Optical Fiber Dispersion Calculator

This calculator estimates pulse broadening contributions and combines them with a root-sum-square method.

• Chromatic broadening: ΔTchrom = |D(λ)| · Δλ · L (ps), where D is in ps/(nm·km), Δλ in nm, and L in km.
• D adjustment with slope: D(λ) ≈ D(λref) + S · (λ − λref).
• Zero-dispersion model: D(λ) ≈ (S0/4) · ( λ − (λ04/λ3) ).
• PMD broadening: ΔTPMD = PMD · √L (ps).
• Modal broadening: ΔTmodal = (Modal ns/km) · L, converted to ps.
• Total (RSS): ΔTtotal = √(ΔTchrom2 + ΔTmodal2 + ΔTPMD2).

1. Select a calculation mode that matches your datasheet values.
2. Enter wavelength, source spectral width, and fiber length.
3. Provide either D (and optionally slope) or λ0 and S0.
4. Optionally add modal dispersion and PMD coefficient if needed.
5. Click Calculate. Results appear above the form.
6. Use Download CSV or Download PDF to save outputs.

1) Dispersion and link performance

Dispersion spreads pulses in time, shrinking eye opening and increasing intersymbol interference. In engineering terms it sets reach, impacts equalization effort, and defines whether you can reuse legacy fiber for higher rates.

2) Chromatic dispersion with practical numbers

Chromatic dispersion is usually given as D in ps/(nm·km). A common single‑mode value near 1550 nm is 16–18 ps/(nm·km), while many fibers sit near zero dispersion around 1310 nm. Because broadening scales as |D|·Δλ·L, doubling length doubles chromatic spread.

3) Dispersion slope and wavelength shifts

D varies with wavelength, so datasheets add a slope S, often about 0.05–0.09 ps/(nm²·km). A 10 nm shift from the reference can change D by roughly S×10, which becomes meaningful on long spans or dense WDM plans.

4) Using λ0 and S0 when D is not provided

Some specifications give the zero‑dispersion wavelength λ0 and the zero‑dispersion slope S0. The calculator uses a standard approximation to estimate D(λ) across a wide band, letting you see how dispersion flips sign around λ0 while the timing spread depends on |D|.

5) Modal dispersion for multimode fibers

Modal dispersion arises when different spatial modes travel different paths. It is often stated in ns/km. For scale, 1 ns/km equals 1000 ps/km, so even a few kilometers can add nanoseconds of spreading, dominating chromatic dispersion in many multimode links.

6) Polarization mode dispersion trends

PMD is modeled as a random process, so its broadening grows with √L. Modern fibers may show around 0.05–0.2 ps/√km, but older routes can be higher. PMD becomes more noticeable as symbol periods shrink at high data rates.

7) Why the calculator uses RSS combining

Chromatic, modal, and PMD contributions are reported separately and then combined with root‑sum‑square (RSS). RSS is a common engineering approach when mechanisms are largely independent. The D·L output in ps/nm is also useful when sizing dispersion compensation or comparing spans.

8) Interpreting the bitrate screening value

The displayed NRZ bitrate limit is a quick screening metric, not a strict maximum. Real limits depend on modulation format, filtering, receiver bandwidth, and FEC. Use it to compare scenarios such as narrower linewidth, shorter length, or different wavelength windows. For coherent links, digital dispersion compensation can handle large D·L, shifting focus to OSNR and nonlinear penalties.

1) What units does the dispersion parameter D use?

D is typically given in ps/(nm·km). Multiply by spectral width in nm and length in km to estimate chromatic pulse broadening in picoseconds.

2) Why does the calculator use absolute value for chromatic broadening?

The sign of D indicates whether longer wavelengths arrive earlier or later. Broadening magnitude depends on how much the pulse spreads, so |D| is used for timing spread estimates.

3) When should I enter modal dispersion?

Enter modal dispersion for multimode fibers or legacy short-reach links where modal delay dominates. For most modern single-mode long-haul links, modal dispersion is negligible and can be left blank.

4) Why does PMD scale with the square root of length?

PMD behaves like a random process along the fiber, similar to a random walk. Independent birefringence sections add statistically, producing a √L dependence for the overall differential group delay.

5) Is the NRZ bit-rate limit output a strict maximum?

No. It is a rough screening estimate based on total broadening. Actual limits depend on modulation, equalization, filtering, and FEC. Use it to compare cases and identify risky dispersion conditions.

6) How do I model a WDM system with different channels?

Run one calculation per channel wavelength using the same fiber length. If you have slope or λ0 and S0, D(λ) can be updated per channel to compare channel-to-channel dispersion differences.

7) What should I do if the total broadening is too large?

Common options include reducing spectral width, shortening spans, choosing a different wavelength window, adding dispersion compensation, or adopting coherent modulation with digital dispersion compensation.