Link budget

A link budget is the systematic accounting of optical power, gain, and loss between transmitter and receiver in an optical communications link. The fundamental purpose is to verify that the received signal power exceeds the receiver sensitivity by a specified margin under worst-case conditions. The link budget is the basic design tool used to specify components, choose fiber type, count amplification stages, and predict reliability.

Basic equation. In dB form (additive):

$$\begin{array}{rl}{\displaystyle P_{\text{rx}}=}& {\displaystyle P_{\text{tx}}+G_{\text{amp}}-L_{\text{fiber}}}\\ {\displaystyle }& {\displaystyle -L_{\text{connectors}}-L_{\text{components}}-L_{\text{margin}},}\end{array}$$

where:

• $P_{\text{rx}}$ = received power [dBm]
• $P_{\text{tx}}$ = transmitter output power [dBm]
• $G_{\text{amp}}$ = total amplifier gain [dB]
• $L_{\text{fiber}}$ = fiber attenuation [dB]
• $L_{\text{connectors}}$ = total connector loss [dB]
• $L_{\text{components}}$ = component-by-component insertion losses [dB]
• $L_{\text{margin}}$ = engineering margin [dB]

For the link to meet specifications, $P_{\text{rx}}$ must exceed the receiver sensitivity by the required Q-factor or OSNR margin.

Standard loss contributions.

Two basic scenarios.

1. Unamplified short-reach link (a 10 km 10G link with an APD receiver):

• $P_{\text{tx}}=-8$ dBm (typical DFB laser launch)
• $L_{\text{fiber}}$ = 10 km × 0.35 dB/km = 3.5 dB (at 1310 nm)
• 2 connectors: 1 dB total
• Component loss (filter, mux): 2 dB
• Margin: 3 dB
• $P_{\text{rx}}=-8-3.5-1-2-3=-17.5$ dBm
• Receiver sensitivity: $-23$ dBm
• Excess margin: 5.5 dB ✓

2. Long-haul amplified link (e.g., 100G coherent over 1000 km):

• $P_{\text{tx}}=0$ dBm (per channel)
• 10 EDFA spans of 100 km each
• Each span: 20 dB fiber loss + 20 dB EDFA gain = 0 dB net
• Total span gain: 0 dB
• $P_{\text{rx}}$ at end: $0+0-$ (output amp & WDM demux losses, say 5 dB) = $-5$ dBm/channel
• BUT: the limit is OSNR, not power. Each amp has $\sim$ 5 dB NF; for 10 identical spans: ${\text{OSNR}}_{\text{end}}\approx 58+P_{\text{ch}}-\text{NF}-L_{\text{span}}-10{\log }_{10}(N_{\text{spans}})$ $\approx 58+0-5-20-10\approx 23\text{ dB}\mathrm{/}0.1\text{nm at end}\mathrm{.}$
• For 100G coherent DP-QPSK: required OSNR 13 to 16 dB/0.1 nm at the FEC threshold; margin 7 to 10 dB ✓

Power budget vs OSNR budget. Two complementary calculations:

Modern long-haul systems are usually OSNR-limited, not power-limited. Adding more gain doesn't help if it adds noise too.

Wavelength dependence. Loss budget components depend on wavelength:

• Fiber attenuation: lowest at 1550 nm (0.20 dB/km), rises at shorter wavelengths
• Bend loss: more sensitive at longer wavelengths
• EDFA gain: 1530 – 1565 nm (C-band) or 1570 – 1610 nm (L-band, with L-band EDFA)
• Component losses: typically increase as wavelength moves away from design

DWDM systems use power-per-channel budgets at the worst-case channel (typically band edges).

Engineering margin. Realistic links allocate 3 – 6 dB of margin to account for:

• Manufacturing tolerance: components specified by datasheet upper-bound losses
• Connector wear and degradation: 0.1 – 0.5 dB increase over 25-year deployment
• Splice aging: 0.05 – 0.1 dB increase
• Fiber loss aging: ~0.005 dB/km/year (Rayleigh scattering is permanent; impurities can slowly increase)
• Temperature variations: ~10% variation in some losses
• Cable bending/installation stress: variable
• Repair splices: each in-field splice adds 0.05 – 0.5 dB

Submarine cables, with 25 – 30 year design life and very expensive repair, typically use 6 – 8 dB margin. Terrestrial systems use 3 – 5 dB.

Coherent system link budget. For modern 400G/600G coherent transmission:

• Net OSNR requirement: 17 – 25 dB/0.1nm depending on modulation format
• Per-channel launch power: $0$ to $+3$ dBm
• Maximum reach without regeneration: 1500 – 4000 km depending on rate and modulation
• Frequency-domain DSP at the receiver compensates for chromatic dispersion (eliminates need for inline DCM)
• DSP also compensates polarization-mode dispersion within limits

Subsea link budget. Submarine cables push the limits:

• 6500 km transatlantic: typically 80 EDFA spans of 80 km
• Per-channel launch: $-2$ to $+1$ dBm to limit nonlinear penalty
• Cumulative ASE noise: $\sim 19$ dB/0.1nm OSNR at receiver
• Modulation: 64 Gbaud DP-16QAM for 400G (at OSNR limits)
• Modulation: 64 Gbaud DP-QPSK for 200G (more robust)

Link-budget software tools. Modern network design uses dedicated tools that integrate the link budget:

• VPI Photonics, Photonic Modules, Ciena's tools
• Standard model: per-span loss, NF, $P_{\text{nonlinear}}$, calculated end-to-end OSNR and Q-factor
• Output: required margin vs distance, mod-format vs reach trade-off

Practical link-budget checklist.

1. Transmitter launch power (worst-case from datasheet)
2. Fiber loss × distance (use worst-case fiber spec, add cable splices)
3. Connectors at all interconnect points
4. Other components: multiplexers, isolators, attenuators
5. Amplifier gain (use worst-case low end)
6. Amplifier noise figure (use worst-case high end)
7. ASE accumulation across amplification chain
8. Receiver sensitivity (specified at required BER)
9. Engineering margin

If the calculation doesn't close (received power or OSNR below threshold), redesign: shorter span, lower-loss fiber, more amplifiers, better receiver, or modulation format with lower OSNR threshold.

For short-reach datacenter classes specifically (400G/800G/1.6T DR, FR, and LR), the Datacenter Link Budget Explorer computes the loss waterfall, per-lane chromatic dispersion, and the pluggable/LPO/CPO energy comparison interactively.

The floor of the budget is the receiver sensitivity, and impairments enter it as power penalties; the Datacenter Link Budget Explorer computes the waterfall live for the 400G to 1.6T classes.

The free-space counterpart, where loss grows with the square of range and the atmosphere sets availability, is covered in The free-space optical link budget.

Common questions

What is an optical link budget?

It is the accounting, in decibels, of the power leaving a transmitter and every gain and loss between it and the receiver, ending in a comparison against the receiver's sensitivity. If the received power exceeds the sensitivity by the required margin, the link closes; if not, something in the design has to change. Amplified links carry out the same accounting a second time for the optical signal-to-noise ratio.

Why is a link budget important?

Because it is the only place where the transmitter, the fiber, the connectors, and the receiver are compared on one scale before anything is bought or installed, and because it is where the margin lives. A link that closes with 3 dB to spare survives 25 years of connector wear (0.1 to 0.5 dB), splice aging, temperature, and a repair splice or two; a link with no margin fails on the first dirty connector. It also turns data sheets into decisions: which fiber, how many amplifiers, which modulation format.

How much margin should a link budget have?

3 to 6 dB for terrestrial links, and 6 to 8 dB for submarine systems where repair is slow and expensive. The margin covers worst-case component tolerances, aging of connectors, splices, and fiber, temperature, and repair splices added over the life of the plant.

What is the difference between a power budget and an OSNR budget?

A power budget compares received power against receiver sensitivity and governs unamplified links, which are limited by the receiver's own noise. An OSNR budget compares the signal against the amplified spontaneous emission accumulated from every amplifier and governs multi-span amplified links, where adding gain adds noise as well. In the worked long-haul example above, a 100G coherent receiver that needs 13 to 16 dB/0.1 nm receives about 23 dB after ten 100 km spans, a margin of 7 to 10 dB. The Datacenter Link Budget Explorer works the short-reach power budget live.

References: Agrawal, Fiber-Optic Communication Systems (4th ed., 2010), Ch. 5 (comprehensive link-budget treatment); ITU-T G.957/G.691 series for standardized link-budget formats; Telcordia GR-468-CORE for component reliability and aging factors.