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PHOTONS AND FIBRES
Lecturer : Professor Laurie Cahill
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PHOTONS AND FIBRES
What is light?
Is light a particle or a wave?
What is a photon?
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HISTORY
Newton - Light is a stream of corpuscles
Huygens, Maxwell,Young - Light travels in waves
Planck - Thermal radiation comprises discrete packets of energy called quanta
Einstein
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EINSTEIN (1905)
Light is similarly quantised
hfE E is the energy of the light quanta, later called photonsh is Planck’s constantf is the frequency of the light
This explains the photoelectric effect
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THE PHOTOELECTRIC EFFECT
The maximum KE of each emitted electron depends on the frequency of the incident light, not the intensity.More photons produce more emitted electrons
(Source: D. A. Neamen)
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WORK FUNCTION OF A METAL SURFACE
(Source : A. Beiser)
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De BROGLIE (1924)
hc
hfc
Ep
Momentum of a photon
Wavelength of a particle Hence matter waves and the Wave-
Particle Duality p
h
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DOUBLE SLIT EXPERIMENT
Similar experimental result (over time) if we use photons or electrons
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WAVES OR PARTICLES?
If light only consists of waves, how come we can only generate and detect discrete photons?If light consists only of particles, how does a photon passing through one slit know about the other slit being open?
Feynman - Consider all possible paths and assign amplitudes and probabilities to “particles” .
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COMMUNICATION USING LIGHT
END VIEWAXIAL CROSS-SECTION
Consider a light ray entering a glass rod
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RAYS IN A CIRCULAR FIBRE
Exercise: Find an expression for the acceptance angle for rays through the axis (Hint apply Snell’s Law).
a
0n 1n2n
1n2n
Cladding
Core
END VIEWAXIAL CROSS-SECTION
Answer : 2
122
21 )()sin( nna
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COMPARISON OF MULTIMODE AND SINGLE MODE FIBRE GEOMETRIES
Can not use ray analysis for SMF - use only modal analysis
Can use ray analysis
Not a ray!
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CAUSES OF FIBRE LOSS
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FIBRE ATTENUATION
Loss in db =
in
out
P
P10log10
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OPTICAL FIBRE LINK
(Source: D. M. Spirit & M. J. Mahoney)
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COMPARISON OF MULTIMODE AND SINGLE MODE FIBRE GEOMETRIES
Can not use ray analysis for SMF - use only modal analysis
Can use ray analysis
Not a ray!
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DIFFERENT PATH LENGTHS IN MULTIMODE FIBRES
The velocity in the z direction depends on the angle of the ray
A spread of path lengths gives a spread of arrival times
This spreads a sharp pulse and limits the bit-rate
1n
c
zV
cos1n
cvz
Use ray analysis:
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ADVANTAGES OF OPTICAL FIBRES
1. LOW LOSS2. HIGH BANDWIDTH3. LOW MATERIAL PRICE4. LOW WEIGHT5. LOW EMI (INTERFERENCE)
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GENERATION AND DETECTION OF PHOTONS
Emission
Stimulated Emission
Detection
hfEE
hfE
12
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ENERGY LEVELS IN A SUITABLE
SEMICONDUCTOR
Photodiode LED Laser Diode
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DETECTION OF PHOTONS
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REQUIREMENTS FOR LASING
Source: Ghatak
•Population inversion
•Optical gain
•Mirrors
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EARLY LASER DIODE
Source: Ghatak
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LIGHT CONCENTRATED IN ACTIVE REGION OF LASER
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LIGHT POWER VS CURRENT
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SPECTRA OF LED AND LASER DIODE
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SINGLE AND MULTIMODE LASER DIODES
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DISPERSION (PULSE SPREADING) IN SMF
Since velocity depends on n, a small change in n with wavelength can affect the arrival time of signals of differentwavelengths and cause them to overlap.
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SINGLE MODE FIBRE DISPERSION
(Source: D. M. Spirit & M. J. Mahoney)
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INTERSYMBOL INTERFERENCE
Dispersion (pulse spreading) can cause pulses to overlap and limit bit-rate
(Source: G. Keiser)
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PHOTONS AND FIBRES
What is a photon?
“All these fifty years of conscious brooding have brought me no nearer to the answer to the question, “What are light quanta (photons)?”
Albert Einstein