Transcript Slide 1

AS Physics Unit 1
Lasers Extension
Mr D Powell
Which is laser light & why?
Mr Powell 2008
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Light Amplification Stimulated Emission Radiation
A laser is simply a mechanically constructed tool
which amplifies the process of “stimulated
emission”.
Emission (already covered) is when an electron
jumps down a quantum energy level and emits
light of energy E = hf.
Stimulated emission is when a second photon
hits an already excited atom releasing two
photons instead of one. That is the original one
and the one formed from the transition of an
energy level
This means that if we can get a medium excited
enough or “pumped” where lots of atoms are
excited or “population inversion” then we can
create a cascade effect and produce a large
number of photons in phase
NB. The time photons spend in
an excited state is usually
measured as an average of
many emissions.
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Mr Powell 2008
Stimulated Emission Summary
Excited atoms can give out two photons at once in an emission instead of
the usual one in “Stimulated Emission”.
We can also work out the wavelength of photons as we know that that; c =

NB: in this case “” is used to represent frequency it is not v = velocity You will only see the
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version with f in your exam!
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Stimulated Emission TASK
Using your own version of energy level diagrams. Try and explain the idea of
“Stimulated Emission” and how it can help make lasers work. Make bullet
points to explain how it works.
Skill Levels E - A
1. Labelled diagram showing absorption & emission of photons
2. + compared to stimulated emission.
3. + comparison of the two concepts and their differences
4. + explanation of way this creates a population inversion
5. + links to formulae and how a substitution can be made to work out the  of
any emitted light.
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Light Amplification Stimulated Emission Radiation
This picture shows you in detail the
process step by step.
In this case we have a ruby rod
(694.3nm) which produces pulses of
light (about 1ms)
We stimulate the atoms until we
have population inversion and allow
the stimulated emission to build,
then when the power reaches a
certain point it pulses out in phase
as laser light.
But what is this population
inversion?
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Population Inversion
Atoms can have lots of different energy levels which
have different possible E =hf or transitions. Some
are easier to pump or produce a population inversion
than others.
It basically works on the time spent by an electron on
a level. If they constantly fall down a level all the time
then they will not be able to produce the effect of
stimulated emission.
A two level system cannot invert the population
without massive energy input. (pointless)
Better examples are three energy level systems (ruby)
or four level (Helium Neon diagram is combined – see
textbook).
Helium gas is mixed with Neon at a ratio of 10:1. The
helium is pumped to the correct value and transfers
energy via collisions to the Neon. The Neon then
drops from a metastable state to lase
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Helium Neon
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He Ne Summary
Learn this time idea
Learn where it comes
from
Learn this transition
from memory
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Laser Summary
We put energy into the atom and excite an
electron to pump it to Level 4
The energy gets quickly transferred to the
Neon metastable state. Population Inversion!
All electrons move down at the same time
producing photons in phase E = hf
according to transition.
The key point you need to know for your
exam is that the “pumped” state is short
lived i.e. 10-8s but the metastable state is
much longer i.e. 10-3s so the electrons stack
up here and produce a “population
inversion”
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Applet
http://phet.colorado.edu/
• So the challenge is can you get the laser animation to lase using the
information you have just learned!
• Try changing the controls to fit the model to what we have learned.
• Look at what we said about metastable states.
• Look at what we said about energy absorption and emission
• Now lets look at some lasers for real on video from Youtube!
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Laser TASK
Can you build upon your previous answer and use the information in the
previous slides to try and explain how a laser works.
Skill Levels E - A
1. Talk about stimulated emission (diagram)
2. + add something about energy levels and the need for three or four states
3. + Concept of population inversion
4. + Metastable states and their uses
5. + Mirrors and ideas of phase.
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Eye Surgery – Lasers in Context - http://www.ultralase.com/
•
Laser eye surgery involves the precise reshaping of the cornea, the transparent window that covers
the coloured part of the eye. For treatment to be permanent, it must take place beneath the thin,
protective outer layer. This layer is gently moved aside in order to let the laser do its work.
•
There are two fundamental types of laser eye treatment: LASIK and LASEK. They differ only in the way
the eye is prepared for treatment. The actual re-shaping process is exactly the same in both cases. All
Ultralase LASIK laser eye surgery treatments are now blade-free, exclusively utilising Intralase® for
safety and accuracy reasons instead.
•
With LASIK laser eye surgery, the protective surface layer of the cornea is lifted and the laser is used to
reshape the layer underneath. The surface is then gently replaced, the whole procedure taking no
more than 20 minutes. Traditionally, the protective layer is lifted with the use of a surgical blade and,
understandably, this can cause concern. At Ultralase, we don't use blades: we use Intralase® as
standard. IntraLase® means that a very, very fine laser is used to lift the protective layer. This makes
Ultralase the ONLY national provider in the UK to move completely away from the use of blades in
vision correction treatment.
•
With LASEK laser eye surgery, the protective surface of the cornea is softened and then folded aside.
The laser then reshapes the layer beneath. Then, the surface is smoothed back into place and a special
contact lens is placed on the eye for 5-7 days while the protective layer heals. You can expect to be in
the Treatment Room for no more than around 20 minutes.
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Further Reading
• There are many different types of lasers. The laser medium can be a
solid, gas, liquid or semiconductor. Lasers are commonly designated by
the type of lasing material employed: Solid-state lasers have lasing
material distributed in a solid matrix (such as the ruby or neodymium:
yttrium-aluminum garnet "Yag" lasers). The neodymium-Yag laser emits
infrared light at 1,064 nanometers (nm). A nanometer is 1x10-9 meters.
• Gas lasers (helium and helium-neon, He Ne, are the most common gas
lasers) have a primary output of visible red light. CO2 lasers emit
energy in the far-infrared, and are used for cutting hard materials.
• Excimer lasers (the name is derived from the terms excited and dimers)
use reactive gases, such as chlorine and fluorine, mixed with inert gases
such as argon, krypton or xenon. When electrically stimulated, a
pseudo molecule (dimer) is produced. When lased, the dimer produces
light in the ultraviolet range.
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Further Reading II
• Dye lasers use complex organic dyes, such as rhodamine 6G, in
liquid solution or suspension as lasing media. They are tunable
over a broad range of wavelengths.
• Semiconductor lasers, sometimes called diode lasers, are not
solid-state lasers. These electronic devices are generally very
small and use low power. They may be built into larger arrays,
such as the writing source in some laser printers or CD players.
• Try this link for an example of a CO2 laser
http://www.synrad.com/
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Wavelength examples...
•
A ruby laser is a solid-state laser and emits at a
wavelength of 694 nm. Other lasing mediums can
be selected based on the desired emission
wavelength (see table below), power needed, and
pulse duration. Some lasers are very powerful, such
as the CO2 laser, which can cut through steel. The
reason that the CO2 laser is so dangerous is because
it emits laser light in the infrared and microwave
region of the spectrum. Infrared radiation is heat,
and this laser basically melts through whatever it is
focused upon. Other lasers, such as diode lasers,
are very weak and are used in today’s pocket laser
pointers. These lasers typically emit a red beam of
light that has a wavelength between 630 nm and
680 nm. Lasers are utilised in industry and research
to do many things, including using intense laser
light to excite other molecules to observe what
happens to them.
Laser Type
Wavelength (nm)
Argon fluoride (UV)
193
Krypton fluoride (UV)
248
Xenon chloride (UV)
308
Nitrogen (UV)
Argon (blue)
Argon (green)
337
488
514
Helium neon (green)
543
Helium neon (red)
Rhodamine 6G dye
(tunable)
633
570-650
Ruby (CrAlO3) (red)
694
Nd:Yag (NIR)
1064
Carbon dioxide (FIR)
10600
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Quick Summary Check
1) What is stimulated emission?
2) Why are lasers more powerful than normal light?
3) Why is a two level laser inefficient compared to 3 /4
system
4) Give a low power example, medium power example and a
high power example.
5) Give some examples of “lasing” mediums
6) What is the formulae that you can use to work out the
wavelength of a emitted light. Can you give any examples
(see the textbook)
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Summary
E = hf
P = nhf
n = number of
photons arriving per
second
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Obi Wan or Obi Non!
• What a great use for a LASER
• Great idea for cutting metals,
fighting and generally chopping
up any undesirables!
• However, what the Jedi Knights
did not reckon on is E = hf.
TASK
• Can you describe using a
Quantum Physics explanation
why this is a load of “Hoki
Magic” and what would happen
if light could behave this way?
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Applications of Lasers
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Applications of Lasers II - Coms
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Laser Diffraction!
Model = 1 x 10-6m
Experimental 5.2 x 10-6m
• Can you work out the spacing of
lines on a CD if light is 632.8
nanometres. (0.5 to 0.9mW Power)
L = 229.5cm
Opp =
55.5cm
57.1cm
58.3cm
n
d
sin 
opp
 t an
adj
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