Optical Communications Laboratory
FT221/4
Laboratory Project:
Power Budget Modelling for Optical Fibre Systems
Objective:
To develop and utilise a spreadsheet based loss budget model for a
singlemode optical fibre transmission link. The model is to be created in
Microsoft Excel. Singlemode fibre is assumed to allow long distance systems
to be modelled and to avoid the need to include modal dispersion.
Model Overview
It should be possible for the user to define the following system specifications:

Bit rate

Material dispersion

System span in km

Transmitter output power

Receiver sensitivity

Average inter-splice distance

Splice loss attentuation

Number of connectors

Connector attenuation.
The model should allow for either worst case or statistical analysis or a
combination of both to be carried out. This will mean for example that
connector attenuation will have to be specified with worst case, average and
standard deviation values as appropriate.
The model outputs should be:

Total fibre attenuation

Total splice attenuation

Total connector attenuation

Dispersion penalty

Received power

Power margin
Outputs must be identified as worst case or statistical as appropriate
After completion of the above extensions to the basic model may include:

Optical amplifiers

Optical splitter and other passive components

Other penalties and margins
Sample and Background:
A simple sample model, based on a worst case analysis is provided in
Appendix A.
Background material, including example power budget calculations, are
contained in Appendix B. Further information is contained in the associated
lecture notes.
Appendix A
Simple Sample Model
Table 1. below presents a simple power budget calculation for the link shown in Figure 1,
which derives the available power margin as an answer. In the columns A and B is the
available or given information, while in columns C and D is the derived or calculated
information.
A
B
C
D
Basic Information
Value
Derived Information
Value
System Span in km
70.00
Transmitter Output Power (dBm)
0.00
Number of Connectors
2.00
Connector Loss (dB)
0.20
Total Connector Loss (dB)
0.40
Fibre Attenuation
0.25
Total fibre attenuation (dB)
17.50
Maximum fibre length available (km)
0.90
No. of fibre lengths needed
77.78
Average loss per splice (dB)
0.08
Total splice loss (dB)
6.14
Receiver sensitivity (dBm)
-28.00
Available power margin (dB)
3.96
Table 1. Power budget calculation
Notice that a single length of fibre does not span the distance between the connectors at the
transmitter and receiver. Instead fixed lengths of fibre are used, joined by fusion splices. This
is taken into account in the model.
Transmitter
Receiver
Legend
Optical Fibre Connector
Optical Fibre Splice
Optical Fibre
Figure 1. Optical transmission system model
Using the Simple Sample Model
Using the spreadsheet model solve the following problems:
1. What is the maximum system span in km which can be catered for?
2. Assuming that a fibre repair margin of at least 2 dB is required, what is the maximum
system span?
3. If the maximum fibre length available is altered to 1.4 km, without altering any other
parameters, how does the power margin change?
4. Use the model to determine and plot the sensitivity of the power margin to changes in the
following parameters:





Fibre attenuation
Connector loss
Transmitter output power
Splice loss
System span
For example for fibre attenuation increase the attenuation from in 2% steps, noting the value
of the power margin each time, then plot the value of the power margin as a function of the
percentage increase in fibre attenuation.
Appendix B
Support notes on power budgeting
The performance of optical transmission systems
In an optical transmission system the distance between the transmitter and the receiver is
spanned by an optical fibre. If the optical fibre was a perfect transmission medium then both
the operating distance between the transmitter and receiver and the bit rate could be
arbitrarily large. In practice however optical fibre possesses inherent limitations that restrict
both the maximum distance and the maximum bit rate.
If the optical power available at the input to the receiver is too low then the error probability
will become unacceptable. This can be seen in figure 1 which plots the error probability
against the received optical power for a typical optical transmission system. In this figure as
the received optical power increases, for example due to a reduction in fibre attenuation, the
error probability gets smaller, which is an improvement.
Figure 1 Variation of error probability with received optical power
One of the major specifications laid down when an optical transmission system is under
development is the acceptable error probability at the planned bit rate. Based on this
specification and on a knowledge of the receiver it is possible to specify a minimum
acceptable received optical power, below which the error rate specification will not be met.
Given this figure for the minimum received optical power the system designer can then
estimate the tolerable reduction or attenuation in optical power caused by the fibre and other
components such as optical fibre connectors and fibre fusion splices. This process is called
power budgeting for an optical transmission system and is considered in the next section.
Power budgeting for optical transmission systems
The development of a power budget at the planning stage of an optical transmission system
is preceded by the specification of a number of important system parameters, which include:




The operating bit rate,
The acceptable error probability,
The distance over which the system is to operate,
The potential for expansion in the future, for example by an increase in the bit
rate.
Based on these parameters the system designer can begin to select the basic components
that make up the system. In some cases the designer may have to specify a new component
or sub-system that will then have to be designed and produced. During this process the
designer checks that the system will operate successfully by calculating a power budget for
the optical transmission system. The whole process is iterative and may involve the
comparison of a wide range of solutions using many power budget calculations.
As an example a simple power budget calculation will now be carried out based on the model
of an optical transmission system shown in figure. This may represent the entire system or a
link between regenerators. The fibre from the transmitter is joined by a connector to a length
of fibre, which in turn is joined to other lengths of fibre using fusion splices. At the end of the
link a connector joins the fibre to the fibre that enters the receiver.
Transmitter
Receiver
Legend
Optical Fibre Connector
Optical Fibre Splice
Optical Fibre
Figure 2 Optical transmission system model
Suppose the output power of the transmitter is +3 dBm and the minimum input power needed
in the receiver to maintain a specified error probability is -41 dBm. This means that a total
reduction in power of 44 dB can be tolerated between the transmitter and receiver. The
reduction is the power budget for the optical transmission system. The power budget is
calculated as:
BudgetdB
=
OutputdBm - Receiver input powerdBm
=
+ 3 dBm
=
44 dB
- ( - 41 dBm )
The power budget of 44 dB is partially used up over the link by attenuation in the optical fibre,
connectors and fusion splices. The remainder is used by a number of other factors, the most
common of which is the power margin. This power margin itself consists of a number of
factors, one of which is a cable repair margin. This is an attenuation value that is included at
the planning stage to allow for repairs to be carried out in the future by the addition of more
fusion splices.
As an example assume that a total distance of 55 km is to be covered and that the system
designer wishes to know the permissible fibre attenuation. If an attenuation of 0.5 dB is
allowed per connector and there are two connectors then the power budget is reduced to 43
dB. If a power margin of 10 dB is to be allowed for, the total loss permitted for the optical fibre
and fusion splices is 33 dB. If fibre is available in 1 km lengths then for the specified distance
55 lengths of fibre will need to be spliced together. This will involve 54 fusion splices, each of
which will have an average loss of 0.03 dB for singlemode fibre. The total loss caused by
fusion splices is therefore 1.6 dB. The total loss in the fibre is therefore 33 dB - 1.6 dB which
equals 31.4 dB. Since the total length of fibre is 55 km, the attenuation per km must be no
greater than 0.57 dB. The complete power budget calculation is shown in summary below.
Power BudgetdB
=
44 dB
Less:
Connector attenuation =
Power margin
=
Total splice loss
1 dB
10 dB
=
1.6 dB
---------
Therefore:
Total fibre loss allowed
=
31.4 dB
A power budget calculation can be used in a variety of ways, for example if the fibre
attenuation was known then a system designer could use a power budget calculation to
determine the maximum distance between the transmitter and receiver or between
regenerators. This distance is called the power-limited distance of the link.
In practice power budget calculations may become significantly more complex for two
reasons:
(i) A number of extra factors may be included in the power budget calculation. For example a
margin may be included to account for variations with temperature and age. The transmitter
output power for example may vary with temperature and decrease with time over a number
of years. To account for this a transmitter degradation margin is included, which is typically 1
dB. This appears in the power budget calculation as if it is a "real" attenuation, whereas in
practice when the system first becomes operational and assuming the temperature is correct
the received power would be 1 dB more than that predicted by the power budget calculation.
(ii) All the power level and attenuation values used in the power budget calculation above are
average values. When the optical transmission system is installed some variation in these
values will naturally take place. The system designer has little control over such variations
and therefore must resort to statistical techniques to ensure that the power budget calculation
is realistic. In a statistical power budget calculation the designer will not only use an average
value for a quantity, but also a value called the standard deviation , which is a statistical
measure of how actual values vary from the average value. How the standard deviation is
used depends on the quantity in question, however for attenuation values the system
designer is concerned with values that are above average. For example 84.13% of the values
are contained between zero and one standard deviation (1) above average, 97.73% within 2
 above average and 99.87% within 3 above average. In power budget calculations,
generally, the 2 value is considered the worst case value.
Ignoring the statistical nature of component performance by using worst case values, in every
case can create extremely over-conservative designs. If for example, in finding the total loss
caused by fusion splices, the worst case loss for a fusion splice is simply multiplied by the
number of splices involved, the result would be a figure for the total splice loss that would
virtually never occur in practice.