Lasers and LEDs Lasers and LEDs Lasers produce narrow beams of - - PDF document

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Lasers and LEDs 1

Lasers and LEDs Lasers and LEDs

Lasers and LEDs 2

Observations about Lasers Observations about Lasers and LEDs and LEDs

Lasers and LEDs often have pure colors

Lasers and LEDs often have pure colors

Lasers produce narrow beams of intense light

Lasers produce narrow beams of intense light

Lasers are dangerous to eyes

Lasers are dangerous to eyes

Reflected laser light has a funny speckled look

Reflected laser light has a funny speckled look

Lasers and LEDs 3

3 Questions about 3 Questions about Lasers and LEDs Lasers and LEDs

• How does laser light differ from regular light?

How does laser light differ from regular light?

How does a laser produce coherent light?

How does a laser produce coherent light?

How does an LED produce its light?

How does an LED produce its light?

Lasers and LEDs 4

Question 1 Question 1

How does laser light differ from regular light?

How does laser light differ from regular light?

Lasers and LEDs 5

Light: Photons and Waves Light: Photons and Waves

• Electrons obey the Pauli exclusion principle

Electrons obey the Pauli exclusion principle

Each wave mode can have only one unique electron.

Each wave mode can have only one unique electron.

That result gives structure to atoms and materials

That result gives structure to atoms and materials

Ph d ’ b h P li l i i i l Ph d ’ b h P li l i i i l

Photons don’t obey the Pauli exclusion principle

Photons don’t obey the Pauli exclusion principle

Each wave mode can have many photons

Each wave mode can have many photons

A radio wave has many photons in a single wave

A radio wave has many photons in a single wave

Most light sources produce photons randomly

Most light sources produce photons randomly

Each photon usually has its own wave mode

Each photon usually has its own wave mode

But laser

But laser light is an exception! light is an exception!

Lasers and LEDs 6

Spontaneous Emission Spontaneous Emission

Excited atoms normally emit light

Excited atoms normally emit light spontaneously spontaneously

These photons are uncorrelated and independent

These photons are uncorrelated and independent

Each photons has its own wave mode

Each photons has its own wave mode

These independent waves are incoherent light

These independent waves are incoherent light

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Lasers and LEDs 7

Stimulated Emission Stimulated Emission

Excited atoms can be

Excited atoms can be stimulated stimulated into duplicating into duplicating passing light passing light

These photons are correlated and identical

These photons are correlated and identical Th h ll h h d Th h ll h h d

The photons all have the same wave mode

The photons all have the same wave mode

This single, giant wave is

This single, giant wave is coherent light coherent light

Lasers and LEDs 8

Question 2 Question 2

How does a laser produce coherent light?

How does a laser produce coherent light?

Lasers and LEDs 9

Laser Amplification Laser Amplification

Light can be amplified using stimulated emission

Light can be amplified using stimulated emission

Excited atom

Excited atom-

• like systems can act as a laser medium

like systems can act as a laser medium

That medium will duplicate any photons that have the

That medium will duplicate any photons that have the right wavelength polarization and orientation right wavelength polarization and orientation right wavelength, polarization, and orientation right wavelength, polarization, and orientation

This duplication is perfect: the photons are true clones

This duplication is perfect: the photons are true clones

This light amplification is the basis for lasers

This light amplification is the basis for lasers

Lasers and LEDs 10

Laser Oscillation Laser Oscillation

A laser medium can amplify its own light

A laser medium can amplify its own light

A laser medium in a resonator acts as an oscillator

A laser medium in a resonator acts as an oscillator

It duplicates its one of its own spontaneous photons

It duplicates its one of its own spontaneous photons

D pli

t d ph t n l k fr m mitr n p r nt mirr r D pli t d ph t n l k fr m mitr n p r nt mirr r

Duplicated photons leak from semitransparent mirror

Duplicated photons leak from semitransparent mirror

The photons from this oscillator are identical

The photons from this oscillator are identical

Lasers and LEDs 11

Properties of Laser Light Properties of Laser Light

Coherent

Coherent – – identical photons identical photons

Controllable wavelength/frequency

Controllable wavelength/frequency – – colors colors

Controllable spatial structure

Controllable spatial structure – – narrow beams narrow beams

Controllable temporal structure

Controllable temporal structure – – short pulses short pulses

Energy storage and retrieval

Energy storage and retrieval – – intense pulses intense pulses

Giant interference effects

Giant interference effects

But apart from all this, laser light is still just light

But apart from all this, laser light is still just light

Lasers and LEDs 12

Examples of Lasers Examples of Lasers

Gas lasers (powered by discharges)

Gas lasers (powered by discharges)

Helium

Helium-

• neon lasers (red, green, yellow)

neon lasers (red, green, yellow)

Carbon dioxide lasers (infrared)

Carbon dioxide lasers (infrared)

S lid l ( d b li h ) S lid l ( d b li h )

Solid state lasers (powered by current or light)

Solid state lasers (powered by current or light)

Diode lasers (red, blue, infrared)

Diode lasers (red, blue, infrared)

Ruby lasers (red)

Ruby lasers (red)

Nd:YAG

Nd:YAG lasers (infrared) lasers (infrared)

Ti:Sapphire

Ti:Sapphire lasers (infrared) lasers (infrared)

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Lasers and LEDs 13

Question 3 Question 3

How does an LED produce its light?

How does an LED produce its light?

Lasers and LEDs 14

Light Light-

• Emitting Diodes

Emitting Diodes

LEDs are Light

LEDs are Light-

• Emitting Diodes

Emitting Diodes

They conduct current only in one direction

They conduct current only in one direction

Each charge releases energy on crossing

Each charge releases energy on crossing pn pn-

• junction

junction

Th t n r

i ft n mitt d ph t n f li ht Th t n r i ft n mitt d ph t n f li ht

That energy is often emitted as a photon of light

That energy is often emitted as a photon of light

For LEDs to emit higher

For LEDs to emit higher-

• energy photons,

energy photons,

they must be designed to have larger band gaps

they must be designed to have larger band gaps

they must be supplied with larger voltage drops

they must be supplied with larger voltage drops

Laser diodes are LEDs that can amplify light.

Laser diodes are LEDs that can amplify light.

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Summary about Lasers and LEDs Summary about Lasers and LEDs

Lasers produce coherent light by amplification

Lasers produce coherent light by amplification

Coherent light contains many identical photons

Coherent light contains many identical photons

Laser amplifiers and oscillators are common

Laser amplifiers and oscillators are common

LEDs are incoherent, light

LEDs are incoherent, light-

• emitting diodes

emitting diodes