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# T320 Calculating Power Margin for Fiber-Optic Cable

After calculating a link's optical power budget you can calculate the optical power margin (PM), which represents the amount of power available after subtracting attenuation or link loss (LL) from the optical power budget (PB). A worst-case estimate of PM assumes maximum LL:

PM = PB – LL

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

Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion, connectors, splices, and fiber attenuation. Table 1 lists an estimated amount of loss for the factors used in the following sample calculations. For information about the actual amount of signal loss caused by equipment and other factors, refer to vendor documentation.

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

The following example uses the estimated values in Table 1 to calculate link loss (LL) for a 2 km-long multimode link with an optical power budget (PB) of 13 dB:

• 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 (PM) is calculated 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

The following sample calculation for an 8 km-long single-mode link with an optical power budget (PB) of 13 dB uses the estimated values from Table 1 to calculate link loss (LL) as 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). The optical power margin (PM) is calculated as follows:

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

#### Published: 2012-07-10 