Glossary entry

Grating coupler

A periodic structure etched into the surface of a photonic integrated circuit waveguide, used to diffract light between the in-plane waveguide mode and a near-vertical free-space mode for fiber I/O.

A surface grating coupler interfaces an in-plane waveguide on a photonic integrated circuit (PIC) to an external optical fiber positioned near-vertically above the chip. The grating diffracts light between the slab waveguide mode and a near-vertical free-space mode. For a uniform first-order grating of period $\Lambda$ on a waveguide of effective index $n_\text{eff}$ , the central output angle in the cladding of index $n_c$ at free-space wavelength $\lambda_0$ is

\sin\theta \;=\; \frac{n_\text{eff}}{n_c} - \frac{\lambda_0}{n_c \, \Lambda}.

Typical silicon-on-insulator (SOI) grating couplers operating at 1550 nm in TE polarization use $\Lambda \approx 630$ nm and produce $\theta \approx 10^\circ$ . The grating is typically detuned a few degrees off normal to suppress second-order back-reflection into the waveguide.

Standard performance characteristics:

Design	Peak coupling efficiency	1 dB bandwidth
Uniform SOI grating, TE	25–50% (3–6 dB loss)	30–40 nm
Apodized SOI grating with backside reflector	60–80% (1–2 dB loss)	30–40 nm
Silicon nitride grating, TE	25–40%	40–60 nm
2D grating coupler (polarization splitter)	$\sim$ 25% per polarization	20–30 nm

Grating couplers are wavelength-sensitive and polarization-sensitive. The wavelength bandwidth is fundamentally limited by the grating dispersion; polarization sensitivity arises because the design optimizes the mode overlap for a specific polarization (typically TE). Edge couplers — a separate device class — provide broadband and polarization-insensitive coupling at the cost of requiring polished facets and precise lateral fiber alignment.

Alignment procedure is covered in Active Fiber Alignment to Surface Grating Couplers.