Inverse taper
A waveguide geometry that narrows toward the chip facet to expand the optical mode for fiber coupling. The standard silicon photonic edge coupler design.
An inverse taper is a waveguide structure that narrows in width as it approaches a chip facet. The narrowing reduces transverse mode confinement, causing the optical field to expand laterally and vertically into the surrounding cladding. This produces an enlarged mode profile at the facet that more closely matches the mode of an external optical fiber.
Standard silicon-on-insulator (SOI) inverse taper:
- Base waveguide: 500 nm wide × 220 nm tall
- Tip width at facet: 100 – 200 nm
- Taper length: 100 – 300 μm
- Mode field diameter at tip: 2 – 6 μm depending on tip width and cladding thickness
Coupling efficiency to external fiber depends on mode matching between the enlarged tip mode and the fiber mode:
| Fiber type | Per-coupler efficiency (typical) |
|---|---|
| Lensed single-mode fiber (3 μm focal spot) | 60 – 80% (1 – 2 dB loss) |
| Standard SMF-28 (10.4 μm MFD at 1550 nm) | 10 – 30% (5 – 10 dB loss) |
| High-NA fiber | 70 – 85% |
For wider mode expansion and lower loss to SMF-28, more sophisticated designs are used:
- Cantilever inverse taper — base waveguide narrows, with surrounding cladding removed to reduce substrate leakage at the tip
- Subwavelength grating taper — periodic structure creates an engineered effective medium for improved mode expansion
- Bilayer inverse taper — two stacked taper sections expand the mode in both transverse dimensions
- Multi-stage spot-size converter — cascaded tapers and intermediate waveguide sections, achieving dB to SMF-28
Inverse tapers dominate over surface grating couplers where broadband or polarization-insensitive coupling is required, at the cost of needing polished chip facets. See Edge Coupler Alignment to Photonic Integrated Circuits for the alignment procedure.