---
title: "Building a Proper Optical Link Budget Calculator: From Component Losses to EDFA Placement"
slug: "optical-budget-calculator-guide-dwdm-span"
type: tutorial
category: "Engineering & Design"
tags: ["link budget", "optical power budget", "EDFA", "DWDM", "attenuation", "splice loss", "optical design"]
seo_focus_keyword: "optical link budget calculator DWDM"
---

An optical link budget is a power accounting exercise—add up all the losses a signal encounters between transmitter and receiver, subtract that from the transmitter's launch power, and verify that the result exceeds the receiver's sensitivity floor by an acceptable margin. The concept is simple. The practice requires accounting for variables that are individually small but collectively determine whether a link works on day one and continues to work five years later after fiber aging, connector degradation, and splices that were optimistic at acceptance testing.

## The Component-by-Component Loss Model

A complete link budget starts with the transmitter launch power. For a standard 100GBASE-ER4 QSFP28 (80 km application), the specified minimum launch power is +1 dBm per lane (the ER4 uses 4 lanes at 25G over CWDM wavelengths). Receiver sensitivity is specified at -14 dBm minimum (with FEC), giving a nominal budget of 15 dB before any system effects.

Working through the loss components:

**Fiber attenuation**: G.652D fiber at 1310 nm (the operating band for CWDM-based ER4) has typical attenuation of 0.34–0.36 dB/km. For 80 km, that's 27.2–28.8 dB of fiber loss alone—which immediately tells you that 100GBASE-ER4 without amplification cannot reach 80 km on 1310 nm-based CWDM. ER4 is a CWDM optic using wavelengths 1295/1310/1325/1340 nm where conventional EDFAs don't operate. ER4's 40 km reach (not 80 km—I'll correct this) on OM4 is based on the actual spec. The 80 km application over single-mode uses ZR or extended-range coherent optics.

Let me recalibrate to a more instructive example: a 100G DP-QPSK ZR optic deployed on a DWDM system at 1550 nm wavelength (C-band, EDFA-amplified), targeting 80 km without inline amplification.

A 100G ZR QSFP28 module specifies a launch power of approximately +1.5 dBm (depending on manufacturer, typically 0 to +3 dBm) and a receiver sensitivity of approximately -22 dBm (with soft-decision FEC). Nominal budget: approximately 23.5 dB.

**Fiber attenuation** at 1550 nm over G.652D: 0.19 dB/km typical, 0.22 dB/km maximum. For 80 km: 15.2–17.6 dB. Using 0.20 dB/km as a design value: 16 dB.

**Splice loss**: properly fusion-spliced G.652D connections produce 0.02–0.05 dB per splice. For an 80 km run with splices every 2 km, that's approximately 40 splices at 0.03 dB average: 1.2 dB.

**Connector insertion loss**: 2 connectors at each end (one on the optic, one at the patch panel) at 0.3 dB each: 0.6 dB × 2 ends = 1.2 dB for 4 total connector pairs. Using 0.3 dB per mating for well-maintained LC APC connectors.

**Total channel insertion loss**: 16 + 1.2 + 1.2 = 18.4 dB.

Budget remaining: 23.5 - 18.4 = 5.1 dB total margin. Against the 23.5 dB budget, that's 5.1 dB of margin before the link fails.

## Aging and Temperature Margin

A link budget that shows 5.1 dB total margin at installation is comfortable today but needs to remain viable over the fiber plant's service life. Several degradation mechanisms consume margin over time:

**Fiber aging**: G.652D fiber increases in attenuation at approximately 0.001 dB/km/year for the first few years, then stabilizes. Over 10 years, this adds 0.01 dB/km, or 0.8 dB for an 80 km span—a meaningful consumption of margin.

**Connector degradation**: connectors that are maintained properly (cleaned, capped when not in use) degrade negligibly. Connectors in poorly maintained environments can increase from 0.3 dB to 1.0 dB or more over 5–7 years. Budget 0.3 dB of additional connector loss per connection point over the system life as an aging allowance—0.6 dB total for 4 connector pairs on our 80 km example.

**Temperature effects**: optical power levels in transceivers vary with temperature. SFF-8636 specifies that QSFP28 modules must operate to specification across 0°C to 70°C case temperature. Launch power at 70°C case temperature may be 1–2 dB lower than at 25°C for some module designs. Budget a temperature derating of 1 dB.

**Safety margin**: standard practice is to include 3 dB of safety margin for unaccounted losses—measurement uncertainties, OTDR dead zones, repair splices after future fiber cuts, and the inevitable "where did that 0.5 dB go" at commissioning.

Total system margin requirement: aging (1.4 dB) + temperature (1.0 dB) + safety (3.0 dB) = 5.4 dB.

In our 80 km example, the available margin of 5.1 dB is less than the required system margin of 5.4 dB. The link is marginally under-designed for the 80 km distance. The solution is either to reduce span length by 5–8 km (available margin then becomes ≈6.2 dB), accept slightly lower average connector quality as 0.25 dB instead of 0.30 dB (requires verified connector quality), or reconsider whether an amplified design is appropriate.

## The 80 km DWDM Span with EDFA: Worked Example

For longer-reach DWDM applications where a single amplified span is justified, the budget methodology extends to include EDFA characteristics.

Consider a 120 km DWDM span using a single EDFA at the midpoint (60 km from each end). The same 100G ZR optic with 23.5 dB budget launches into the first 60 km segment.

**First fiber segment** (0–60 km): 60 km × 0.20 dB/km = 12.0 dB, plus 30 splices × 0.03 dB = 0.9 dB, plus two connector pairs = 0.6 dB. First segment loss: 13.5 dB.

**EDFA parameters**: a typical C-band EDFA for 100G coherent applications provides gain of 20–23 dB with noise figure of 5–6 dB. Using 20 dB gain (conservative, to minimize gain tilt on a loaded C-band system) and 5.5 dB noise figure.

**OSNR calculation**: the OSNR at the EDFA output must be sufficient for the second fiber segment to still meet the receiver's OSNR sensitivity floor. OSNR into the EDFA is:

OSNR_in = P_in (dBm) - NF(dB) - 10·log10(h·ν·Bref)

where h·ν is the photon energy at 1550 nm (~1.28 × 10^-19 J) and Bref is the reference bandwidth (12.5 GHz for 0.1 nm reference bandwidth convention). The noise floor term 10·log10(h·ν·Bref) evaluates to approximately -58 dBm. After a 13.5 dB loss segment, with launch power +1.5 dBm, EDFA input power is -12 dBm. OSNR out of the EDFA is approximately -12 - 5.5 + 58 - (20 - 13.5) = 23.5 dB (simplified single-span approximation).

**Second fiber segment** (60–120 km): 12.0 dB fiber + 0.9 dB splices + 0.6 dB connectors = 13.5 dB. EDFA output power after 20 dB gain and 13.5 dB second segment loss arrives at the receiver at approximately: -12 + 20 - 13.5 = -5.5 dBm.

Receiver sensitivity for 100G ZR is approximately -22 dBm, giving 16.5 dB of power margin—but OSNR is the actual constraint for coherent systems. A single EDFA span at these parameters will deliver approximately 18–20 dB OSNR at the receiver, and 100G DP-QPSK ZR requires approximately 13–14 dB OSNR for BER below the FEC threshold. OSNR margin is approximately 4–6 dB, which is adequate but not lavish.

## Putting the Calculator Together

A functional optical link budget spreadsheet for single-mode systems needs five inputs per segment: span length, fiber attenuation coefficient, average splice spacing and loss, connector count and loss, and any passive splitting or filtering losses. The outputs are: total segment loss, available power budget, operating margin, and recommended EDFA placement if margin is insufficient.

The aging margin (0.01 dB/km/year × planned life in years), temperature margin (1 dB for standard commercial transceivers), and safety margin (3 dB minimum, 4 dB for carrier-grade applications) are constants applied to the available budget.

For EDFA-amplified spans, the additional inputs are EDFA gain, noise figure, and the OSNR sensitivity floor for the modulation format—which changes from 13 dB for 100G DP-QPSK to approximately 21 dB for 400G DP-16QAM. For multi-amplifier spans or cascaded EDFA designs, OSNR accumulates additively (in linear scale) and the calculation extends iteratively per span, which is where dedicated DWDM planning tools (Ciena's GreenPlanner, Cisco's Network Planner, or open-source alternatives) add value over manual spreadsheet calculations.

The manual worked-example approach remains valuable for understanding what the tools are actually computing and for validating their outputs against known engineering principles.
