Step Index Fiber Calculator

Estimate fiber NA, V number, and modes quickly. Compare core and cladding indices with radius. Validate single mode limits for practical optical links today.

Calculator

Core refractive index (n1)

Typical silica core is around 1.45 to 1.47.

Cladding refractive index (n2)

Must be lower than n1 for guiding.

External medium index (n0)

Use 1.000 for air, ~1.33 for water.

Core radius (a)

Radius, not diameter. Multimode fibers often use 25 µm radius.

Operating wavelength (λ)

Common telecom wavelengths are 1310 nm and 1550 nm.

Fiber length (L)

Used for a simple step-index modal dispersion estimate.

Tip: keep units consistent. The calculator converts everything to meters internally.

Formula used

This tool uses standard step-index fiber relationships for guiding and modal behavior.

Numerical aperture: NA = √(n1² − n2²)
Relative index difference: Δ = (n1 − n2)/n1
Acceptance half-angle: θa = asin(NA/n0)
Normalized frequency: V = (2πa/λ)·NA
Mode count (approx.): M ≈ V²/2 for large V
Single-mode cutoff: V ≈ 2.405, so λc = (2πa·NA)/2.405
Modal delay spread: ΔT ≈ (n1·Δ/c)·L (rough step-index estimate)
Bandwidth estimate: B ≈ 0.5/ΔT (very approximate)

How to use this calculator

Enter n1 and n2 for your fiber at the operating wavelength.
Choose the core radius and its unit. Use radius, not diameter.
Enter the wavelength and unit. Telecom defaults are often 1310 nm or 1550 nm.
Set n0 for the surrounding medium to estimate the acceptance cone.
Add fiber length to estimate a basic modal delay spread and bandwidth limit.
Click Calculate. Results appear above the form for quick review.
Use Download CSV or Download PDF to keep records.

Example data table

Case	n1	n2	a (µm)	λ (nm)	NA	V	Regime
Multimode (short reach)	1.4682	1.4620	25	850	≈0.135	≈24.9	Multimode
Smaller core (closer to cutoff)	1.4682	1.4660	4.1	1310	≈0.080	≈1.57	Single-mode (approx.)
Telecom style (large V example)	1.4682	1.4633	4.1	1550	≈0.120	≈2.00	Single-mode (approx.)

Values are illustrative and depend on wavelength-specific refractive indices.

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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.