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Lasers
Light Amplification by Stimulated Emission of Radiation
What is stimulated emission?
Well, there are two types of light emission that can occur with atoms!
The kind that we have been talking about is the first type.
• spontaneous emission
Einstein postulated the second type and gets credit for “discovering”
it. (But it had always been occurring!)
• stimulated emission
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Stimulated Emission
Lasers
Spontaneous Emission
• atom is stimulated to emit
photon by an incoming photon
• atom emits photon on its own
• emitted in random direction
• incoming photon must have the
same energy as the one that
will be emitted
• the two photons leave in the
same direction
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Lasers
Possible Idea!
If we could get a bunch of excited atoms and have each one be
stimulated to emit a photon, then we could get a whole bunch of
photons with the same energy and wavelength travelling in the same
direction. This would give us a bright output beam…..a laser beam!
Problem!
For most atomic energy levels, spontaneous emission occurs before
stimulated emission. Spontaneous emission will not give us a nice
beam and just one wavelength.
Solution!
Find atoms that have an energy level where stimulated emission is
more likely to occur. This state is called the metastable state.
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Lasers
Three Level Laser Operation
1.Pump atom from 1 to 3 using
excitation source.
2. Quick decay from 3 to 2.
3
2
3. Atom “waits” in metastable
state for incoming photon*.
4. Incoming photon stimulates
emission from 2 to 1.
5. Repeat Step 1.
1
* When there are more atoms at level 2 than level 1, we have a
population inversion. Lasing action will now occur.
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Lasers
So what do you need to build a laser?
1. Gain Medium - a material with atoms that have a metastable
state and that can achieve population inversion
2. Excitation Source - a way to supply energy to excite the atoms
3. Resonating Cavity (Mirrors) - allows the stimulated photons to
travel back and forth through the gain medium so they
can stimulate more photons (light amplification)
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Lasers
full mirror
partial mirror
gain medium
• allows some photons to
escape to produce beam
• solid
• liquid
excitation source
• gas
• light, laser for solid lasers
• laser for liquid lasers
• high voltage for gas lasers
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Lasers
Light Characteristics
• tight beam
• nearly one wavelength (monochromatic)
• relatively high output power, especially in pulsed wave operation
Operation Schemes
• continuous wave (cw) - beam is on all the time
- scanning, optical ranging, light displays
• pulsed wave – short pulses are emitted
-high-power applications like surgery or cutting
-low-power applications like communication
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Lasers
Some Examples:
Type
Wavelength
Excitation
Comments
SOLIDS
Ruby
Nd:YAG
Ti:Sapphire
red (694 nm)
IR (1.06 m)
visible
flashlamp
laser
laser
first laser built (1960)
medical applications
tunable to different colors
LIQUIDS
Dye (different
dyes available)
visible-near IR
(400 - 900 nm)
laser
hard to work with, usually only for research
GASES
He-Ne
red (633 nm)
green (543 nm)
blue (488 nm)
green (514 nm)
Far IR (10.6 m)
high volatge
UV
high voltage
very common visible laser, being replaced by red
semiconductor lasers
relatively high power (~10 W), often used as
excitation source for solid & dye lasers
large, extremely high power (up to ~1000 W),
medical applications, welding, cutting
high power (~10 W) in pulsed operation, medical
applications
+
Argon ion (Ar )
Carbon Dioxide
(CO2)
Excimer (ArF,
KrF)
high voltage
high voltage
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Lasers
Semiconductor Lasers (Laser Diodes)
• made from semiconductor solid crystals
• operation is similar to the previous lasers we discussed but there
are significant differences
• use a flow of electrons (current) for the excitation source
• very small but with medium to high output powers
• laser light can be tuned sightly to different wavelengths
• used for:
– optical reading & writing (dvd, cd, laser printer, bar code)
– laser pointers
– optical fiber communication
– excitation sources for other lasers
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Lasers
SEMICONDUCTOR
Material
AlGaAs
Tunable Range
near IR
Excitation
current
AlGaInAsP
near IR
current
optical communication
AlGaInP
red
current
replaced He- Ne lasers, read & write technologies
InGaN
green to blue
current
read & write technologies
first semiconductor lasers
This is a AlGaInP/GaInP laser emitting light at 676 nm. It is
0.03 mm wide, 0.3 mm long, and about 0.1 mm thick.
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Laser Applications
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Laser Applications
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Laser Applications
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