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 MIL-HDBK-1012/1
the present state of the art, analytical expressions for signal distortion in
optical sources are not available; but even though distortion effects can not
be considered in minute detail, they can be measured and specified
independently. SNR and bandwidth analyses can be performed. For digital
systems, the design goals are specified in terms of Bit Error Rate (BER) and
bandwidth. For both types of systems, calculations that pertain to the
overall SNR or BER performance can be performed. To summarize the
interactions between modules, a system link budget should be prepared.
h) Link budget. To determine whether or not the system has
adequate optical power to meet the design goals for SNR or BER, the engineer
can use a link budget. The link budget starts with the total source power.
All of the system losses (the input coupling losses) are listed, along with
the received optical power. The difference between the losses and the
received power required to meet the SNR or BER design goals is the power
margin available for system degradation. A sample link budget is shown below:
LINK BUDGET
dBm
Available Power Source
mW
dBm
Input Coupling Loss
Fiber Loss
dBm
Splice Loss
dBm
Connector Loss
dBm
Output Coupling Loss
dBm
dBm
Receiver Power
mW
Target BER
Required Receiver Power
mW
Margin
i) System bandwidth calculation. The designer must be aware of
the overall system bandwidth to ensure that his design goal has been met.
This is particularly true in analog systems. The main concern is that
selection of the wrong fiber can lead to bandwidth limitations. (It is
assumed that bandwidth concerns have been accounted for in the transmitter and
receiver designs.) The calculation can be performed by first taking the
square root of the sum of the squares of the individual module rise times:
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