1.3 Decibel Notation
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Table 1.3 Examples of
optical power levels and their
dBm equivalents
Power
dBm equivalent
200 mW
23
100 mW
20
10 mW
10
1 mW
0
100 μW
−10
10 μW
−20
1 μW
−30
100 nW
−40
10 nW
−50
1 nW
−60
100 pW
−70
10 pW
−80
1 pW
−90
Because the decibel is used to refer to ratios or relative units, it gives no indication
of the absolute power level. However, a derived unit can be used for this purpose. Such
a unit that is particularly common in optical fiber communications is the dBm (simply
pronounced dee bee em). This unit expresses the power level P as a logarithmic ratio
of P referred to 1 mW. In this case, the power in dBm is an absolute value defined by
Power level (in dBm) = 10 log
P(in mW)
1 mW
(1.5)
An important rule-of-thumb relationship to remember for optical fiber communications is 0 dBm = 1 mW. Therefore, positive values of dBm are greater than 1 mW
and negative values are less than 1 mW.
Example 1.5 Consider three different light sources having the following optical
output powers: 50, 1, and 50 mW. What are the power levels in dBm units?
Solution Using Eq. (1.5) to express the light levels in dBm units shows that the
output powers of these sources are −13 dBm, 0 dBm, and +17 dBm, respectively.
Example 1.6 Consider a product data sheet for a photodetector that states that an
optical power level of −32 dBm is needed at the photodetector to satisfy a specific
performance requirement. What is the power level in nW (nanowatt) units?
Solution Equation (1.5) shows that −32 dBm corresponds to a power in nW of
P = 10
−32/10 mW = 0.631 μW = 631 nW
Table 1.3 lists some examples of optical power levels and their dBm equivalents.
13
Table 1.3 Examples of
optical power levels and their
dBm equivalents
Power
dBm equivalent
200 mW
23
100 mW
20
10 mW
10
1 mW
0
100 μW
−10
10 μW
−20
1 μW
−30
100 nW
−40
10 nW
−50
1 nW
−60
100 pW
−70
10 pW
−80
1 pW
−90
Because the decibel is used to refer to ratios or relative units, it gives no indication
of the absolute power level. However, a derived unit can be used for this purpose. Such
a unit that is particularly common in optical fiber communications is the dBm (simply
pronounced dee bee em). This unit expresses the power level P as a logarithmic ratio
of P referred to 1 mW. In this case, the power in dBm is an absolute value defined by
Power level (in dBm) = 10 log
P(in mW)
1 mW
(1.5)
An important rule-of-thumb relationship to remember for optical fiber communications is 0 dBm = 1 mW. Therefore, positive values of dBm are greater than 1 mW
and negative values are less than 1 mW.
Example 1.5 Consider three different light sources having the following optical
output powers: 50, 1, and 50 mW. What are the power levels in dBm units?
Solution Using Eq. (1.5) to express the light levels in dBm units shows that the
output powers of these sources are −13 dBm, 0 dBm, and +17 dBm, respectively.
Example 1.6 Consider a product data sheet for a photodetector that states that an
optical power level of −32 dBm is needed at the photodetector to satisfy a specific
performance requirement. What is the power level in nW (nanowatt) units?
Solution Equation (1.5) shows that −32 dBm corresponds to a power in nW of
P = 10
−32/10 mW = 0.631 μW = 631 nW
Table 1.3 lists some examples of optical power levels and their dBm equivalents.