Download SECTION 6 RAY ANALYSIS OF MULTIMODE CIRCULAR …

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
E  hf
E is the energy of the light quanta, later called photons
h is Planck’s constant
f 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)

Momentum of a photon
E
p
c
hf

c
h




h

p
Wavelength of a particle
Hence matter waves and the WaveParticle Duality
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
Consider a light ray entering a glass rod


AXIAL CROSS-SECTION
END VIEW
RAYS IN A CIRCULAR FIBRE
Exercise: Find an expression for the acceptance
angle  a for rays through the axis (Hint apply
Snell’s Law).
Cladding
n0
n2
n1
n2


n1

AXIAL CROSS-SECTION
Answer :
END VIEW
1
2 2
2
sin(  a )  (n12  n )
Core
COMPARISON OF MULTIMODE AND
SINGLE MODE FIBRE GEOMETRIES
Can use ray analysis
Not a ray!
Can not use ray analysis for SMF - use only modal analysis
CAUSES OF FIBRE LOSS
FIBRE ATTENUATION
 Pout 

Loss in db =  10 log 10 
 Pin 
OPTICAL FIBRE LINK
(Source: D. M. Spirit & M. J. Mahoney)
COMPARISON OF MULTIMODE AND
SINGLE MODE FIBRE GEOMETRIES
Can use ray analysis
Not a ray!
Can not use ray analysis for SMF - use only modal analysis
DIFFERENT PATH LENGTHS IN
MULTIMODE FIBRES
Use ray analysis:
c
v z  cos 
n1



c
n1

Vz
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 bitrate
ADVANTAGES OF OPTICAL
FIBRES
1.
2.
3.
4.
5.
LOW LOSS
HIGH BANDWIDTH
LOW MATERIAL PRICE
LOW WEIGHT
LOW EMI (INTERFERENCE)
GENERATION AND
DETECTION OF PHOTONS
Emission
Stimulated
Emission
Detection
E  hf
E2  E1  hf
ENERGY LEVELS IN A
SUITABLE SEMICONDUCTOR
Photodiode
LED
Laser Diode
DETECTION OF PHOTONS
REQUIREMENTS FOR LASING
•Population inversion
•Optical gain
•Mirrors
Source: Ghatak
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 different
wavelengths 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