7. Optical Fiber
Communication Systems
Inter-Continental Optical Fiber
Communications
Optical Fiber Communication Systems and Fiber
Sensors (Example of Measuring the Gap of Bridge)
Attenuation and Dispersion in
Silica Fiber
Erbium Doped Fiber Amplifier (EDFA)
• Er3+ ions doped into the core
region of an optical fiber to
achieve light amplification
need (stimulated emission >
absorption) population
inversion required (N2> N1)
• Optical gain, Gop=K(N2 –N1),
where K is a constant depends
on pumping intensity
• Er3+-doped fiber is usually
inserted into the fiber
communication line by splicing
• Gain efficiency : 8-10 dB/mW
Classification of Optical Fibers
Theory of Step-index Fiber
Theory of Step-index Fiber (Cont’)
Single-mode Operation of Stepindex Fiber
Coupling from LD to Fiber
Fiber Lens to Improve Coupling Efficiency
Optical Fiber Splice and Mismatches
in a Fiber-to-fiber Connection
Digital Signals—RZ and NRZ Formats
• A return-to-zero (RZ)
format is the signal
returns to zero between
each pulse. The signal is
self-clocking.
• A non-return-to-zero
(NRZ) format does not
have a rest state. NRZ is
not inherently a selfsynchronizing code, thus
some additional
synchronization
technique must be used
for avoiding bit slip.
Some Digital Modulations in
Communication
•
•
•
•
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature Amplitude Modulation (QAM)
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature PSK (QPSK)
• In the example shown
in the right figure, 00
is represented by
sin(2πf0t), 10 is
represented by
sin(2πf0t+π), 01 is
represented by
sin(2πf0t+π/2), and 11
is represented by
sin(2πf0t+3π/2).
Analog Quadrature Amplitude Modulation (AQAM)
A low-pass filter removes the high frequency terms
containing cos(4πf0t), leaving only the I(t) term.
Example of Digital 8-QAM
• In this 8-QAM example,
eight different modulation
states using 4 phase
angles and 2 amplitudes:
one at 50% modulation;
the other at 100%. (4
phases X 2 amplitudes =
8 modulation states).
• In QAM, the constellation
diagram is a useful
representation. The
constellation points are
usually arranged in a
square grid.
Example of Constellation Diagram in 16-QAM
Examples of Constellation Diagrams in
QAMs
Example of 8-QAM Optical Fiber
Communication System
Eye Diagram
Eye Diagram (Cont’)