Adjust attenuation, length, launch power, splice count, connector count, receiver sensitivity, and dispersion to estimate received power, margin, bandwidth, and maximum reach.
Fiber & System Settings
Attenuation α (dB/km)
dB/km
Fiber length L (km)
km
Transmit power P_in (dBm)
dBm
Splice count N_s
Connector count N_c
Receiver sensitivity (dBm)
dBm
Spectral linewidth Δλ (nm)
nm
While paused, move the sliders to update the result instantly.
Signal propagation & link power budget (live)
Results
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Received power (dBm)
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Power margin (dB)
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Link bandwidth (GHz)
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Max reach (km)
Received power vs distance (dBm)
Bandwidth vs distance
Theory & Key Formulas
$$P_{\text{dBm}}= P_{\text{in}}- \alpha L - N_s l_s - N_c l_c$$
$$\sigma_t = D \cdot \Delta\lambda \cdot L$$
$$\text{BW}\approx \frac{0.44}{\sigma_t}$$
Splice loss ls=0.1 dB, connector loss lc=0.5 dB. SMF chromatic dispersion D=17 ps/(nm·km) at 1550 nm.
What is an Optical Fiber Link Budget?
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What does the received power value tell me in this simulator?
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It is the estimated optical power arriving at the receiver after fiber attenuation, splice loss, and connector loss are subtracted from the launch power. The power margin compares that received power with the receiver sensitivity.
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So a positive margin means the link should work?
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Usually yes, but a design should keep margin for aging, temperature drift, connector contamination, repair splices, and measurement uncertainty. A few dB of margin is a practical minimum for many links.
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Why does the bandwidth chart change when I move fiber length or linewidth?
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Longer fiber and wider source linewidth increase chromatic dispersion, which broadens pulses and lowers usable bandwidth. That is why a link can have enough optical power but still fail at the desired data rate.
Physical Model & Key Equations
The link budget subtracts all loss terms from the transmitter launch power. Fiber attenuation scales with distance, while splice and connector losses scale with the number of joints in the route.
$$P_{rx}=P_{in}-\alpha L-N_s l_s-N_c l_c$$
Prx and Pin are in dBm, α is attenuation in dB/km, L is fiber length, and Ns/Nc are splice and connector counts.
The power margin is the difference between received power and receiver sensitivity. Positive margin means the receiver is above its minimum specified optical input.
$$\text{Margin}=P_{rx}-P_{\text{sensitivity}}$$
A higher margin gives the design more tolerance for dirty connectors, repair splices, aging, and environmental variation.
As the guided light pulse travels, it loses power due to absorption and scattering in the glass. This signal loss is characterized by attenuation α. The output power P(L) after traveling a length L is calculated from the input power P₀.
$$ P(L) = P_0 \cdot 10^{-\alpha L / 10} $$
P(L) is the output power (W), P₀ is the input power (W), α is the attenuation coefficient (dB/km), and L is the fiber length (km). The factor of 10 in the exponent is because attenuation is defined in decibels, a logarithmic unit.
Frequently Asked Questions
Consider increasing the transmission power, shortening the fiber length, reducing the number of splices and connectors, or switching to low-loss connectors (e.g., from 0.1 dB to 0.05 dB). Additionally, replacing the fiber with a product that has a lower attenuation coefficient (e.g., 0.2 dB/km) is also effective.
Typical splice loss is 0.1 dB per splice, and typical connector loss is 0.5 dB per connector (e.g., SC connector). If you do not have measured values, use these standard values. However, if high accuracy is required, manufacturer specifications or measured data are recommended.
The link power margin is the difference (in dB) between the received power and the receiver's minimum sensitivity. Generally, a margin of 3 dB or more ensures stable operation against aging and temperature changes. If the margin is small, system reliability may decrease, so caution is needed.
No, for long distances, bandwidth limitations (modal dispersion and chromatic dispersion) become important. This tool primarily focuses on loss calculation, but in actual system design, you should also check dispersion constraints based on the transmission rate. For example, at 10 Gbps or higher, dispersion compensation may be necessary.
Real-World Applications
Long-Haul Telecommunications: This is the backbone of the global internet. Signals encoded as light pulses can travel hundreds of kilometers in ultra-low-loss fibers (α ≈ 0.2 dB/km) before needing amplification. Transoceanic cables use this technology to connect continents.
Medical Endoscopy: In flexible medical scopes, coherent bundles of optical fibers transmit an image from inside the body to an eyepiece or camera. The high NA of the fibers allows efficient light collection from the illuminated tissue.
Industrial Sensing & Lighting: Fibers are used to deliver bright light to hard-to-reach places, like inside jet engines for inspection, or to carry laser light for precision cutting and welding. They can also be sensors themselves, with changes in transmitted light indicating strain or temperature.
Data Centers: Within and between server racks, high-bandwidth multimode fibers with large NA and cores rapidly transmit vast amounts of data over short distances. The trade-off of higher dispersion for easier coupling is acceptable in these short links.
Common Misconceptions and Points to Note
First, do not confuse "dBm" and "dB". dBm is "an absolute power value where 1mW is 0 dBm", while dB is "a relative value indicating the ratio between two powers". If you set "Transmit Power 0 dBm" in the tool and enter "Connector Loss 0.5 dB", the received power becomes -0.5 dBm. It is not 0.5 dBm. Next, typical parameter values change depending on the situation. For example, the default connector loss of 0.5dB is realistic for a clean, new LC connector, but for a dirty SC connector, exceeding 1dB is not uncommon. If you think "let's design with margin" when using the tool, a practical tip is to calculate by applying a margin of about 1.5 times to each loss value. Finally, understand that bandwidth calculation is a separate constraint independent of "loss". Even if a link is feasible based on loss calculation alone, if the bandwidth is insufficient, communication cannot occur at the intended speed. For example, when using OM3 fiber for 10GbE, while transmission over several hundred meters might be possible from a loss perspective, the maximum transmission distance specified by the standard is around 300m. With this tool, you can see how "the stricter of these two different constraints (loss and bandwidth) determines the final maximum distance".
