Attenuation is being measured in dB/km, referring to the loss in signal strength or power of the light. Loss happens due the light traveling through the single-mode or multimode fiber.
There is a variety of wavelengths with which light is traveling. Original wavelength was 580 nm due to support of original LED and detector. As the technology was evolving, the 1310 nm region got its popularity. It allowed lower dispersion with power loss decrease. As the wavelength region is increasing – performance and costs are increasing too. Such as 1550 nm wavelength that is requiring no repeaters.
Size of the fiber affects the optical loss. The single mode is using 9/125 um while multimode – 62.5/125 um or 50/125 um. Wavelength is affecting fiber loss – higher wavelength is having lower loss while lower wavelength – higher loss.
Table 1. Wavelength 1310 nm.
Attenuation/ Km (dB/Km) [a]
Attenuation/optical connector (dB) [C]
Attenuation/joint (dB) [J]
Min
0.3
0.4
0.02
Best Conditions
Average
0.38
0.6
0.1
Normal
Max
0.5
1
0.2
Worst situation
Figure 1. Typical optical fiber scenario.
Lets calculate attenuation for scenario shown in Figure 1. We want to find total attenuation (TA) by this formula:
TA = n x C + c x J + L x a + M
Values:
n – number of optical connectors;
c – number of splices;
L – total length of optical cable;
M – system margin (~3 dB).
Values from Table 1:
C – attenuation for one optical connector (dB);
J – attenuation for one splice (dB);
a – attenuation for optical cable (dB/km);
For 1310 nm, Normal:
TA = n x C + c x J + L x a + M = 2 x 0.6dB + 4x 0.1dB + 20.5Km x 0.38dB/Km + 3dB = 12.39dB
For 1310nm, Worst Situation:
TA = n x C + c x J + L x a + M = 2 x 1dB + 4x 0.2dB + 20.5Km x 0.5dB/Km + 3dB = 16.05dB
If the optical card is providing power budget between 27 dB and 17 dB, our worst situation is giving value of 16.05 dB, making optical link able to work with no problem.
