Physics Lecture 3b

8/8/2019 Physics Lecture 3b

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A LECTURE BY DANIEL WONG

Photonics


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What is photonics?

Photonics is thescience of generating

and manipulatinglight for the purpose

of transmittinginformation.

WojPob, Wikiexpert in physics andthe history of Nazi Germany

The photon is the basic unit oflight. It exhibits wave-particleduality.

The light used in photonics

includes: Visible light (380 760 nm)

Infrared light (760 1000 nm)

Ultraviolet light (10 380 nm)

cf=

1f

from http://en.wikipedia.org/wiki/index.php?curid=36496


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How do we produce light?

Two common types:

from http://www.freefoto.com/images/11/12/11_12_52---Electric-Light-Bulb_web.jpg

from http://commons.wikimedia.org/wiki/File:Fluorescent_light_bulbs_09.JPG

Incandescent Fluorescent


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Incandescence

Emission of light from a body due to its temperature.How does it work? Hot objects have moving atoms. There are collisions between atoms.

These collisions cause valence electrons to accelerate. Principle: accelerating charged particles emit EMR. Accelerating electrons produce light.

The spectrum ofthermal radiation produced is

continuous and exhibits a peak wavelength.The peak wavelength depends on the temperature of theobject.


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Thermal spectrum

from http://www.northeastblacksmiths.org/ashokantext/gallery/harris/20.jpg

from http://upload.wikimedia.org/wikipedia/commons/a/a2/Wiens_law.svg

The greater the temperature,the lower the peak wavelengthand the greater the amount oflight emitted.


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Incandescent objects

When current passes through the tungstenfilament, it heats up and its atoms vibrate.

Collisions between atoms causes outer-shellelectrons to accelerate and emit light.

from http://commons.wikimedia.org/wiki/File:Incandescent_light_bulb_%28no_labels%29.svg

from http://ryanericsongcanlas.files.wordpress.com/2009/08/candle.jpg

from http://www.enterstageright.com/archive/articles/0707/070907sun.jpg


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Fluorescence

Emission of light due to atoms being excited to higherenergy levels and then falling back to ground state.

SAY-WHAA?

from http://www.masterschool.org/page.cfm?p=366


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The energy states of atoms

When atoms absorb energy they enter higher energy states.

Ground stateLowest energy

Excited stateElectrons have more energy


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Spontaneous emission

When excited atoms return to the ground state, they emitphotons of discrete energy equal to the difference in energylevels.

Excited state

Higher energy

Ground state

Lower energy


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Fluorescent tubes

The tube contains a gas at low pressure.

Electrons pass through the gas between electrodes at eitherend of the tube.

As electrons pass through the gas, atoms in the gas aretransferred to higher energy levels (become excited).

The excited atoms release light of discrete wavelengths.

As they do not involve heating objects, they can be rapidlyturned on and off.


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Line emission spectrum

Hence fluorescent light sources have discrete, rather thancontinuous, emission spectra.

from http://en.wikipedia.org/wiki/File:Yellow_fluorescent_light_spectrum.png
http://upload.wikimedia.org/wikipedia/en/7/78/Yellow_fluorescent_light_spectrum.png


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Light-emitting diodes (LEDs)

An LED is a small semiconductor diode that emits lightwhen a current passes through it.

from http://en.wikipedia.org/wiki/LED from http://en.wikipedia.org/wiki/LED


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Energy bands

When n atoms come close together, their energy levels splitinto n parts. These divided energy levels form an energyband.

Materials have a valence band and a conduction band:

Energy

level

Conduction band

Valence band

Lower energy levels


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Energy bands

Conductors are materials for which the conduction band is closeto the valence band (it requires little energy to move electronsfrom the valence band to the conduction band).

The band gap is the difference in energy between the valence andconduction bands.

from http://en.wikipedia.org/wiki/File:Isolator-metal.svg
http://upload.wikimedia.org/wikipedia/commons/c/c7/Isolator-metal.svg


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How do LEDs produce light?

In LEDs, electrons in the conduction band fall intothe valence band and EMR is released (spontaneousemission).

The wavelength of the photons released is given by:

gE

hc=

Where:h is Plancks constantc is the speed of lightEg is the band gap


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Laser

Light

Amplification by

Stimulated

Emission ofRadiation from

http://www.wired.com/images_blogs/photos/uncategorized/2007/07/05/superlaser_01ds.jpg

fromhttp://ericlightborn.files.wordpress.com/2009/06/dr_evil_laser.jpg


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How do lasers produce light?

A current excites atoms of the gain medium to a higherenergy state.

More atoms are in a higher energy state rather than theground state (population inversion).

Spontaneous emission of a photon occurs when one atomreturns to the ground state and this triggers a chainreaction of stimulated emission.


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Stimulated emission

A photon interacting with an excited atom produces twophotons of the same wavelength, direction of motion andphase (they are coherent).

from http://en.wikipedia.org/wiki/File:Stimulated_Emission.svg


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Laser structure

In the resonator,amplification of lightoccurs to produce a beamthat is: Coherent (in phase)

Monochromatic (samewavelength/colour)

Parallel (not divergent)

from http://media-2.web.britannica.com/eb-media/45/95645-004-32D0C70A.jpg


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Laser safety

Key knowledge: identify and apply safe andresponsible practices when working with photonicsequipment.

Due to their high energy, some lasers can cause eyedamage or even burn skin. Some lasers that producelight outside the visible light spectrum can causedamage to the retina.

Wear safety goggles and avoid shining lasers intopeoples eyes.


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Optical fibres

Uses total internal reflection to carry light along thecore.

from http://upload.wikimedia.org/wikipedia/commons/b/b2/Stomach_endoscopy_2.jpgfrom http://en.wikipedia.org/wiki/Fibre-optics


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Reflection and refraction

LMT: When light enters amaterial of greaterrefractive index (greaterdensity) it refracts towards

the normal.Snells law:

n1sin

1= n

2sin

2

Some light is also reflectedand the angle of reflectionequals the angle ofincidence.from http://misclab.umeoce.maine.edu/

boss/classes/SMS_491_2003/refraction.gif


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The critical angle and TIR

When the angle of incidence equals the critical angle, the angle if refractionis 900. Beyond this, only reflection occurs (total internal reflection).

The angle of incidence is given by n1sin1 = n2sin2 where sin2 = 1 (as 2 =

900). We get:

from http://en.wikipedia.org/wiki/Total_internal_reflection

1

21sin

n

n

c

=


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Basic structure of optical fibres

Consists of a core through which the light travels.

The surrounding cladding has a lower refractiveindex so that TIR of the light occurs.

from http://en.wikipedia.org/wiki/Fibre-optics


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Numerical aperture

The light-gathering ability of an optical fibre.

The acceptance angle () is made by the edge of theacceptance cone and the axis of the optical fibre.

2

2

2

1 nnNA =( )22211sin nn = sin=NA


Where n1 refers to the core and n2 refers to the cladding.
http://upload.wikimedia.org/wikipedia/commons/4/46/Optical-fibre.svghttp://upload.wikimedia.org/wikipedia/commons/4/46/Optical-fibre.svg


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Acceptance angle

A better illustration:

from http://www.timbercon.com/Fiber-Optic-Glossary/images/Acceptance-Angle.gif


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Factors affecting NA

The greater the diameter of the core, the greater thenumerical aperture.

The lower the critical angle for the interface between

the core and the cladding, the greater the numericalaperture.

from http://en.wikipedia.org/wiki/Total_internal_reflection


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Dispersion through optical fibres

Dispersion is the spreading out of a light signal as ittravels through an optical fibre.

There are two kinds:

Modal dispersion Material dispersion


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Modal dispersion

A mode is a particular pathway that a ray of light canfollow as it is transmitted along an optical fibre.

from http://en.wikipedia.org/wiki/File:OF-na.svg
http://upload.wikimedia.org/wikipedia/en/3/36/OF-na.svg


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Material dispersion

Different wavelengths of light light travel at differentspeeds through a non-vacuum medium.

Light of greater wavelength travels faster.

As the pulse is widened, a lower frequency must beused.

from http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2996


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Solutions

http://upload.wikimedia.org/wikipedia/commons/0/0e/Optical_fiber_types.svg


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Attenuation

Attenuation is the loss of power of a signal.

It is measured in decibels (dB).

Attenuation in dB

Two main causes of attenuation in optical fibres are: Rayleigh scattering

Absorption

in

out

P

P10

log10=


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Rayleigh scattering

The scattering of light in random directions due tosmall imperfections in the core of an optical fibre.

Worse for shorter wavelengths of light.

from http://atmoz.org/img/rayleigh_scattering.jpg


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Absorption

Molecules (including impurities) in the core absorblight energy and transform it into heat.

At longer wavelengths, absorption rather than

scattering is the main cause of attenuation.


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Bundles of optical fibres

from http://en.wikipedia.org/wiki/File:Flexibles_Endoskop.jpg

Coherent bundleIncoherent bundle

from http://www.resilientnetwork.com/NitroFiber/Images/Index-fiber-bundle.jpg


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The End

Daniel WongContact: [email protected]

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Physics Lecture 3b