Calculator
Example data table
| Fiber type | D (ps/(nm·km)) | Length (km) | Δλ (nm) | Estimated spreading (ps) |
|---|---|---|---|---|
| SMF (1550 nm typical) | 17.0 | 2 | 0.1 | 3.4 |
| SMF long run | 17.0 | 20 | 0.2 | 68 |
| Low-dispersion fiber | 3.5 | 10 | 0.2 | 7 |
| Mixed link (avg) | 10.0 | 5 | 0.4 | 20 |
| Wide source bandwidth | 17.0 | 1 | 2.0 | 34 |
Formula used
Chromatic dispersion causes different wavelengths to travel at different speeds. The calculator estimates time spreading from the dispersion parameter:
- D is dispersion parameter in ps/(nm·km).
- L is link length in km.
- Δλ is the source spectral width in nm.
For a segmented link, total dispersion is the sum of each segment:
Maximum length and maximum spectral width rearrange the same relationship.
How to use this calculator
- Select Single segment or Segmented link.
- Choose a calculation type: spreading, maximum length, or maximum width.
- Enter D, route length, and source spectral width.
- If using limits, enter your allowable spreading value.
- Press Calculate to show results above the form.
Why chromatic dispersion matters on construction sites
Fiber links used for CCTV backbones, building automation, and temporary network spines can fail at higher data rates when chromatic dispersion spreads pulses beyond the receiver timing window. Long pulls through risers, tunnels, and outdoor duct banks increase length quickly, so dispersion should be checked early—before splicing, tray loading, and handover testing. For acceptance testing, compare OTDR and link test lengths against the modeled length so results align. This helps avoid rework later.
Reference values you can validate from vendor sheets
Single‑mode fiber near 1550 nm often lists D around +16 to +18 ps/(nm·km), while near 1310 nm it can be close to zero. Many “low‑dispersion” or specialty fibers publish smaller magnitudes, which improves tolerance to wider sources. Always confirm the wavelength, because the same cable can have very different dispersion across operating windows.
Relating spreading to bandwidth and timing margin
A practical check is comparing calculated spreading Δt to symbol time. For example, a 10 Gb/s link has a 100 ps symbol period. If dispersion broadening is 10 ps, that is roughly 10% of the period and typically manageable with margin. If broadening approaches 30–50 ps, you should reduce length, narrow Δλ, or select a different fiber grade.
Example data for common jobsite planning scenarios
| Scenario | D (ps/(nm·km)) | L (km) | Δλ (nm) | Δt = |D|×L×Δλ (ps) |
|---|---|---|---|---|
| Campus duct bank pull | 17.0 | 12 | 0.2 | 40.8 |
| High‑rise riser backbone | 17.0 | 3 | 0.1 | 5.1 |
| Low‑dispersion section | 3.5 | 8 | 0.2 | 5.6 |
| Wide source bandwidth | 17.0 | 2 | 1.0 | 34.0 |
Segmented runs and documentation expectations
Mixed routes are common: indoor riser cable, outdoor loose‑tube, and patch leads in the same end‑to‑end path. Use segmented mode so each section contributes D·L realistically. Capture the inputs, exports, and assumptions in your closeout package to support commissioning, troubleshooting, and future extensions.
FAQs
What does the dispersion parameter D represent?
D describes how much group delay changes with wavelength, normalized by distance. It is commonly given in ps/(nm·km) and depends strongly on operating wavelength and fiber design.
Why does the calculator use absolute values?
Dispersion can be positive or negative depending on wavelength, but pulse spreading magnitude is what affects timing. Using |D| or |ΣD·L| reports the spreading size without sign confusion.
Which inputs should come from field data versus datasheets?
Use route length from drawings or measured pulls, including slack and patching. Use D and recommended operating wavelength from vendor datasheets. Use Δλ from the transmitter specification or module class.
How do I choose an allowable spreading limit?
A simple rule is to keep spreading under 10–20% of the symbol time for your data rate. Tight systems, long links, or older optics may require a smaller limit and extra design margin.
When should I use segmented mode?
Use segmented mode when parts of the path use different fiber types, wavelengths, or route lengths. The calculator sums D·L per segment, which better matches mixed risers, outdoor runs, and patch fields.
Does chromatic dispersion replace link budget checks?
No. Dispersion addresses timing spread, while link budget addresses optical power and losses. You typically verify both: attenuation/splice loss for power margin and dispersion for bandwidth and error‑rate stability.
Can this tool help with troubleshooting after installation?
Yes. If a link passes power tests but fails at higher speeds, compare actual length and optics Δλ to your design. A high predicted Δt suggests reducing length, narrowing the source, or selecting different optics.
Notes for construction deployments
- Use vendor datasheets for D at your operating wavelength.
- Include patch panels and slack loops in route length.
- For mixed fiber types, use segmented links for better estimates.
- Keep a margin for temperature, splices, and future extensions.