Tool · Datacenter optics

Datacenter Link Budget Explorer

Does a given datacenter optical link close on power, how much of each symbol does chromatic dispersion smear away, and what does the module cost in energy per bit? The explorer steps the transmitted power through fiber, connector and multiplexer losses to a receiver margin, computes the worst-case dispersion on every lane from the G.652 fiber curve, and compares pluggable, LPO and CPO module power at fleet scale, with class presets for 400G, 800G and 1.6T. Background: link budget, chromatic dispersion, and co-packaged optics.

Link

Class preset

Representative teaching values, not compliance numbers: check the exact IEEE clause before signing anything. Every field below is editable.

Fiber length (km)

Wavelength band

Fiber attenuation (dB/km)

Connectors in path

Loss per connector (dB)

Mux + demux loss (dB)

Zero for parallel-fiber (DR) links; a few dB round trip for WDM.

TX OMA per lane (dBm)

RX sensitivity, OMA (dBm)

Sensitivity at the KP4 pre-FEC threshold (BER 2.4 × 10⁻⁴).

Module & fleet

Architecture

Pluggable retains the full DSP; LPO removes it (linear drive); CPO moves the optics next to the switch ASIC. Powers below are representative published figures; edit them.

Module power (W)

Ports in fleet

Learn with it

Four two-minute experiments. Each one sets the controls, tells you where to look, and asks you to predict before it shows you.

Power budget · the waterfall

Every lane sees the same losses, so this waterfall applies per lane. Margin here is loss margin at the FEC threshold; dispersion is accounted separately below, the way the specs do it (as an allocation, not a simple dB). The margin does not subtract transmitter penalties such as TDECQ or any other penalty allocation; a real budget must hold those on top, so a link that shows a small positive margin here can still fail.

Chromatic dispersion · G.652 curve and your lanes

Energy per bit · architectures at this speed

Readouts

Checked against

These validation rows run in your browser on every load: the dispersion model against the textbook G.652 numbers, the budget arithmetic against exact identities, and the energy math against hand calculation.

Check	Expected	Computed	Tolerance

The table checks the fiber dispersion model against the textbook G.652 values and the budget, unit-interval and energy arithmetic against hand calculation. A ± tolerance is the largest absolute difference that still passes, in the units of the Expected column; “condition” means the Expected entry is itself the test, and “exact” means the outcome must match.

The model

The power ledger is a sum in decibels. With transmitter OMA $P_{t x}$ per lane, fiber attenuation $α$ over length $L$ , $N_{c}$ connectors of loss $ℓ_{c}$ each and a combined multiplexer and demultiplexer loss $ℓ_{m}$ , the received OMA and the margin against the sensitivity $S_{r x}$ are

P_{r x} = P_{t x} - α L - N_{c} ℓ_{c} - ℓ_{m}

M = P_{r x} - S_{r x}

The fiber dispersion follows the G.652 form with slope $S_{0} = 0.092 p s / (n m^{2} k m)$ , and each lane is charged with the worse of the two ends of the zero-dispersion range $λ_{0} = 1300$ to 1324 nm:

D (λ) = \frac{S_{0}}{4} (λ - \frac{λ_{0}^{4}}{λ^{3}})

The signal bandwidth is taken as the symbol rate $B$ , a serviceable estimate for PAM4, and converted to a wavelength width; the pulse spread is then compared with the unit interval $T = 1 / B$ :

Δ λ = λ^{2} B / c

Δ τ = ∣ D L ∣ Δ λ

Energy per bit is module power divided by line rate. The LPO and CPO bars scale the pluggable figure by fixed ratios (0.571 and 0.464) that are assumptions of the tool, not measurements. The model leaves out transmitter penalties such as TDECQ, reflections, polarization effects and laser chirp, so the margin is a loss margin only; the 30% and 80% boundaries on the spread bars are rules of thumb for where equalization starts to matter and where the eye is closing, not specification limits.

Worked example

Select the 800G FR4 preset: four CWDM lanes at 1271, 1291, 1311 and 1331 nm, each 106.25 GBd, over 2 km of fiber at 0.35 dB/km, with two 0.25 dB connectors, 3 dB of multiplexing loss, 1.0 dBm launch and a sensitivity of −5.5 dBm. The losses add to 0.70 + 0.50 + 3.00 = 4.20 dB, so 1.0 − 4.20 = −3.20 dBm arrives and the margin is +2.30 dB. The worst lane is 1271 nm with $λ_{0} = 1324$ nm: D = −5.19 ps/(nm·km), or −10.38 ps/nm over 2 km. The signal occupies $Δ λ$ = 1271² × 106.25×10⁹ / 2.998×10¹⁷ = 0.573 nm, so the spread is 5.94 ps, 63.1% of the 9.41 ps unit interval. A 15 W module at 800 Gb/s spends 18.75 pJ/bit (shown rounded as 18.8), and 100,000 ports draw 1.50 MW. For comparison, IEEE 802.3 clause 124 allows a 400GBASE-DR4 channel a maximum insertion loss of 3 dB over 500 m. Transmitter penalties are treated in the PAM4 eye and TDECQ explorer.

References: ITU-T Recommendation G.652, “Characteristics of a single-mode optical fibre and cable” (2016). IEEE Std 802.3-2022, IEEE Standard for Ethernet, clause 124. G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010), ch. 2.