Ultra-low loss SiN edge coupler interfacing with a single-mode fiber

Area of Science:

• Photonics
• Materials Science
• Optical Engineering

Background:

• Edge couplers are critical for interfacing optical fibers with photonic integrated circuits.
• Existing designs often focus on the core structure, overlooking cladding's role in mode confinement.

Purpose of the Study:

• To design and fabricate an ultra-low loss silicon nitride (SiN) edge coupler.
• To investigate the impact of cladding structure on mode characteristics and coupling efficiency.

Main Methods:

• Optimization of up-cladding thickness for vertical mode expansion.
• Incorporation of air trenches for lateral mode confinement.
• Refractive index tuning of the up-cladding layer to minimize substrate leakage.

Main Results:

• Achieved a coupling loss of 0.67 dB/facet for TE mode at 1630 nm.
• Measured 0.85 dB/facet (TE) and 1.09 dB/facet (TM) at 1550 nm, yielding a polarization-dependent loss of 0.24 dB.
• Demonstrated the effectiveness of the cladding-focused design on a SiN platform.

Conclusions:

• The cladding structure is crucial for determining mode size and shape in edge couplers.
• The developed SiN edge coupler offers ultra-low loss and low polarization dependence.
• The design principles are transferable to silicon-on-insulator platforms.