Measurement Units: "dB"
and "dBm"
Whenever tests are performed on fiber optic networks, the
results are displayed on a meter readout in “dB.” Optical
loss is measured in “dB” which is a relative measurement,
while optical power is measured in “dBm,” which is
relative to 1mw optical power Loss is a negative number
(like –3.2 dB) while power measurements can be either
positive (greater than the reference) or negative (less
than the reference.) Should a meter be set for dB or dBm?
Confused? Many fiber optic techs are too. Let’s see if we
can clear up some of the confusion.
Measuring Power
When we make fiber optic measurements, we are measuring
the power in the light coming out of the end of a fiber.
The primary calibration standards we use for power
measurements, maintained by NIST (the US National
Institute of Standards and Technology,) are actually
determined by the heating effect of the power of the light
as it is absorbed in a detector. Every fiber optic power
meter sold is calibrated traceable to the NIST standard in
the US or similar primary standards worldwide so different
meters should measure the same power, within the limits of
calibration uncertainty.
Optical power in fiber optics is similar to the heating
power of a light bulb, just at much lower power levels.
While a light bulb may put out 100 watts, most fiber optic
sources are in the milliwatt range (0.001 watts), so you
won’t feel the power coming out of a fiber and it’s
generally not harmful. (Except for DWDM systems with fiber
amplifiers or lasers used for surgery or welding. DWDM
systems with fiber amplifiers can carry enough power to
pit the fiber end or even ignite cables if a fiber in
cracked!) Remember you can't see this light either because
it is in the infrared wavelengths beyond the sensitivity
of the human eye.
Read more on optical
power measurements.
Understanding dB
In the very early days of fiber optics, source output
power was usually measured in milliwatts and loss was
calculated in dB or deciBels. Over the years, all
measurements migrated to dB for convenience. This was when
the confusion began.
Loss measurements were generally measured in dB since dB
is a ratio of two power levels, one of which is considered
the reference value  that's "0 dB" for loss measurements.
dB is a logarithmic scale (remember “logs” from high
school math?) where each 10 dB represents a ratio of 10
times.
The actual equation used to calculate dB when the power is
measured in watts is:
dB = 10 log (measured power / reference power).
Using this equation, 10 dB is a ratio of 10 times (either
10 times as much or onetenth as much), 20 dB is a ratio
of 100, 30 dB is a ratio of 1000, etc. When the two
optical powers compared are equal, dB = 0, a result of the
log scale used in dB but a convenient value that’s easily
remembered.
In most fiber optic power meters, the readings are in dB,
not watts, so the measurement of dB is expressed more
simply  no logs:
dB = measured power(dB)  reference power (dB)
The
table below shows the ratio of power for dB differences
in power:
dB
(gain)

Power
ratio

dB
(loss)

Power
ratio

0

1.000

0

1.000

0.1

1.023

0.1

0.977

0.2

1.047

0.2

0.955

0.3

1.072

0.3

0.933

0.4

1.096

0.4

0.912

0.5

1.122

0.5

0.891

0.6

1.148

0.6

0.871

0.7

1.175

0.7

0.851

0.8

1.202

0.8

0.832

0.9

1.230

0.9

0.813

1

1.259

1

0.794

2

1.585

2

0.631

3

1.995

3

0.501

4

2.512

4

0.398

5

3.162

5

0.316

6

3.981

6

0.251

7

5.012

7

0.200

8

6.310

8

0.158

9

7.943

9

0.126

10

10

10

0.1

20

100

20

0.01

30

1000

30

0.001

40

10000

40

0.0001

50

100000

50

0.00001

60

1000000

60

0.000001

Compare
the positive and negative dB across the rows. The ratio
of the positive dB is the inverse of the negative dB,
e.g. +10dB is a ratio of 10 times and 10 dB is a ratio
of 1/10 or 0.1. Thus 10 dB is a ratio of 10 times: +10
dB means the power measured is 10 times greater than the
reference power and 10 dB is onetenth as much. Some of
the numbers are easy to remember and may be useful. For
example, +3 dB is a factor of two in power and 3 dB is
a factor of onehalf.
When
the two optical powers compared are equal, dB = 0, a
convenient value that is easily remembered. If the
measured power is higher than the reference power, dB
will be a positive number, but if it is lower than the
reference power, it will be negative. Thus measurements
of loss are expressed as negative numbers.
Here
is an Excel spreadsheet that calculates dB/power ratio
and dBm/milliwatts.
Measuring Power
Measurements of optical power are expressed in units
of dBm. The “m” in dBm refers to the reference power
which is 1 milliwatt. Thus a source with a power level
of 0 dBm has a power of 1 milliwatt. Likewise, 10 dBm
is 0.1 milliwatt and +10 dBm is 10 milliwatts. Fiber
optic sources may vary from 20dBm to +20dBm and
receiver power may go as low as 40dBm.
dBm
= 10 log (measured power / 1mw)
When the power measured is 1mw, the equation
becomes:
dBm
= 10 log (1mw / 1mw) = 10 log (1) = 0 dBm
or
dBm
= measured power(dB)  reference power
(0dB) = dB
= measured power(dB)  0
If the power
is greater than 1mw, say 2mw, the equation
becomes:
dBm
= 10 log (2mw/ 1mw) = 10 log (2) =
+3dBm (rounded off a little)
If
the power is less than
1mw, say 0.5mw, the
equation becomes:
dBm
= 10 log
(0.5mw/ 1mw) =
10 log (0.5) =
3dBm (rounded
off a little)
That's
not hard to
remember.
Positive dBm
means power
greater than
1mw and
negative means
less than 1mw.
A good laser
source for a
singlemode
link will have
a power output
of ~ +3 to +6
dBm  24mw 
coupled into
the fiber. A
VCSEL for
multimode
links should
have a power
around 0dBm 
1mw. And a
LED, used in
older
multimode
links, has a
typical power
of 10 dBm 
0.1mw or
100microwatts.
Measuring
Loss
If we have loss in a fiber optic system, the measured
power is less than the reference power, so the ratio of
measured power to reference power is less than 1 and the
log is negative, making dB a negative number. When we set
the reference value, the meter reads “0 dB” because the
reference value we set and the value the meter is
measuring is the same. Then when we measure loss, the
power measured is less, so the meter will read “ – 3.0 dB”
for example, if the tested power is half the reference
value. Although meters measure a negative number for loss,
convention has us saying the loss is a positive number, so
we say the loss is 3.0 dB when the meter reads – 3.0
dB.
Here is a short movie of what happens when we induce loss
in a cable by stressing it and watch the display of a
power meter. We start at 20.0dBm and after stress is
added to the cable to cause loss, the power level goes
down to 22.3dBm, showing our stress on the cable caused
2.3dB loss.
Here is the math of calculating this loss:
dB
= measured power(dB)  reference power (dB) = 22.3
dBm (20dBm) = 22.3 + 20 = 2.3 dB (remember
that subtracting a negative number has two minuses with
becomes a +.)
Look at this animated simulation of a
laser/singlemode system with 1mw power from the source
and watch the meter reading.
Watch carefully as the transmitter couples a
signal into the fiber. As the signal pulse travels down
the fiber, it is attenuated by the fiber, suffers more
loss in the connection, then is attenuated more until it
reaches the receiver. See how the power in the signal
decreases as it travels down the fiber, becoming more
negative when measured in dBm.
Note: If you are used to making measurements of loss
with a light source and power meter, you are used to
loss being a negative number. But some manufacturers
of optical loss test sets, which include a source and
meter, show dB loss as a positive number. They were
probably confused by the fact that everybody says "the
loss is X dB" not "the loss is X dB. And they never
looked at the math. Or learned math.
PowerMeasuring Instruments
Instruments that measure in dB can be either optical power
meters or optical loss test sets (OLTS). The optical power
meter usually reads in dBm for power measurements or dB
with respect to a userset reference value for loss. While
most power meters have ranges of +3 to –50 dBm, most
sources are in the range of 0 to –10 dBm for lasers and
–10 to –20 dBm for LEDs. Only lasers used in CATV or
longhaul telephone systems with fiber amplifiers have
powers high enough to be really dangerous, up to +20 dBm –
that’s 100 milliwatts or a tenth of a watt!
The OLTS or the power meter on the dB scale measures
relative power or loss with respect to the reference level
set by the user. The range they measure will be determined
by the output power of the source in the unit and the
sensitivity of the detector. For multimode fiber, an OLTS
using a LED source will usually measure over a range of
030 dB, more than adequate for most multimode cable
plants which are under 10 dB loss. Singlemode networks use
lasers and may have loss ranges of up to 3040 dB for
longhaul telecom systems, but campus cabling using
singlemode may only have 13 dB loss. Thus a singlemode
OLTS may be different for short and long systems.
Read
more on fiber optic instruments.
Conclusion
If you remember that dB is for measuring loss, dBm is for
measuring power and the more negative a number is, the
higher the loss, it’s hard to go wrong. Set your zero
before measuring loss and check it occasionally while
making measurements.
Here is an Excel spreadsheet
that calculates dB/power ratio and dBm/milliwatts.
More
on calibration
and metrology
(the science of measurements) in fiber optics.
More
pages of information on fiber optic measurements
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