Glossary entry

Link budget

The accounting of optical power, gain, and loss across a transmission link, used to verify that the received signal exceeds the receiver sensitivity with sufficient margin. The basic design tool for any point-to-point optical link.

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):

P_\text{rx} \;=\; P_\text{tx} + G_\text{amp} - L_\text{fiber} - L_\text{connectors} - L_\text{components} - L_\text{margin},

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.

Loss source	Typical value
SMF-28 fiber @ 1550 nm	0.20 dB/km
SMF-28 fiber @ 1310 nm	0.35 dB/km
Multimode fiber OM4 @ 850 nm	2.3 dB/km
LC/UPC connector (good)	0.2 dB per mating
LC/UPC connector (typical)	0.3 – 0.5 dB
LC/APC connector	0.3 dB
Mechanical splice	0.5 – 1 dB
Fusion splice (single mode)	0.05 – 0.1 dB
Polarization controller	0.5 – 1 dB
Variable optical attenuator (set to 0 dB)	0.5 – 1 dB
Optical circulator	0.6 – 1 dB per port
Optical isolator	0.5 – 1 dB
WDM multiplexer (per channel)	1 – 5 dB depending on technology
EDFA (typical gain)	$+20$ to $+30$ dB
Receiver sensitivity (10G NRZ)	$-23$ dBm
Receiver sensitivity (100G coherent)	$-25$ dBm
Margin (engineering reserve)	3 – 6 dB

Two basic scenarios.

1. Unamplified short-reach link (e.g., 10G-LR Ethernet):

$P_\text{tx} = -8$ 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 + 22 dB EDFA gain = $+2$ dB
Total span gain: +20 dB
$P_\text{rx}$ at end: $+0 + 20 -$ (output amp & WDM demux losses, say 5 dB) = $+15$ dBm/channel
BUT: the limit is OSNR, not power. Each amp adds $\sim$ 5 dB NF noise; 10 amps: $\text{OSNR}_\text{end} \approx \text{OSNR}_\text{input} - 10\log_{10}(10) - \text{NF} - L_\text{span} + 58$ $\approx 50 - 10 - 5 - 20 + 58 \approx 73 \text{ dB}/\text{nm at input, } \approx 23 \text{ dB}/0.1\text{nm at end}.$
For 100G coherent: OSNR threshold $\sim 17$ dB/0.1nm; margin $\sim 6$ dB ✓

Power budget vs OSNR budget. Two complementary calculations:

Power budget	OSNR budget
Tracks signal power vs receiver sensitivity	Tracks signal-to-ASE ratio vs OSNR threshold
Relevant for unamplified or single-amp links	Relevant for amplified multi-span links
Limited by thermal/dark noise at receiver	Limited by ASE accumulation
Margin: typically 3 – 6 dB	Margin: typically 3 – 6 dB OSNR

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: 32 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.

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

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.