What is chromatic dispersion in optical fiber?

Chromatic dispersion causes different wavelengths of light to travel at slightly different speeds through the fiber. The dispersion coefficient D (ps/nm·km) quantifies this. A source with spectral width Δλ (nm) over a span L (km) produces pulse broadening σ = |D| × Δλ × L (ps).

What is the difference between SMF, DSF, and NZDSF?

Standard SMF (G.652) has D ≈ 17 ps/nm·km at 1550 nm — well-understood, widely deployed. Dispersion-Shifted Fiber (DSF, G.653) is designed for D = 0 at 1550 nm, but causes four-wave mixing issues in WDM. Non-Zero DSF (NZDSF, G.655) has a small nonzero D (typically 2–6 ps/nm·km) to suppress FWM while keeping dispersion manageable.

How does spectral width affect pulse broadening?

Pulse broadening scales linearly with spectral width. A DFB laser with Δλ = 0.1 nm produces 10× less broadening than an FP laser with Δλ = 1 nm over the same fiber. Narrow-linewidth sources are essential for high bit-rate, long-distance links.

What dispersion-limited bit rate does this tool report?

The tool reports the maximum NRZ bit rate where pulse broadening stays below 1/(4 × bit_rate) — a common rule of thumb ensuring the intersymbol interference penalty stays under ~1 dB. For RZ modulation, the actual limit is roughly twice as tight.

Can I use this for multimode fiber?

This tool focuses on chromatic dispersion, which dominates in single-mode fiber. Multimode fiber is primarily limited by modal dispersion; use the Fiber Optic Bandwidth Calculator for that scenario.

Fiber Optic Dispersion Calculator

Enter fiber type, length, dispersion coefficient, and spectral width to calculate pulse broadening and dispersion-limited bandwidth.

Fiber Parameters

Fiber Type

Dispersion Coefficient D (ps/nm·km)

Standard SMF at 1550 nm — positive D (anomalous-dispersion regime).

Fiber Length (km)

Source Spectral Width Δλ — FWHM (nm)

DFB laser ≈ 0.1 nm · FP laser ≈ 1–5 nm · LED ≈ 30–60 nm

Center Wavelength (nm)

Target Bit Rate

Enter parameters on the left and click Calculate.

Summary

Enter fiber type, length, dispersion coefficient, and spectral width to calculate pulse broadening and dispersion-limited bandwidth.

How it works

Select fiber type: standard SMF (D ≈ 17 ps/nm·km at 1550 nm), DSF (D ≈ 0), NZDSF, or custom.
Enter the fiber span length in kilometers.
Enter the dispersion coefficient D in ps/nm·km (auto-filled for standard fiber types).
Enter the source spectral width (FWHM) in nanometers — e.g., 0.1 nm for a DFB laser, 1–5 nm for an FP laser.
Select target bit rate to evaluate pulse broadening against standard bit periods.
Click Calculate to get total chromatic dispersion, RMS pulse broadening, and maximum dispersion-limited bit rate.

Use cases

Estimate pulse broadening for a long-haul single-mode DWDM link before deploying dispersion compensation.
Determine whether a Fabry-Perot laser source needs replacement with a DFB laser to meet bit-rate requirements.
Size dispersion compensating fiber (DCF) or FBG modules for a given span.
Compare G.652, G.653, and G.655 fiber performance at different wavelengths.
Support optical communications coursework or lab calculations for fiber system design.
Quickly sanity-check vendor dispersion specs against system rise-time budgets.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu