OSNR and EDFA Span Calculator for Amplified Links
In an amplified DWDM link every EDFA adds noise, and optical signal-to-noise ratio is the quantity that decides whether a coherent or direct-detect receiver can still decode the signal. This calculator uses the standard 58 dB rule to estimate OSNR after N spans.
Your numbers
Per-channel power at the amplifier output.
About 0.20 to 0.25 dB/km at 1550 nm for G.652.
Typical EDFA noise figure is 4.5 to 6 dB; check the vendor datasheet.
Receiver requirement measured in 0.1 nm reference bandwidth, from the transponder datasheet, plus implementation margin.
Your results
Linear-regime estimate with identical spans and no ROADM, filtering or nonlinear penalty. Use the line system vendor's planning tool for design sign-off.
OSNR vs number of spans (dB)
Sweeps Number of spans from 1 to 30, holding your other inputs fixed.
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How to use the calculator
Enter the per-channel launch power, span length and loss figures, the amplifier noise figure and the number of spans. The tool returns span loss, OSNR at the end of the chain, the margin over the receiver requirement and the largest span count with positive margin. The chart plots OSNR as the span count rises against the required OSNR line, which makes the crossover point easy to see.
- Use the real loss measured per span, including patch connectors
- Use the noise figure for the operating gain, not the best case
- Add penalties for ROADMs, filtering and nonlinearity separately
The formula
For N identical spans, OSNR in a 0.1 nm reference bandwidth near 1550 nm is approximately 58 + Pch - span loss - NF - 10 x log10(N), with Pch in dBm and the loss and NF in dB. The constant 58 dB comes from the photon energy at 1550 nm and the 0.1 nm bandwidth. Doubling the number of spans costs 3 dB of OSNR, and every extra dB of span loss costs 1 dB.
Worked example
A span of 80 km at 0.22 dB/km plus 2 dB of connectors has 19.6 dB of loss. With 0 dBm per channel and a 5.5 dB noise figure, one span gives 58 - 19.6 - 5.5 = 32.9 dB. After 5 spans this is 32.9 - 7.0 = 25.9 dB, which is 10.9 dB above a 15 dB requirement. The same link supports about 12 spans at a 15 dB requirement, and then nonlinear effects and ROADM penalties will usually set the practical limit.
Common mistakes
The 58 dB formula assumes the reference bandwidth is 0.1 nm and the signal is at 1550 nm; quoting OSNR in a 12.5 GHz or 0.2 nm bandwidth changes the number by about 3 dB. Raising launch power does not help indefinitely because fiber nonlinearity causes distortion, and the optimum power depends on span loss and fiber type. Also remember that per-channel power falls when more channels share a fixed amplifier output.
Frequently Asked Questions
Why is the constant 58 dB?
It comes from the ASE noise power of an ideal amplifier at 1550 nm in a 0.1 nm bandwidth, which is about -58 dBm per dB of gain-limited noise. Per-channel power in dBm then gives OSNR in dB.
What OSNR does a receiver need?
It depends on the modulation and FEC. Coherent 100G DP-QPSK needs roughly 12 to 15 dB in 0.1 nm, and higher-order formats need more. Take the figure from the transponder datasheet.
Does this include nonlinear effects?
No. At high launch power or with many channels, cross-phase modulation and four-wave mixing add penalties. Use vendor planning software for final design.
Can I use different span lengths?
This tool assumes identical spans. For unequal spans, sum the linear noise contributions, 10 to the power of the negative OSNR over 10, for each span and convert back.
Netray can help keep optical design assumptions and as-built measurements in a single record linked to your assets and projects.
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