UNIT 4 APPENDICES
Your FiberMeter meter has a decibel(dB) range that spans 75dB. The whole purpose of your meter is to measure light
energy in decibels. But what exactly is a decibel? It is not uncommon for a technician to be using an optical power meter
and not understand what a decibel really is. If you struggle a little with this review, you will find the concept not so
threatening.
This is what happens when we test a link: we send light of a specified wavelength from a stable light source down the fiber
optic cabling. The meter then captures this light and measures it. As light travels down this path, however, its photons
encounter impurities, connector gaps, and even the glass atoms themselves. All of these factors cause attenuation in the
fiber. If there is too much attenuation, your fiber optic link will fail, because it does not have enough power to trigger the
circuitry in the active receiver equipment. No communications can occur in this scenario.
Simply put, a decibel is a measure of the ratio of the power received at the end of a link relative to the power you started
with, which is known. This fraction calculates the relative power. This calculation is similar to test scores or percentages.
If test results were kept in percent or fractions, the average person wouldn’t have difficulty in grasping the concept of
decibels. Unfortunately, in scientific measurement, graphing power measurements has become too cumbersome due to
their wide range of possible values; a person would need to paste lots of sheets of graph paper together to plot these power
functions by hand. So scientists came up with a little trick to “squeeze” the graph down to a more presentable size. The
trick uses logarithms (logs). Logs are fairly simple -- most of us have already encountered logs in pre-algebra class. They
didn’t seem so important then, but they are important to the understanding of decibels. Logs expressed in Base 10 are just
a way to state how many zeros there are in a number.
For example, log(10)=1, log(100)=2, log(1000)=3. Notice the pattern. Logs simply express how many places there
are after the first number. Let’s work with a number that isn’t 10, 100, or 1000. For instance the log(2014)=3.304.
Notice that 2014 has three digits beyond the first, thus the “3” in the 3.304. Also notice how it relates to the “3” in
log(1000)=3. Remember, logs are a tool we use to “squeeze” our measurements down so they will fit on graph paper.
Power is measured in Watts (W), therefore:
Mathematicians and scientists must have noticed that the shrinking effect of the graph was a little excessive. Logs in optical
decibels get multiplied by 10. Thus we finally have:
Remember: Decibels are simply the ratio of the power received by the receiver over the power sent out by the light
source at the start of link.
=
x100
Relative power out as a %
Amount of optical power left over at the end of the link
Amount of optical power at the beginning of the link
Appendix A - Decibel Review
RELATIVE POWER IN dB
END OF LINK
BEGINNING OF LINK
=
10 LOG(
W
W
)
RELATIVE POWER AS
A POWER OF 10
END OF LINK
BEGINNING OF LINK
=
LOG(
W
W
)
=
RELATIVE POWER AS
A POWER OF 10
LOG(
)
Amount of optical power left over at the end of the link
Amount of optical power at the beginning of the link
4-1
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