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LOGARITHMS/DB MEASUREMENT UNITS
Some instruments have a display that responds in a logarithmic scale, where the
measurement is related numerically to the base 10 logarithm (Log) of the change in optical
power. For example, a radiometer has an input power of 1 mW (a reference level in the
fi ber optic industry), and the display is adjusted to indicate zero. If the input power is now
increased by a factor of 10 to 10 mW, the Log of 10 mW divided by 1 mW is equal to 1.
This ratiometric change of 10 is referred to as 1 Bel (abbreviated B). In practice, a smaller
fraction of a Bel is most used: 1/10 of a Bel or decibel (abbreviated dB), where one Bel
is equal to 10 decibels. Therefore, in the example above, the display would indicate a 10
dB change. If the input power was raised to 100 mW, the display would indicate a 20 dB
change. Note that all these readings are relative, since one power level is ratioed to another
with no indication of the absolute power in the reading.
You can see that the dB scale shows
large power variations with small
changes in the displayed reading. An
example where the decibel scale is
used is in the measurement of fi ber
attenuation, where the standard
unit is dB/km (decibels of loss-per-
kilometer length of fi ber).
An important variation on the dB
scale was hinted at earlier. 1 mW is
a standard power level reference in
fi ber optic and other industries, and
is displayed as a reading of 0 dBm
(recall that the Log of 1 is zero).
Note that the small “m” following
the dB unit refers to the 1 mW
power reference. Optical power
measurements can be compared as
before, but now the result shows
the absolute power. For example, 10
dBm equals an absolute power of 10
mW, or 10 dB more than the 1 mW
reference.
Table 4. Conversion from dBm
to radiometric power
dBm
Optical Power
+20 dBm
100 mW
+13 dBm
20 mW
+10 dBm
10 mW
+7 dBm
5 mW
+3 dBm
2 mW
0 dBm
1 mW
-3 dBm
500
μ
W
-10 dBm
100
μ
W
-13 dBm
50
μ
W
-20 dBm
10
Μ
W
-23 dBm
5
μ
W
-30 dbm
1
μ
W
