Ring Resonator FSR Calculator

Design optical rings with fast, reliable spacing estimates. Review frequency, wavelength, round-trip, and resonance outputs. Plot trends, save data, and document photonic designs confidently.

Calculator Inputs

Formula Used

Round-trip length from radius:
L = 2πR
Free spectral range in frequency:
FSRf = c / (ngL)
Local free spectral range in wavelength:
FSRλ ≈ λ² / (ngL)
Round-trip time:
τ = ngL / c
Approximate resonance order:
m ≈ neffL / λ

Use the group index for FSR calculations because it captures dispersion and determines spacing between resonant modes more accurately than the effective index.

How to Use This Calculator

  1. Select whether you want to enter radius or total round-trip length.
  2. Enter the geometric value using the appropriate unit.
  3. Provide the group index of the guided mode.
  4. Enter the center wavelength where you want the local FSR.
  5. Optionally add effective index to estimate resonance order.
  6. Click Calculate FSR to show results above the form.
  7. Review the graph and resonance table for nearby mode spacing.
  8. Use the export buttons to save the results in CSV or PDF format.

Example Data Table

Case Radius (µm) Group Index Center Wavelength (nm) Round-Trip Length (µm) FSR (GHz) FSR (nm)
Silicon photonics ring 10 4.2 1550 62.832 1136.821 9.104
Low-index contrast ring 25 2.1 1310 157.080 909.457 5.201
Visible-band resonator 50 1.9 850 314.159 502.308 1.209

Example values are rounded and intended for comparison and design screening.

FAQs

1) What is FSR in a ring resonator?

FSR is the spacing between adjacent resonant modes. It is usually expressed in frequency and sometimes approximated in wavelength around a chosen center wavelength.

2) Why does the calculator use group index?

Group index determines how optical pulses and resonance spacing behave with dispersion. That makes it the correct parameter for estimating free spectral range.

3) When should I enter effective index?

Enter effective index when you want an approximate resonance order. It is not required for FSR itself, but it helps with mode-number interpretation.

4) Why is wavelength-domain FSR only approximate?

The wavelength formula is a local conversion around the selected wavelength. Exact mode spacing is fundamentally uniform in frequency, not in wavelength.

5) Does a larger ring reduce FSR?

Yes. Increasing radius increases the round-trip length, which lowers frequency spacing. Smaller rings generally produce larger FSR values.

6) Can I use circumference instead of radius?

Yes. The calculator lets you enter total round-trip length directly. It converts that value into an equivalent radius for convenience.

7) What units are supported?

Length inputs support nanometers, micrometers, millimeters, centimeters, and meters. Wavelength inputs support the same unit set for flexible photonic design work.

8) Are these results suitable for fabrication signoff?

They are useful for design estimation and quick checking. Final fabrication decisions should also include dispersion models, coupling effects, loss, temperature, and process variation.

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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.