Fiber Coupling Efficiency Calculator

Calculate fiber coupling efficiency with mode mismatch, lateral offset, and angular misalignment losses. Computes optimal focal length for beam-to-fiber coupling.

Coupling a free-space Gaussian beam into a single-mode fiber requires matching the beam's spatial mode to the fiber's guided mode, described by its mode field diameter (MFD). Three independent loss mechanisms reduce coupling efficiency below unity: mode mismatch (beam waist radius w₀ ≠ fiber mode radius w_f), lateral offset of the beam from the fiber axis, and angular misalignment between the beam and fiber axis. This tool computes each contribution separately and combines them into a total coupling efficiency (%) and insertion loss (dB). It also calculates the optimal focal length for a coupling lens that maximizes mode overlap at the fiber face — a function of beam waist, fiber MFD, and wavelength. Default parameters reflect a 1550 nm single-mode fiber (SMF-28, MFD 10.4 µm) with a 3.5 µm beam waist.

Beam Parameters
Fiber Parameters
Misalignment
Coupling Efficiency
Total efficiency (η)
85.8%
Total loss
0.66dB
Loss Breakdown
Mode mismatch loss
0.66dB
Lateral offset loss
0.00dB
Angular misalignment loss
0.00dB
Optimal Coupling
Optimal focal length (f)
0.04mm
Abridged Optics — Fiber Coupling Efficiency Calculator v1.0η = (2w₀wᶠ/(w₀²+wᶠ²))² × exp(−2δ²/(w₀²+wᶠ²)) × angular term. Loss = −10log₁₀(η).

All information, equations, and calculations have been compiled and verified to the best of our ability. For mission-critical applications, we recommend independent verification of all values. If you find an error, please let us know.