FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

MAXIMUM ALLOWABLE RISETIME = 0.7/RN~ = 100 nS FIBEROPTIC LINR OPTICAL POWER BUDGET
(Fig. 6.16)
RISETIME BUD(ZT MARGIN = 7fI ns
LINK DESCRIPTION
The 74 ns margin between the maximum allowable
risetime based on the required signaling rate and
Link length, L
1.5km
Data Rate, R
7.0 Mblsec
the system risetime for all the components can be used
Operating wavelength
0.830 nm
in many ways. For example, the cable could be made
longer, the signaling rate could be increased, or a
cheaper cable with a greater material or modal dispersion
per unit length could be used. However, other
budgets, such as the power budget, may place limits on
theae changes. Perhaps the one single fact to remember
in regard to risetime is that it represents reciprocal
dollars, i.e., the longer the risetime, the cheaper
the component. Therefore, except for a power safety
margin, all allowable risetime should be used up for
the performance requirement in a given application.
6.2.3.2 Optical Power Budget Analysis
The optical power requirement of a given
fiberoptic telemetry system is also a matter of distribution
of power over a serial or a parallel set of
fiberoptic channels. For example, if there are star
couplers, there must be sufficient input power to satisfy
the input power requirement for each parallel output
channel. Serially connected components, such as
cables, splices, and couplers, will each have an inser -
tion loss. Repeaters will insert a gain. The losses
and gains are added to obtain the source-to-receiver
overall loss. The optical power dissipation that can
be allowed is the difference between the sum of the
transmitter and repeater optical power outputs and the
required receiver (photodetector) optical power input
in order to keep the bit error rate (BER) below, or
signal-to-noise ratio (S/N) above, a specified value.
A typical allowable optical power loss between transmitter
and receiver for various data rates at an allowable
BER of 10-9 is shown in Fig. 6.18. Normally one
TRANSMITTER POWER, pT
Light source type
Average source optical power
Fiber coupling loss
Average launched optical power,
REQUIRED RECEIVER POWRR, pREQ
Detector type
Required bit error rate, BER
Receiver bandwidth
Receiver sensitivity
Link margin
‘REQ
ALLOWABLE LINK LOSS OVER MARGIN, PAL
pm = PT - PmQ = -14 -(-48) = 34 dB
LINR LOSS COMPUTATION
LED 0.1 mW
-10 dBm
-4 dB
-14 dBm
PIN-FET
1(3-9
50 MHz
-55 dBm
+ 7 dB
-48 dllm
Contributor Unit Quantity Total
Recr coupling 3.0 dB 1 3.0 dB
Connectors 2.0 2 4.0
Splices 0.5 4 2.0
Cable 5.0 dB/km 1.5 km 7.5
Splitters
Margin 2.0 dB 2.0
Total link loss
18.5 dB
REMAINING POWER MARGIN
(Allowable - Total link loss) = 34- 18.5
= 15.5 dB
=
II
g
n
Fig. 6.18
o
-lo
-20
-30
-40
-50
-60
-70
t
LASER-LAUNCHED
POWER RANGE
LED-LAUNCHED
--..POWER RANGE _______ -14
I
-80
-901 1 1 1
1 10 100 1000
DATA RATE (Mb/s)
BER =10-9
optical power loss versus data rates for
the fiberoptic data link used in the optical
power budget analysis in Section 6.1.3.
would not use up all the available power, but maintain
a safety margin of a few dB to insure satisfactory performance.
The allowable loss is then distributed over
the fiberoptic link components as shown in Fig. 6.16
and given in the following example:
Since a link margin of 7 dB and a link loss
computation margin of 2 dB have already been allowed,
the entire 34 dB may be used up. The loss in the link
of the given design is 18.5 dB. Therefore, there is an
excess total available optical power margin of 15.5 dB.
It may be cheaper to use up this excess margin by
selecting cheaper cable components with greater losses
or it may be cheaper to choose a lower-power source.
Link power is like dollars, it is best to get by with
the least power, low power sources being cheaper. However,
link loss is more like reciprocal dollars, the
greater the power loss, the cheaper the components.
Therefore, there is a trade-off to be made in order to
minimize the overall cost of a fiberoptic link.
6.2.3.3 Cost Budget Analysis
The cost of a fiberoptic telemetry system
will be based on judicious maximal use of available
risetime and minimal use of power as well as many other
factors. Fiberoptic cable designs, repeater spacing,
multiplexing schemes, allowable BERs, cable routings,
the use of overhead versus subterranean cables, availability
of components, simplicity of design and fabrication,
and ease of maintenance, are just a few of the
many factors that will enter into the overall cost of
the system.
6.2.4 Fiberoptic Telemetry System Specific
Configurations
An example of a set of the final design para-
6-9