P
M
= P
B
– LL
P
M
= 13 dB – 2 km (1 dB/km) – 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB
P
M
= 13 dB – 2 dB – 2.5 dB – 1 dB – 0.5 dB
P
M
= 7 dB
The following sample calculation for an 8-km-long single-mode link with a power budget (P
B
) of 13 dB
uses the estimated values from Table 41 on page 150 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 power
margin (P
M
) is calculated as follows:
P
M
= P
B
– LL
P
M
= 13 dB – 8 km (0.5 dB/km) – 7(0.5 dB)
P
M
= 13 dB – 4 dB – 3.5 dB
P
M
= 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.
Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
IN THIS SECTION
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable | 151
Attenuation and Dispersion in Fiber-Optic Cable | 152
This topic describes signal loss, attenuation, and dispersion in fiber-optic cable.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
Multimode fiber is large enough in diameter to allow rays of light to reflect internally (bounce off the walls
of the fiber). Interfaces with multimode optics typically use LEDs as light sources. However, LEDs are not
coherent sources. They spray varying wavelengths of light into the multimode fiber, which reflects the
light at different angles. Light rays travel in jagged lines through a multimode fiber, causing signal dispersion.
When light traveling in the fiber core radiates into the fiber cladding, higher-order mode loss results.
151
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