Predict Brillouin onset for robust optical designs fast. Compare effective length methods and correction factors. Export results, validate inputs, and document calculations easily here.
Provide fiber and source parameters. Use either direct effective length or attenuation-based length.
A widely used engineering estimate for the onset of stimulated Brillouin scattering (SBS) in a fiber is:
Pth \(\approx\) 21 \(\cdot\) Aeff / (gB \(\cdot\) Leff) \(\cdot\) F
If you choose attenuation-based length, the tool converts α from dB/km to nepers per meter and uses Leff = (1 − e−αL)/α.
| Aeff (μm²) | gB (m/W) | α (dB/km) | L (km) | Leff (m) | ΔνB (MHz) | ΔνL (MHz) | F | Pth (W) | Pth (dBm) |
|---|---|---|---|---|---|---|---|---|---|
| 80 | 5.0E-11 | 0.2 | 5 | 4466.112 | 30 | 1 | 1 | 0.007523322 | 8.764 |
| 80 | 5.0E-11 | 0.2 | 10 | 8013.681 | 30 | 1 | 1 | 0.00419283 | 6.225 |
| 80 | 5.0E-11 | 0.2 | 25 | 14848.014 | 30 | 1 | 1 | 0.002262929 | 3.547 |
| 60 | 5.0E-11 | 0.25 | 10 | 7602.935 | 30 | 50 | 1.667 | 0.005524182 | 7.423 |
These rows illustrate typical single-mode values and a broadened-linewidth case.
Stimulated Brillouin scattering (SBS) is a narrowband nonlinear process where light couples to acoustic waves and generates a backward Stokes wave. When launched power exceeds a threshold, backscatter rises sharply and depletes forward power. In practice, SBS can cap transmit power in long, low-loss single‑mode fibers and in high-gain amplifier chains.
This calculator applies the common rule-of-thumb threshold model Pth ≈ 21·Aeff/(gB·Leff)·F. The constant 21 absorbs polarization and spectral details into a conservative margin for many silica systems. The model is most useful for comparing scenarios quickly, not for replacing full system characterization.
The effective area Aeff captures how tightly the optical mode is confined. Larger Aeff reduces intensity and increases threshold power roughly linearly. Typical values for standard single‑mode fiber are about 70–90 μm² at 1550 nm, while large‑effective‑area designs can exceed 110 μm². The gain coefficient gB depends on material composition, temperature, and wavelength and is often around 4–6×10⁻¹¹ m/W for silica.
SBS accumulates over an interaction length that saturates when attenuation is significant. For a physical length L and power‑attenuation coefficient α (in nepers per meter), the tool uses Leff = (1 − e−αL)/α. With low loss (for example 0.20 dB/km), Leff approaches several kilometers quickly, so long spans can still behave like a shorter “effective” segment in the threshold equation.
SBS gain is narrow, typically ΔνB ≈ 20–50 MHz in silica fiber. If the laser spectrum is broadened, the peak gain is reduced and the threshold increases. The calculator uses F = max(1, ΔνL/ΔνB). For example, broadening a 1 MHz source is negligible, while a 200 MHz broadened source can raise the threshold by roughly a factor of 200/30 ≈ 6.7 when ΔνB is 30 MHz.
Results are shown in watts and dBm to match laboratory instruments and link budgets. As a reference, 1 W equals 30 dBm, 100 mW equals 20 dBm, and 10 mW equals 10 dBm. Compare the computed threshold against your planned launch power at the fiber input, including amplifier output power and connector losses upstream of the span.
Common mitigation methods include increasing Aeff (large‑mode‑area fiber), reducing Leff (shorter spans or higher loss where acceptable), and increasing F via phase modulation or chirp. Temperature and strain can shift Brillouin frequency and slightly modify gain, while depolarization can change effective coupling. In systems with multiple amplifiers, power management across spans is often the most practical control.
Validate the estimate by monitoring backward power on an optical circulator or tap while sweeping launched power. A clear SBS onset often appears as a knee in backscatter growth and a flattening of forward power. Use the calculator to bracket safe operating power, then confirm margins with real fiber reels, connectors, and polarization conditions.
1) What does the threshold power represent?
It is an approximate launch power where SBS backscatter increases rapidly. Above this point, forward power can be depleted and noise can rise. Treat it as a planning limit with margin, not an absolute boundary.
2) Why is the constant 21 used in the model?
It is a widely used engineering factor that bundles polarization, spectral shape, and practical margins for typical silica fibers. Different setups may shift the effective constant, so measurements are recommended for final validation.
3) Should I enter physical length or effective length?
If you know attenuation and span length, select the attenuation method so the tool computes Leff. If you already calculated Leff from your own model or vendor data, use the direct option.
4) How does laser linewidth affect SBS?
SBS gain is narrow, so broadening the source reduces peak gain. The calculator applies F = max(1, ΔνL/ΔνB). Larger linewidth usually increases the threshold roughly in proportion to that ratio.
5) What typical values should I start with?
Standard single‑mode fiber often has Aeff near 70–90 μm², gB around 4–6×10⁻¹¹ m/W, and attenuation near 0.18–0.25 dB/km at 1550 nm. Brillouin bandwidth is frequently 20–50 MHz.
6) Why can very long spans still have limited Leff?
Attenuation causes the interaction to saturate. As L grows, the exponential term e−αL becomes small and Leff approaches 1/α. This is why increasing span length eventually changes the threshold more slowly.
7) Does this cover pulsed or multi-channel systems?
It is primarily a continuous-wave estimate. Pulses, modulation formats, and WDM channel interactions can change effective linewidth and power distribution. Use this as a first check, then confirm with system-specific testing and detailed modeling.
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.