Single Mode Fiber Calculation Calculator

Model core and cladding properties with precision. Estimate cutoff wavelength, acceptance angle, and attenuation accurately. See fast outputs, exports, graphs, and helpful guidance today.

Calculator Inputs

Core Radius (µm)

Core Refractive Index (n1)

Cladding Refractive Index (n2)

Operating Wavelength (nm)

Fiber Length (km)

Attenuation (dB/km)

Use a core index greater than the cladding index. For step-index single mode fiber, single mode operation occurs when V < 2.405.

Example Data Table

Core Radius (µm)	n1	n2	Wavelength (nm)	Length (km)	Attenuation (dB/km)	V-Number	Cutoff Wavelength (nm)	Mode Status
4.10	1.4500	1.4460	1310	10	0.35	2.1165	1,152.86	Single Mode
4.10	1.4500	1.4460	1550	25	0.20	1.7888	1,152.86	Single Mode
5.00	1.4600	1.4500	850	2	2.50	6.3049	2,228.34	Not Single Mode

Formula Used

1) Numerical Aperture

NA = √(n1² - n2²)

2) Relative Index Difference

Δ% = ((n1 - n2) / n1) × 100

3) Normalized Frequency or V-Number

V = (2πa / λ) × NA

4) Cutoff Wavelength for Single Mode Operation

λc = (2πa × NA) / 2.405

5) Acceptance Angle in Air

θa = sin⁻¹(NA)

6) Total Attenuation Loss

Loss(dB) = Fiber Length × Attenuation per km

7) Remaining Power Ratio

Power Ratio = 10^(-Loss/10)

In these formulas, a is core radius and λ is operating wavelength. Use the same length unit for both when calculating V-number and cutoff wavelength.

How to Use This Calculator

Enter the core radius in micrometers.
Enter the core refractive index and cladding refractive index.
Enter the operating wavelength in nanometers.
Optionally include fiber length and attenuation to estimate loss.
Click Calculate Fiber Properties.
Read the result card above the form for fiber status and derived values.
Review the graph to see how V-number varies with wavelength.
Use the export buttons to download the result as CSV or PDF.

FAQs

1) What makes a fiber single mode?

A step-index optical fiber is single mode when its normalized frequency is below 2.405. That condition limits propagation to the fundamental mode only.

2) Why is the V-number important?

The V-number combines radius, wavelength, and numerical aperture into one measure. It tells you whether the fiber is single mode, near cutoff, or capable of carrying multiple modes.

3) What happens if n1 is not greater than n2?

Guided propagation requires the core index to exceed the cladding index. If n1 is not larger, total internal reflection will not occur correctly in a standard step-index fiber.

4) Why does increasing wavelength reduce V-number?

V-number is inversely proportional to wavelength. As wavelength increases, the normalized frequency falls, which pushes the fiber closer to single mode operation.

5) What is cutoff wavelength?

Cutoff wavelength is the wavelength at which V reaches 2.405. Operating above that wavelength usually supports single mode transmission in a step-index fiber.

6) What does numerical aperture describe?

Numerical aperture measures how much light the fiber can accept. A larger numerical aperture means a larger acceptance cone and stronger light-gathering capability.

7) Can this calculator estimate link loss?

Yes. Enter fiber length and attenuation per kilometer. The tool multiplies them to find total loss and then estimates the remaining power percentage.

8) Is this calculator valid for all fiber types?

It is best suited to step-index single mode fiber calculations. Specialty fibers, graded-index fibers, or advanced dispersion models may need more detailed equations.