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