Chapter 3 Communication with Optics
ERRATA
1. Add the following missing reference to Chapter 3.
Ref. 3.7: Dennis Derickson Ed., Fiber Optic Test and Measurement, Prentice Hall PTR, New
Jersey, 1998.
2. Use the following updated figures and figure captions to replace the corresponding
original figures.
Fig. 3.7. Fiber Preform OVD fabrication method.
Fig. 3.19 Illustration of 1 Tbps WDM optical networks.
Fig. 3.20 An illustration of output spectrum of 1 Tbps WDM optical networks.
Fig. 3.21. An illustration of basic process for a digital fiber-optic communication link
Fig. 3.22. A schematic diagram of bit error rate measurements and functional test.
3. Update the following text
Original text in Italian “To ensure the good performance, it is very important to test the
performance of the optics networks. The most important parameter of a digital system is the rate
at which errors occur in the system. A common evaluation is the bit error ratio (BER) test as
shown in Fig. 3.4. 7. A custom digital pattern is injected into the system. It is important to use a
data pattern that simulates data sequences most likely to cause system errors. A pseudo-random
binary sequence (PRBS) is often used to simulate a wide range of bit patterns. The PRBS
sequence is a “random” sequence of bits that repeats itself after a set number of bits. A common
pattern is 2 23  1 bits in length. The output of the link under test is compared to the known input
with an error detector. The error detector records the number of errors and then ratios this to
the number of bits transmitted. A BER of 10-9 is often considered the minimum acceptable bit
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Chapter 3 Communication with Optics
error ratio for telecommunication applications. A BER of 10-13 is often considered the minimum
acceptable bit ratio for data communications.
Bit error ratio measurements provide a pass/fail criteria for the system and can often identify
particular bits that are in error. It is then necessary to trouble shoot a digital link to find the
cause of the error onto find the margin in performance that system provides. Digital waveforms
at the input and output of the system can be viewed with high –speed oscilloscope to identify and
troubleshoot problem bit patterns. In general, the eye diagram is used.
Another link evaluation is clock jitter measurement. A perfect clock waveform would have a
uniform bit period (unit interval) over all time. The fiber optic system can add variability to the
unit interval period that is referred to as jitter. The jitter causes bit errors by preventing the
clock recovery circuit in the receiver from sampling the digital signal at the optimum instant in
time.
The jitter originates primarily from noise generated by the regenerator electronic
components.”
Updated text.
“To ensure the good performance, it is very important to test and monitor the performance of the
optics networks. The most important parameter of a digital system is the bit error rate (BER).
To test the BER, in general a pseudo-random binary sequence (PRBS) is used. A common
pattern is 2 23  1 bits in length. The PRBS has two outputs. One output connects to the input
end of the fiber network to be tested and the other output is used as the reference and is
connected directly to the reference input port of the BER detector, as illustrated in Fig. 3.22.
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Chapter 3 Communication with Optics
The fiber link under test is compared to the known input with BER detector. The error detector
records the number of errors and then ratios this to the number of bits transmitted. A BER of
10-9 is often considered the minimum acceptable bit error ratio for telecommunication
applications. A lower BER of 10-13 is often considered the minimum acceptable bit ratio for data
communications.
Besides BER, clock jitter is another commonly used evaluation criterion, which is done by
comparing the measured bit period with a perfect reference clock waveform. To minimize the
clock jitter error, the clock regeneration via regenerator electronic component is used in the real
fiber optic networks.”
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Chapter 3 Communication with Optics
Soot preform
Flame
Mandrel
Move in both
direction
Materials
gases
Fig. 3.7
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Chapter 3 Communication with Optics
4x1
Coupler
λ1
…
C-band
OSA
10Gb/s
λ60
Fiber
P
S
P
C
…
λ61
L-band
λ100
AM Amplifie Amplifie
r
r
……
PIN
Rcvt
Fiber
ATT
BERT
BPF Amplifie
r
Fiber
Fiber
CR
Amplifie
r
Monitor
Fig. 3.19
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Chapter 3 Communication with Optics
0
Power (dBm)
10
-20
-30
-40
-50
60
channels
40 channels
-60
1520
1540
1560
1580
Wavelength
(nm)
1600
1620
Fig. 3.20
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Chapter 3 Communication with Optics
E/O
Transmitter
Optical
amplifier
Input data
Optical
fiber
power
time
power
power
time
time
time
Optical
fiber
Recovered
input data
time
time
Recovered
clock
time
O/E
Receiver
Fig. 3.21
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Chapter 3 Communication with Optics
Input pattern/clock
Fiber channel
BER detector
Detector
Reference
channel
Fig. 3.22
8