Estimate cavity and ring spacing with confidence. Switch units, export tables, and inspect plotted relationships. Designed for deeper resonator checks across practical calculation scenarios.
| Type | Length | Group Index | Center Wavelength | FSR (GHz) | Wavelength Spacing (pm) |
|---|---|---|---|---|---|
| Standing cavity | 25 mm | 1.4682 | 1550 nm | 4.083810 | 32.727149 |
| Standing cavity | 50 mm | 1.4682 | 1550 nm | 2.041905 | 16.363574 |
| Ring resonator | 10 mm | 1.9 | 1310 nm | 15.778550 | 90.321053 |
| Ring resonator | 2 mm | 2.1 | 1550 nm | 71.379157 | 572.023810 |
General relation: FSR = 1 / round-trip time
Standing-wave cavity: FSR = c / (2 × ng × L)
Ring resonator: FSR = c / (ng × L)
Wavelength spacing approximation: Δλ ≈ λ² / (ng × Lrt)
Estimated mode order: m ≈ neff × Lrt / λ
In these relations, c is the speed of light, ng is the group index, L is the physical cavity length, Lrt is round-trip length, λ is the selected center wavelength, and neff is the optional effective index used for order estimation.
Free spectral range describes the spacing between neighboring resonances in a cavity or resonator. Engineers, physicists, and photonics students often use it when comparing resonator designs, checking laser mode spacing, or estimating how many resonances fit inside a wavelength band.
This calculator accepts standing-wave cavities and ring resonators because their round-trip paths differ. A standing cavity sends light forward and backward across the physical length, so the round-trip path is twice the listed length. A ring resonator uses a single closed loop, so the round-trip path equals the physical path length.
The most useful output is often frequency-domain FSR, shown here in hertz, megahertz, gigahertz, and terahertz. The page also estimates wavelength spacing around your chosen center wavelength. That wavelength result is an approximation that works best when the spacing is small compared with the center wavelength.
Group index strongly affects the result because FSR depends on round-trip group delay. Larger group index or longer path length lowers the spacing. Shorter resonators produce larger free spectral ranges, which is often useful when wider mode spacing is required.
The optional effective index field supports a rough longitudinal mode order estimate. That value is helpful for deeper checks, but the FSR itself is driven by group delay, so the group index remains the key input for spacing calculations.
Free spectral range is the spacing between neighboring resonant frequencies of a cavity or resonator. It is usually reported in hertz, but it can also be approximated as wavelength spacing near a chosen center wavelength.
Longer cavities take more time for one round trip. Because FSR equals the inverse of round-trip time, a longer resonator produces a smaller frequency spacing, while a shorter resonator produces a larger spacing.
FSR depends on round-trip group delay. Group index captures that delay more accurately than ordinary refractive index when you want mode spacing in the frequency domain, especially in dispersive optical structures.
A standing-wave cavity uses a forward and backward pass over the physical length. A ring resonator uses one loop. That difference changes the round-trip path and therefore changes the computed free spectral range.
The wavelength spacing shown here is an approximation around the selected center wavelength. It is most reliable when the spacing is small compared with the chosen wavelength and dispersion does not vary sharply over the interval.
The optional effective index field estimates resonance order. It is not required for FSR. Leave it blank if you only need mode spacing and wavelength spacing.
Yes. The calculator can help with fiber cavities, Fabry-Perot structures, and ring-style resonators as long as you enter a suitable physical length, center wavelength, and group index for the device.
The CSV and PDF downloads save the current calculated summary. They make it easier to document design checks, compare setups, or share spacing calculations with students, teammates, or clients.
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.