The optical power quota, or power budget, for fiber-optic cables is determined by subtracting the receiver's minimum sensitivity from the transmitter's minimum power and then accounting for losses due to cable attenuation, splices, and connectors.

Determining the Optical Power Budget

1. Identify Transmitter and Receiver Specifications Obtain the minimum transmit power (P_T) of the optical source and the minimum receive sensitivity (P_R) of the receiver. These values are usually provided by the manufacturer and represent worst-case performance, not averages, to ensure reliable operation .
2. Calculate Initial Power Budget The basic formula is: Power Budget (P_B) = P_T – P_R For example, if P_T = –15 dBm and P_R = –28 dBm, then P_B = 13 dB . This represents the maximum allowable loss before the signal becomes unreliable.
3. Account for Cable Losses Fiber-optic cables experience attenuation, typically 0.22–0.5 dB per kilometer depending on fiber type. Multiply the attenuation by the total cable length to estimate total loss. For instance, a 20 km fiber with 0.3 dB/km loss results in 6 dB total attenuation .
4. Include Splice and Connector Losses
   • Splices: Each splice typically adds ~0.1 dB loss.
   • Connectors: Loss per connector is provided by the manufacturer, often 0.2–0.5 dB. Sum these losses and subtract from the initial power budget to determine the available margin .
5. Calculate Power Margin The power margin (P_M) is the remaining power after subtracting all losses: P_M = P_B – (Cable Loss + Splice Loss + Connector Loss) A positive P_M indicates sufficient power for reliable operation .

Practical Considerations

• Bidirectional Links: When connecting devices from different vendors or models, calculate the power budget in both directions and use the smaller value to ensure reliability .
• Safety Margin: Always include a margin to account for aging, temperature variations, or installation imperfections.
• Cable Type: Multimode fibers have higher attenuation and modal dispersion than single-mode fibers, affecting the required power budget .
• Testing: After installation, use an optical power meter to verify actual link loss and ensure the power margin is adequate. By following these steps, you can determine the correct optical power quota for your fiber-optic installation, ensuring signal integrity and preventing underpowered or overpowered links.