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The following sample calculation for an 8-km-long single-mode link with a power budget (P
uses the estimated values from
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
P
= P
– LL
M
B
P
= 13 dB – 8 km (0.5 dB/km) – 7(0.5 dB)
M
P
= 13 dB – 4 dB – 3.5 dB
M
P
= 5.5 dB
M
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.
Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
IN THIS SECTION
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable | 53
Attenuation and Dispersion in Fiber-Optic Cable | 54
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. Together these factors limit the transmission distance of multimode fiber compared
with single-mode fiber.
Single-mode fiber is so small in diameter that rays of light can reflect internally through one layer only.
Interfaces with single-mode optics use lasers as light sources. Lasers generate a single wavelength of
light, which travels in a straight line through the single-mode fiber. Compared with multimode fiber,
single-mode fiber has higher bandwidth and can carry signals for longer distances.
Table 25 on page
52. This example calculates link loss (LL) as the sum of
) is calculated as follows:
M
53
) of 13 dB
B
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