Compute mode field diameter for single-mode fibers. Use core geometry, wavelength, and numerical aperture options. Save results, validate inputs, and share reports quickly anytime.
This tool estimates the mode field diameter for a step-index fiber by computing the normalized frequency (V-number) and an empirical mode radius fit.
| λ (nm) | Core radius a (µm) | NA | V-number | Estimated MFD (µm) |
|---|---|---|---|---|
| 1550 | 4.1 | 0.14 | 2.3268 | 9.2192 |
| 1310 | 4.1 | 0.14 | 2.7539 | 8.3400 |
| 1550 | 4.5 | 0.12 | 2.1897 | 10.0050 |
The mode field diameter (MFD) describes how optical power spreads across the fiber cross-section. It influences splice loss, connector loss, and bending sensitivity. In telecom links, an MFD near 9–11 µm at 1550 nm is common for standard single-mode designs, but specialty fibers can differ widely.
This calculator models the fundamental mode as a Gaussian beam, where the mode-field radius is w and MFD = 2w. That definition aligns with many engineering calculations for coupling and splice estimates. Measurement standards can use alternative methods, so treat results as a design-ready estimate.
The normalized frequency V summarizes whether the fiber behaves as single-mode. A widely used cutoff is V ≈ 2.405 for step-index fibers. When V rises above the cutoff, higher-order modes may propagate, and a single MFD value becomes less representative for system performance.
You can provide NA directly or compute it from indices using NA = √(n²core − n²clad). Typical single-mode NA values often fall around 0.10–0.14, depending on the design. A small NA generally increases mode size, while a larger NA tends to confine light more tightly.
Fiber drawings may list a core diameter, while many formulas use the core radius a. The calculator accepts either and converts internally to micrometers for stability. If you scale a upward or λ downward, V increases, which often reduces the predicted mode size.
Alongside MFD, the tool reports an approximate effective area using Aeff ≈ πw². Larger effective area reduces optical intensity for a given power, helping mitigate nonlinear effects in high-power or long-haul systems. This Gaussian approximation is convenient for comparisons, even when detailed field profiles are unavailable.
Use the displayed V as a quick reasonableness check. If V is far above 2.405, treat the MFD as a fundamental-mode indicator only. If V is very low, the empirical fit can be less accurate, and measured data is preferred.
Engineering workflows often require repeatable documentation. The CSV export supports quick comparison across wavelengths or core sizes, while the PDF export is helpful for test reports and design reviews. For best results, record your wavelength, geometry, and NA assumptions, then keep those consistent across calculations.
Core diameter describes the doped region size. MFD describes the optical field width, which can extend beyond the core. MFD is more predictive for coupling and splice behavior.
V-number indicates modal behavior. For step-index fibers, V ≈ 2.405 is a common single-mode cutoff. Values above that suggest higher-order modes may exist.
Enter NA if you already have a datasheet value. Use indices when you want NA derived from material data. Both routes should give similar results when values are consistent.
It is a practical engineering estimate for many step-index single-mode fibers. Accuracy decreases for very low V, very high V, or fibers with strong grading or unusual profiles.
Switch to the radius mode and enter w. The tool returns MFD = 2w and computes an approximate effective area using πw².
A larger NA generally confines light more strongly, shrinking the fundamental mode size. A smaller NA typically allows the field to spread farther into the cladding.
Core radius, wavelength, and NA drive V-number and thus the predicted mode size. Small changes in radius or wavelength can noticeably shift the result, especially near cutoff.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.