Download EEE440 Modern Communication Systems Optical Fibre

EEE440
Modern Communication Systems
Optical Fibre Communication
Systems
En. Mohd Nazri Mahmud
MPhil (Cambridge, UK)
BEng (Essex, UK)
[email protected]
Room 2.14
Semester 1 2011-2012
Announcement
EEE440
Test on mobile and wireless
Tuesday 6/12/2011;11am to 1pm
DK1
Semester 1 2011-2012
System elements
Semester 1 2011-2012
System elements
Semester 1 2011-2012
Light sources
• Semiconductor light-emitting diodes (LEDs) and laser diodes are
suitable
• Major differences between LED and Laser
– LED has an incoherent optical output whereas Laser produces highly
coherent, monochromatic and directional output because a cavity exist
for wavelength selectivity
• LED
– Generally used for multimode fibre
– For optical communications requiring bit rates less than 100-200 Mb/s
– Best for high-speed local applications which needs many wavelengths
on the same fibre
• Laser diodes
– the best light source for long-hauled fibre-optic links due to brightness,
narrow spectral width and coherence
Semester 1 2011-2012
Light source - LASER
• LASER stands for Light Amplification by Stimulated Emission of
Radiation
• Principle of operation
– Semiconductor material can generate light when current is injected
directly into it due to the stimulated emission of photons in the material
– The stimulated emission of photons occur when an external photon
impinges on an excited laser material
– The direct injection of current causes the particles of the laser materials
to undergo the process of excitation whereby the particles move from a
lower energy level (or ground state) to a higher energy level (or excited
state)
– To initiate the lasing action, the number of particles in the excited state
must be made greater than the number of particles in the ground state
(ie. Population inversion)
Semester 1 2011-2012
Light source - LASER
• Principle of operation
– The excited particles in the population inversion state are
unstable and can return to the stable ground state again and
spontaneously emit photon
– The photons from the spontaneous emission trigger stimulated
emission of other photons resulting in a cascade of stimulated
emission (ie lasing action that generate optical signal)
Semester 1 2011-2012
LASER
Laser diode: principle of operation: (a) Stimulated
(b) light amplification and positive feedback;
(c) pumping to create population inversion
Semester 1 2011-2012
emission;
LASER
Lasing effect: (a) Gain and loss; (b) input-output characteristic;
(c) setup to measure input-output characteristic
Semester 1 2011-2012
Light source - LASER
• There are many semiconductor laser types
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Fabry-Perot laser
Distributed Feedback (DFB) laser
Distributed Bragg Reflector (DBR) laser
Distributed Reflector (DR) Laser
Semester 1 2011-2012
Fabry-Perot Laser
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Consists of a heterojunction-structured semiconductor laser: 2 adjoining
semiconductor materials with different band-gap energies
A pair of flat, partially reflecting mirrors are directed toward each other to
enclose the cavity
When the junction is forward bias, electrons and holes are injected into the
p and n regions
These can recombine and release a photon energy, hv
Semester 1 2011-2012
Fabry-Perot Laser
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The two mirrors and the active medium between them form a laser
Mirrors provide positive feedback: the return of stimulated photons to an
active medium to stimulate more photons
The two mirrors form a resonator with length L
Let an arbitrary wave travel from the left-hand mirror to the right-hand one
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At the right-hand mirror, the wave experiences a 180° phase shift and continues to
propagate. At the left-hand mirror, this wave again has the same phase shift and continues to
travel yielding a stable pattern called a standing wave
Semester 1 2011-2012
Fabry-Perot Laser
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The only difference between the two waves shown in Figures 9.13(b) and 9.13(c) is
their wavelengths. Thus, a resonator can support only a wave with a certain
wavelength, the wave that forms a standing-wave pattern
The resonator supports a wavelength where 2L/N = 1300.8 nm.
But this resonator also supports wavelengths equal to 2L/(N ± 1), 2L/(N ± 2), 2L/(N ±
3), and so forth.
Many wavelengths may exist. Wavelengths selected by a resonator are called
longitudinal modes.
When the length of a resonator increases or decreases, the laser switches from one
longitudinal mode to another. This is called mode hop.
Semester 1 2011-2012
Fabry-Perot Laser
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However, the active medium provides gain within only a small range of wavelengths.
Since a laser is formed by a resonator and an active medium and since radiation is
the result of their interaction, only several resonant wavelengths that fall within the
gain curve might be radiated.
Light generation starts only when gain exceeds loss. Thus, eventually only those
resonant wavelengths that are within the gain-over-loss curve will actually be
radiated.
Waves With N, N±1, and N±2 might be radiated, but only waves with N and N±1 will be the
actual laser output. Modes N±2, depicted in black, are not generated.
Semester 1 2011-2012
DFB Laser
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To reduce the spectral width, we need to make a laser diode merely radiate
only one longitudinal mode with distributed-feedback (DFB) laser diodes
Has the Bragg grating incorporated into its heterostructure in the vicinity of
an active region.
The Bragg grating works like a mirror, selectively reflecting only one
wavelength, B
Semester 1 2011-2012