glass wafers. The waveguide pattern is formed by etching trenches with depths on the order of tens of nanometers into the glass, filling them with silicon nitride, removing the excess layer, and bonding the planarized glass surfaces, thereby forming a thick, symmetric dielectric cladding. As a proof of concept, we fabricated straight waveguides with a core height of 50 nm and widths from 1.3 to 3.5 $\mu$m. With butt coupling to standard SMF-28 single-mode fiber at 1550 nm, transmission of up to 60% was obtained, corresponding to input/output coupling losses of $\sim1$ dB per facet and consistent with numerical estimates. Fabry-Perot analysis of high-resolution spectra measured with AR-coated lensed fibers gave effective propagation losses down to $0.62\pm0.36$ dB/cm, depending on waveguide width and polarization. The proposed approach provides a simple and scalable route to low-confinement glass-encapsulated photonic circuits with passive butt coupling, promising for long delay lines, external-cavity laser feedback circuits, and ring-resonator sensors.