Glossary entry

Mode field diameter (MFD)

The full-width diameter at which the optical field amplitude in a single-mode fiber falls to 1/e of its peak value, equivalently the diameter at which intensity falls to 1/e². The standard parameter for fiber coupling calculations.

Mode field diameter (MFD) describes the transverse extent of the guided mode in a single-mode optical fiber. It is defined as the diameter at which the optical field amplitude falls to $1/e$ of its peak value at the fiber axis, equivalently the diameter at which the intensity (proportional to amplitude squared) falls to $1/e^2 \approx 13.5\%$ .

The MFD is not equal to the fiber's physical core diameter. The guided mode extends measurably into the cladding, and the MFD is typically 10–20% larger than the core. For Corning SMF-28, the core diameter is 8.2 μm but the MFD is 9.2 μm at 1310 nm and 10.4 μm at 1550 nm.

Common single-mode fiber MFDs:

Fiber	980 nm	1310 nm	1550 nm
HI-1060	6.2 μm	—	—
SMF-28	—	9.2 μm	10.4 μm
LEAF (non-zero dispersion shifted)	—	—	9.6 μm
Lensed SMF (focused)	—	2–5 μm	2–5 μm
Photonic crystal fiber (typical)	varies	4–8 μm	4–10 μm

For Gaussian mode-matching to a single-mode fiber, coupling efficiency is maximized when the beam waist diameter equals the fiber MFD. Mismatched diameters introduce excess loss even with perfect lateral and angular alignment:

\eta_\text{size} \;=\; \left( \frac{2 w_b w_f}{w_b^2 + w_f^2} \right)^2,

where $w_b$ and $w_f$ are the beam and fiber mode radii (half the diameter, $1/e^2$ intensity). For a 3:1 ratio, this contributes $\sim 2$ dB of mode-mismatch loss; for a 10:1 ratio, $\sim 7$ dB.

Interactive computation is available in the Fiber Coupling Efficiency Calculator.