Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate...

Area of Science:

• Integrated Optics
• Waveguide Technology
• Nanophotonics

Background:

• High-index-contrast (HIC) waveguides, such as silicon (Si) and silicon nitride (Si3N4), offer compact integration due to their small mode size.
• Significant coupling losses occur between HIC waveguides and single-mode fibers owing to mode mismatching.
• Mode size converters are crucial interfaces for efficient light coupling in integrated optical circuits.

Purpose of the Study:

• To develop an effective mode size converter to minimize coupling loss between HIC waveguides and single-mode fibers.
• To utilize silicon oxynitride (SiON) as an intermediate material for mode matching.
• To optimize the SiON waveguide structure for reduced loss and relaxed fabrication tolerances.

Main Methods:

• Employing silicon oxynitride (SiON) with a refractive index of approximately 1.50 as the intermediate material.
• Optimizing the SiON waveguide structure, incorporating inverse tapers and suspended structures.
• Designing separate SiON structures to interface with cleaved single-mode fiber and HIC waveguides (Si and Si3N4).

Main Results:

• Achieved coupling losses of 1.2 dB/facet (TE) and 1.4 dB/facet (TM) for Si3N4 waveguides with a 200 nm tip, and 1.8 dB/facet (TE) and 2.1 dB/facet (TM) for Si waveguides with a 150 nm tip.
• Demonstrated a 3 dB alignment tolerance of ±3.5 μm for Si3N4.
• Maintained polarization dependence loss (PDL) within 0.5 dB for both Si and Si3N4 platforms.

Conclusions:

• The optimized SiON mode size converter effectively reduces coupling loss between HIC waveguides and single-mode fibers.
• The use of SiON as an intermediate material, combined with inverse tapers and suspended structures, significantly improves coupling efficiency.
• The developed approach offers practical solutions for compact and efficient integrated optical devices with relaxed alignment tolerances.