AN IMPORTANT UNIT IN FIBER OPTICS (THE DECIBEL)
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10 The “DECIBEL” Section is reproduced by permission TECHNICIAN’S GUIDE TO FIBER OPTICS 2E
By Donald J Sterling, Jr. - DELMAR PUBLISHERS, INC., Albany, New York, Copyright 1993
The decibel is an important unit that you will use
continually in fiber optics as well as in electronics. It
is used to express gain or loss in a system or
component. A transistor, for example, can amplify a
signal, making it stronger by increasing its voltage,
current or power. This is called gain. Similarly, loss
is a decrease in voltage, current, or power. The
basic equations for the decibel are:
dB = 20 log
10
(V
1
/V
2
)
dB = 20 log
10
(I
1
/I
2
)
dB = 10 log
10
(P
1
/P
2
)
Where V is voltage, I is current, and P is power. The
decibel then is the ratio of two voltages, currents, or
powers.
Notice that voltage and current are 20
times the logarithmic ratio, and power is 10 times
the ratio.
The basic use of the decibel is to compare the
power entering the system, circuit, or component to
the power leaving it. In fiber optics, we deal mostly
with loss and optical power.
The source emits
optical power. As light travels through the fiber to
the receiver, it loses power.
This power loss is
expressed in decibels. For example, if the source
emits 1,000 microwatts (
µ
W) of power and the
detector receives 20
µ
W, the loss through the
system is about 17dB.
Loss = 10 log
10
(P
r
/P
tr
)
= 10 log
10
(20/1,000)
= -16.989 dB
Where P
tr
is the power transmitted from the source
and P
r
is power received by the receiver. A 10dB
loss represents a loss of 90% of the power; only
10% remains. A useful figure to remember is 3dB,
which represents a loss of one half of the power.
Fiber optic links easily tolerate losses of 30dB,
meaning that 99.9% of the power from the source is
lost before it reaches the detector. If the source
emits 1,000
µ
W of power, only 1
µ
W reaches the
detector. In fiber optics, it is common to omit the
negative sign.
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