Download How lasers work Simulated emission Population Inversion The laser

L 36 Modern Physics [2]
z How
How lasers work
• First we must understand
the difference between
incoherent and
coherent radiation
• Ordinary light sources
(light bulbs, fluorescent
lights, etc) produce
incoherent light
• lasers produce coherent
lightÆ all atoms radiate
in the same manner
lasers work
Medical applications of lasers
z Applications of high power lasers
z
z Medical
imaging techniques
CAT scans
z MRI’s
z
Spontaneous vs Stimulated Emission
• Coherent radiation is
produced when an atom
undergoes stimulated
emission.
• Spontaneous emission occurs
when an electron makes an
unprovoked transition to a
lower energy level
• Stimulated emission occurs
when an incoming photon
induces the electron to
change energy levelsÆ
amplification
Ei (larger energy)
photon
Ef (smaller energy)
Spontaneous emission
Incoming
photon
Stimulated emission
Population Inversion
• In a normal situation (a)
more atoms are in the
lower state than the
upper state
• If an external energy
source is provided to
excite electrons into a
higher energy state, a
population inversion can
be created as in (b)
• this is called pumping
Simulated emission
• With stimulated emission, one photon
comes in and its vibrations cause the
electron to fall to the lower energy level,
emitting another photon (the bridge)
• Thus, one photon goes in and 2 come out,
moving in step and in the same direction
• To get a system of atoms to undergo lasing
action, we must arrange to have many
atoms in the excited state Æ this is called
population inversion.
The laser medium
• The atoms are pumped
into an excited state
• The excited electrons
fall into an intermediate
state (upper laser state)
and stay there until a
photon comes along
and causes them to fall
down to the lower laser
state.
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He Ne Laser (633 nm red)
• A HeNe laser is a gas laser which uses a
medium of 15% Helium and 85% Neon.
• A high voltage discharge is produced in this gas
mixture and this produces the population
inversion.
• The lasing action starts when one atom emits a
photon which then induces another atom to emit
and so on.
• The partially silvered mirror keeps most of the
photons in.
• Photons which are not moving horizontally do no
become part of the laser beam.
Medical Applications of Lasers
Laser surgery to correct for
(a) nearsightedness, and
(b) farsightedness
A Helium-Neon (HeNe) Laser
Applications of High Power Lasers
Using lasers to
Cut metals
Laser
Fusion
Multiple beams of a powerful laser are focused
on a tiny pellet containing fusion fuel. The
laser energy compresses the pellet producing
a mini-hydrogen bomb that produces energy
pellet
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Lasers Diodes
Solid State Laser Diodes
small
Come in a variety of
different colors
Medical Imaging Techniques
• x-rays
• CT and CAT scans (Computerized
Tomography)
• MRI’s (Magnetic Resonance Imaging)
A pineapple and a bananna
• A shadow image can be
misleading
• two shadows taken from
different angles provides
a better picture
• shadows taken at
multiple angles gives a
more complete picture
• this is what a CT or CAT
scan does
• Diode lasers use semiconductor materials
(tiny chips of silicon) as the lasing media
• When current flows through the silicon
chip it emits an intense beam of coherent
light.
• Diode lasers are used to read the
information embedded in the pits in CD’s
and DVD’s, and also to read UPC’s in bar
code scanners and in laser pointers!
X-rays
• very short wavelength
(0.01 – 0.1 nm)
electromagnetic waves
• produced when
energetic electrons
slam into a metal target
• able to penetrate soft
tissue, but not bone
• produces a two
dimensional shadow
image
CAT Scans
X ray images are taken at many different angles
passing through the patient. Some of the cuts
overlap. A full three dimensional image can be
reconstructed using computers.
Æ this procedure is called tomography.
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Computerized Tomography
• A computerized tomography
or CT scan image is formed
by analyzing x-ray shadow
images taken at many
different angles and
positions
• an x-ray source and an array
of electronic detectors
rotates around the patient as
the patient slowly moves
through the ring.
MRI- how does it work?
• MRI works by locating the hydrogen atoms
inside the body. Since the body is mostly
water, there are lots of hydrogen atoms
• the nucleus of a hydrogen atom is a single
protons. Protons behave like tiny bar
magnets with a north pole at one end and
a south pole at the other end.
• If you put a bar magnet in a magnetic field,
it will try to align itself with the field.
Magnetic Resonance Imaging
• The rules of atomic physics (quantum
mechanics) require that the atomic
hydrogen bar magnets can only have 2
orientations when placed in a magnetic
field Æ either parallel or antiparallel to it,
we call this spin-up or spin-down
magnetic
field
spin-down
spin-up
Is there a better medical diagnostic?
• A CAT scan does a good job of imaging
bones, but it does not provide as good an
image of soft tissue
• Also, it requires that the patient receives a
big dose of x-rays, which can be harmful in
themselves Æ it is an invasive diagnostic
• Magnetic resonance imaging (MRI) is a
better method of imaging soft tissue
A bar magnet in a magnetic field
magnetic field
N
N
S
S
Solenoid for producing a strong
magnetic field by passing a large current
through a set of coils
Magnetic Resonance
• Protons has a “spin” that can be either
“up” or “down” relative to the direction of
the magnetic field
• If radio waves (FM) hit the protons, it can
cause it them to flip from one spin state to
the other at a frequency that depends on
the strength of the magnetic field
• These spin flips result in the absorption or
release of radio wave energy that can be
detected electronically
protons
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Magnetic Resonance Imaging
• In effect, the magnetic field makes the
protons act like tiny radio transmitters that
only broadcast their signal when the value
of the magnetic field is just right
• By varying the strength of the magnetic
field as a function of position in the body,
the spin flips can be detected in various
parts of the body
• A computer is used to combine the signals
from various parts of the body to generate
detailed cross-sectional images
MRI DEVICE
Homer
MRI safety considerations
• The magnetic field used in MRI are very
strong – 30,000 times the strength of the
earth’s magnetic field.
• Because the magnet coils are cooled to
liquid helium temperatures, they are
usually kept on all the time
• Because the magnetic field is on, all iron
and steel objects must not be allowed to
enter the room.
http://www.mercola.com/2001/aug/15/mri.htm
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