What is optical power budget and how do I calculate it?

Optical power budget is the difference between the minimum transmit power and the receiver sensitivity — the total "budget" for fiber losses.

Formula

Power Budget = Tx Min Power (dBm) - Rx Sensitivity (dBm)

Example: Tx min = -5 dBm, Rx sensitivity = -20 dBm → Budget = 15 dB

Loss Contributors

Example Calculation

Link: 15km SMF at 1310nm with 4 connectors and 2 fusion splices

• Fiber: 15 × 0.35 = 5.25 dB
• Connectors: 4 × 0.5 = 2.0 dB
• Splices: 2 × 0.1 = 0.2 dB
• Total loss: 7.45 dB

With 15 dB budget, margin = 15 - 7.45 = 7.55 dB (good — should be >3 dB minimum)

Recommended Safety Margins

• 3 dB minimum: For temperature variation, aging
• 5-6 dB preferred: Comfortable margin
• >10 dB: Consider lower-power modules to reduce reflections

Common Mistakes

• Using max Tx power instead of min for budget calc
• Ignoring connector losses at patch panels
• Not accounting for future aging degradation
• Using ideal fiber loss (real fiber may be worse)

Testing

• OTDR: Trace losses along fiber
• Power meter + light source: End-to-end loss
• DDM Rx power: In-service verification