photons and fibres lecturer : professor laurie cahill
TRANSCRIPT
PHOTONS AND FIBRES
Lecturer : Professor Laurie Cahill
PHOTONS AND FIBRES
What is light?
Is light a particle or a wave?
What is a photon?
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
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
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)
WORK FUNCTION OF A METAL SURFACE
(Source : A. Beiser)
De BROGLIE (1924)
hc
hfc
Ep
Momentum of a photon
Wavelength of a particle Hence matter waves and the Wave-
Particle Duality p
h
DOUBLE SLIT EXPERIMENT
Similar experimental result (over time) if we use photons or electrons
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” .
COMMUNICATION USING LIGHT
END VIEWAXIAL CROSS-SECTION
Consider a light ray entering a glass rod
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
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!
CAUSES OF FIBRE LOSS
FIBRE ATTENUATION
Loss in db =
in
out
P
P10log10
OPTICAL FIBRE LINK
(Source: D. M. Spirit & M. J. Mahoney)
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!
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:
ADVANTAGES OF OPTICAL FIBRES
1. LOW LOSS2. HIGH BANDWIDTH3. LOW MATERIAL PRICE4. LOW WEIGHT5. LOW EMI (INTERFERENCE)
GENERATION AND DETECTION OF PHOTONS
Emission
Stimulated Emission
Detection
hfEE
hfE
12
ENERGY LEVELS IN A SUITABLE
SEMICONDUCTOR
Photodiode LED Laser Diode
DETECTION OF PHOTONS
REQUIREMENTS FOR LASING
Source: Ghatak
•Population inversion
•Optical gain
•Mirrors
EARLY LASER DIODE
Source: Ghatak
LIGHT CONCENTRATED IN ACTIVE REGION OF LASER
LIGHT POWER VS CURRENT
SPECTRA OF LED AND LASER DIODE
SINGLE AND MULTIMODE LASER DIODES
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.
SINGLE MODE FIBRE DISPERSION
(Source: D. M. Spirit & M. J. Mahoney)
INTERSYMBOL INTERFERENCE
Dispersion (pulse spreading) can cause pulses to overlap and limit bit-rate
(Source: G. Keiser)
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