1. Do you use dB or dBm for final fiber optics link test
using power meter & light source?
When to use dB and when to use dBm? Most testers get confused
between dB and dBm.
It’s mathematics and logarithmic unit (remember the electronics, university and
the school) dB vs. dBm This article appeared in the November 2002 issue of In
Tech Magazine Decibel (dB) and dB relative to a milliwatt (dBm) represent two
different but related concepts.
dBm is an absolute power value in milliWatts, it’s simply expressed on a
logarithmic scale. We use logs in FO power and loss measurements for a host
of reasons. dB is a relative measurement, actually a tenth of a Bel ( name after
Alexander Graham ) with each Bel being one power of ten.
As dBm (W) is absolute and dB is relative, the following applies. Laser output,
dBm. Received power level, dBm. Receiver sensitivity (a range), max dBm to
min dBm. Loss of a passive system (or gain of an active system), dB. It is
simply the measurement of dBm IN minus dBm OUT, and you get dB. Pin/Pout
= loss ratio. In logs, division is subtraction, so dBm IN minus dBm OUT = dB
loss. dB is simply a ration expressed on a log scale.
dBm - dBm = dB
dBm - ( or + ) dB = dBm
Learning aid: Thing of dBm as “ feet above sea level” and dB as “feet”. If you
fall from 4500 feet ASL to 2500 feet ASL, you fell 2000 feet. If you fall 2000 feet
ASL to Sea Level, you fell 2000 feet.
1. dBm - 5 dB loss = -12 dBm
1. dBm - 5 dB loss = -18 dBm
Sometimes we like to express loss ( diff in power ) simply, so we simple set an
arbitrary “ 0dB “ (relative) to our laser output power, and measure loss in dB
directly from that value. So we define out position (600 feet ASL) as ‘ground
level or 0 AGL. If we fly a kite at 100 feet above us, it’s 100 feet AGL or 700 feet
ASL.
When you deal with instruments, a light source and power meter will measure
loss as the difference between two power levels, the “0 dB reference” and the
actual loss test value. Most power meters have a “0 dB” reference set - a
button that will zero the meter at any value in the dB range so you can read
loss directly. Since the test power will be lower than the reference power (it’s a

2. loss), a power meter will measure loss as a negative number on this test.
Some manufacturers of optical loss test sets (some of whom came into the
fiber optic instrument game recently) make the OLTS read loss as a positive
number, highly confusing. Transmitter and receiver power are measured in
dBm. The loss of the cable plant is in dB.
The power budget loss of cable plant is calculated in dB, but we use dBm
when measured the cable plant loss using power meter and light source.
Therefore to relate the calculated and measure values, we deduct measured
values in dBm and will get the unit back to dB. For example, if the optical
power injected was -22 dBm and the power received at the other end -23 dBm,
then the optical loss of the link would be -22 - (-23) = 1 dB. The calculated
value in dB gives positive number and measured value in dBm gives negative
number.
In general dBm and dB are confusing for many technicians. In fact the answer
is easy. dBm is absolute power measurement and dB is used for calculation
matters only. Also dB is used when you want to know the loss of a link in a
network. When doing so you tell your powermeter, that its result in dBm should
be used as a zero reference. Now put the link in between the both patch cords
still on the source and the power meter and you can read the loss in dB. This is
only to make things easy because if you change to dBm mode again you will
find a result in dBm. If you deduct this value from the original value you will
have the result in dB again. Most confusion comes up when an equipment guy
and a passive guy start talking. The second one thinks in dB, because he has
to guarantee a certain loss in the link. The equipment guy only thinks in dBm
because his equipment has a certain output in dBm and a certain sensitivity in

3. dBm. His power budget is the difference between the output in dBm and the
sensitivity in dBm minus 4 dB(depends on how much reserve you need). So if
the output is 0 dBm and the sensitivity minus 10 dBm he can affort to loose 10
dB minus 4 dB is 6 dB. When measuring with a power meter on the end of the
link the reading should be -6dBm and not -6dB, because you don't know what
is the reference. Here the confusion starts between the two guys, because
they don't think alike. So to make things easy: always talk and measure dBm
for trouble shooting in networks and dB for testing the passive link only, after
you have been able to set a proper reference. If not also use dBm and do the
math yourselves afterwards.
dB is a percentage value and is used for three types of measurements in fiber
optics.
1. Attenuation (power loss) - such as fiber/cable, connectors, splices, splitters
and spans.
1. Gain - optical amplifiers/amplification.
1. Reflectance - This could be optical return loss (ORL) or component
reflectance such as a SC/UPC or APC connector value.
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