27/02/02 dB and dBm overview for Fibre Systems.prz
Dublin Institute of Technology
School of Electronic and
Communications Engineering
Optical Communications Systems
The Decibel
Dr. Gerald Farrell
Unauthorised usage or reproduction strictly prohibited
Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
27/02/02 dB and dBm overview for Fibre Systems.prz
Decibels and Attenuation
Decibels allows us to relate the power at two points, for example:
At the input and output of a complete link
Over a span of fibre
Across a connector
Pin
Pout
Fibre or other component
Attenuation in dB = -10 Log 10
Pout
Pin
By convention the attenuation in a fibre or other optical component is
specified as a positive figure
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Decibel Formulae
Basic decibel power equation relates two absolute powers P1 and P2:
Power ratio in dB = 10 Log [P1/P2]
10
In a fibre or other component with an input power Pin and an output power
Pout the loss is given by:
Loss in dB = 10 Log10[Pout/Pin]
By convention the attenuation in a fibre or other optical component is
specified as a positive figure, so that the above formula becomes:
Attenuation in dB = -10 Log10[Pout/Pin]
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Sample Attenuation Calculation
For the fibre span shown the input power is 2 mW , while the output
power is 250 µ W. What is the total attenuation over the fibre span?
If the fibre span length is 38 km what is the fibre attenuation per km
(db/km)
2 mW
250 µW
Fibre
Attenuation in dB = -10 Log 10
Pout
Pin
Attenuation in dB = -10 Log 10
250 µW
2 mW
Attenuation in dB = -10 Log 10(0.125) = 9.03 dB
or
0.24 dB/km
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Attenuation Exercises #1
For the fibre spans shown below find:
1. the total attenuation over the fibre span and
2. fibre attenuation per km (db/km)
1.1 mW
145 µW
510 µW
45 km Fibre
61 km Fibre
10 km Fibre
90 µW
1.1 µW
250 µW
Answers:
1.
2.
Answers:
1.
2.
Answers:
1.
2.
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Answers to Attenuation
Exercises #1
For the fibre spans shown below find:
1. the total attenuation over the fibre span and
2. fibre attenuation per km (db/km)
1.1 mW
145 µW
510 µW
45 km Fibre
61 km Fibre
10 km Fibre
90 µW
1.1 µW
250 µW
Answers:
1. 10.87 dB
2. 0.24 dB/km
Answers:
1. 21.2 dB
2. 0.35 dB/km
Answers:
1. 3.09 dB
2. 0.309 dB/km
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Summing Attenuation
Three fibre spans are joined by a splice as shown below. What is the
total attenuation in dB if the splice loss is 0.05 dB per splice?
Fibre A 0.55 dB
Fibre B 0.76 dB
Fibre A 0.25 dB
Total Attenuation in dB = (the total fibre attenuation) + (total splice attenuation)
Total Attenuation in dB = (0.55 dB + 0.76 dB + 0.25 dB) + (0.05 dB + 0.05 dB)
Total Attenuation in dB = (1.56 dB) + (0.1 dB)
Total Attenuation in dB = 1.66 dB
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Absolute power in Decibels
It is very useful to be able to specify in dB an absolute power in watts or
mW.
To do this the power P2 in the dB formula is fixed at some agreed reference
value, so the dB value always relates to this reference power level.
Allows for the easy calculation of power at any point in a system
Where the reference power is 1 mW the power in an optical signal with a power level P
is given in dBm as:
Power in dBm = 10 Log 10[P/1mW ]
Where the reference power is 1 µW the power in an optical signal with a power
level P is given in dBµ as:
Power in dBµ = 10 Log [P/1µW]
10
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
Watts to dBm Conversion Table
Power (watts)
Power (dBm)
1W
100 mW
10 mW
5 mW
2 mW
1 mW
500 µW
200 µW
100 µW
50 µW
10 µW
5 µW
1 µW
500 nW
100 nW
+30 dBm
+20 dBm
+10 dBm
+7 dBm
+3 dBm
0 dBm
-3 dBm
- 7 dBm
-10 dBm
-13 dBm
-20 dBm
-23 dBm
-30 dBm
-33 dBm
-40 dBm
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
dBm Calculation (Transmitter)
A transmitter laser has a measured output power of 2.3 mW. What is the
laser diode output power expressed in dBm?
Transmitter Laser
2.3 mW
Power
Power in dBm = 10 Log 10 1 mW
2.3 mW
Power in dBm = 10 Log 10 1 mW
Power in dBm = 10 Log 10(2.3) = + 3.61 dBm
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2
27/02/02 dB and dBm overview for Fibre Systems.prz
dB and dBm Calculations
Combined
dB and dBm can be combined in the same calculation
As shown a fibre span ( including splices etc.) has a total attenuation of 13 dB.
If the transmitter output power is +2 dBm what is the receiver input power in dBm?
+2 dBm
Transmitter
? dBm
Fibre span: attenuation 13 dB
Receiver
Receiver input in dBm = Transmitter output power - Total fibre span attenuation
Receiver input in dBm = +2 dBm - 13 dB
Receiver input in dBm = -11 dBm
Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Source: Master 2_2