Power Margin Calculation

After calculating a link's power budget (using the equation described in Power Budget Calculation), you can calculate the power margin (P_M), which estimates the amount of power available after subtracting attenuation or link loss (LL) from the power budget (P_B). A worst-case estimate of P_M assumes maximum LL:

P_M = P_B - LL

A P_M greater than zero indicates that the power budget is sufficient to operate the receiver.

Table 16 lists the estimated amount of loss for factors that cause link loss.

Table 16: Estimating Link Loss

Link-Loss Factor	Estimated Link-Loss Value
Higher-order mode losses	Single-mode—None Multimode—0.5 dB
Modal and chromatic dispersion	Single-mode—None Multimode—None, if product of bandwidth and distance is less than 500 MHz-km
Connector	0.5 dB
Splice	0.5 dB
Fiber attenuation	Single-mode—0.5 dB/km Multimode—1 dB/km

In the following sample calculation for a 2 km-long multimode link with a power budget (P_B) of 13 dB, the link loss (LL) is the sum of the following factors:

• Fiber attenuation for 2 km @ 1.0 dB/km= 2 dB
• Loss for five connectors @ 0.5 dB per connector = 5(0.5 dB) = 2.5 dB
• Loss for two splices @ 0.5 dB per splice =2(0.5 dB) = 1 dB
• Higher-order loss = 0.5 dB
• Clock recovery module = 1 dB

The power margin is as follows:

PM = PB - LL

PM = 13 dB - 2 km (1.0 dB/km) - 5 (0.5 dB) - 2 (0.5 dB) - 0.5 dB [HOL] - 1 dB [CRM] 

PM = 13 dB - 2 dB - 2.5 dB - 1 dB - 0.5 dB - 1 dB 

PM = 6 dB 

In the following sample calculation for an 8 km-long single-mode link with a power budget (P_B) of 13 dB, the link loss (LL) is the sum of fiber attenuation (8 km @ 0.5 dB/km, or 4 dB) and loss for seven connectors @ 0.5 dB per connector, or 3.5 dB:

PM = PB - LL

PM = 13 dB - 8 km (0.5 dB/km) - 7 (0.5 dB) 

PM = 13 dB - 4 dB - 3.5 dB 

PM = 5.5 dB 

In both examples, the calculated power margin is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.