Calculator inputs
Plotly graph
The chart shows estimated threshold power versus fiber length. It also overlays your entered operating power for visual comparison.
Formula used
This calculator applies a practical stimulated Brillouin scattering threshold estimate for optical fibers:
- Pth = estimated Brillouin threshold power in watts.
- C = empirical threshold constant, commonly near 21.
- K = polarization or system factor.
- Aeff = effective optical mode area in square meters.
- gB = Brillouin gain coefficient in meters per watt.
- Leff = effective interaction length after attenuation.
- Fbw = linewidth broadening factor.
The margin is computed as 10 log10(Pth / Poperating). Positive margin means the operating power is below the estimated threshold.
How to use this calculator
- Enter the physical fiber length in kilometers.
- Provide fiber attenuation in dB/km.
- Enter the effective area of the guided mode in µm².
- Set the Brillouin gain coefficient for your fiber type.
- Enter laser linewidth and Brillouin bandwidth in MHz.
- Keep the threshold constant near 21 unless your model differs.
- Input your operating optical power in watts.
- Press Calculate Threshold to view threshold power, margin, and graph.
- Use the CSV or PDF buttons to export the computed result.
Example data table
| Case | Length (km) | Loss (dB/km) | Aeff (µm²) | gB (m/W) | Linewidth (MHz) | Bandwidth (MHz) | Estimated Threshold (mW) |
|---|---|---|---|---|---|---|---|
| Short link | 10 | 0.20 | 80 | 5.0E-11 | 10 | 30 | 4.19 |
| Longer link | 25 | 0.20 | 80 | 5.0E-11 | 10 | 30 | 2.26 |
| Broad linewidth | 25 | 0.20 | 70 | 5.0E-11 | 150 | 30 | 9.89 |
These examples illustrate how longer effective lengths reduce threshold, while larger linewidth broadening can increase it.
Frequently asked questions
1) What is the Brillouin threshold?
It is the approximate input power where stimulated Brillouin scattering becomes significant. Above this level, backward scattering can grow quickly and limit useful forward transmission.
2) Why does effective fiber length matter?
Stimulated Brillouin scattering builds along the interaction length. As effective length rises, gain accumulates more strongly, so the threshold generally falls.
3) Why is attenuation included?
Attenuation reduces the optical power available to drive the nonlinear process along the fiber. It shortens the effective interaction length used in the threshold estimate.
4) What does the linewidth factor do?
A broader laser spectrum can spread power across frequency, reducing overlap with the Brillouin gain peak. That often raises the effective threshold.
5) Why is the constant often near 21?
Many engineering estimates use a constant near 21 for quick threshold prediction. It is empirical, so detailed systems can require a different factor.
6) Does this replace a full simulation?
No. This is a practical estimate for fast design screening. Precise work can require spectral, thermal, polarization, modulation, and fiber-specific modeling.
7) What units should I enter for area and gain?
Enter effective area in µm² and Brillouin gain in m/W. The calculator converts area internally to square meters before evaluating the formula.
8) How should I interpret the margin result?
Positive margin means the operating power is below the estimated threshold. Negative margin means the operating power exceeds the estimate and SBS risk is higher.