Laser

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Chapter 11

• Laser

June 8, 2005
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Brief review to the last lecture

• The production of x-rays

What is the nature of x-rays????,???
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These are the two ways of producing x-rays. How
does it work based on your knowledge? ???,???,
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The spectrum of X-rays
How does the continuous spectrum produced how
does the line spectrum produced?(???,???) . Why
do we get cut-off frequency? (???, ???)
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• The basic properties of x-rays
• Ionizing function,
• Fluorescence function,
• Actinic (??) function,
• biological effect,
• high penetration capability

• What does the intensity and hardness of x-rays
  depend on? (???, ???)

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• The attenuation of x-rays

x is the thickness and ? is the attenuation
coefficient.
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• Applications of x-rays to medicine
• Treatment,
• Diagnosis,
• Digital subtraction angiography (????????).
• X-CT, CT Computerized Tomography (X???????)

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11.1 The fundamental principles of laser

• Laser
• LASER is an acronym for light amplification by
  stimulated emission of radiation, a
  monochromatic, coherent light beam.
• Atomic Energy levels
• Ground state, Excited states, under thermal
  equilibrium, most atoms are in their ground
  states at room temperature.

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• Lifetime
• Excited states (10-9 10-7 second).
• metastable state (10-3 10-2 second)
• Radiation transition
• There are three interaction processes between
  atoms and radiation. They are absorption,
  spontaneous emission and stimulated emission.

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Ee

1. Absorption

h?
Eg
(2) Spontaneous emission same frequency, not the
same direction, not the same phase, not coherent
light, not amplified!
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Fig.10.1 Three interaction processes between an
atom and a radiation. (a) absorption, (b)
spontaneous emission, (c) stimulated emission.
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(3) stimulated emission This idea was first
proposed by Einstein, shown schematically in
Fig.10.1c. Stimulated emission takes place when
a photon encounters an excited atom and forces
it to emit another photon of the same frequency,
in the same direction, and in the same phase.
The two photons go off together as coherent
radiation. So it is amplified!
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• Maxwell-Boltzman distribution law
• Consider a large number of atoms
• No radiation, thermal equilibrium
• most in the ground state no and few in the
  excited state nE
• Distributions are based on the Maxwell- Boltzman
  distribution law.

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For example, Neon, at room temperature, on 3S
and 1S states
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• Population inversion

EE

• Strong beam of light irradiate on the atoms
• making nE gt n0

h?
Eg
Not easy, as both the absorption of normal atoms
and the stimulated emission of excited atoms
exist! This is like a two-way street! Metastable
state!
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The necessary population inversion can be
achieved in a variety of ways. As an example, we
consider the helium-neon (?-?) laser, a simple,
inexpensive laser available in many universities.
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• Helium-neon Laser machine
• 7 helium to 1 neon atoms (Z10) at a low pressure
  of the order of 10(-3) atm are sealed in a glass
  tube provided with two electrodes. When a
  sufficiently high voltage is applied, a glow
  discharge (????) occurs. Collisions between
  ionized atoms and electrons carrying the
  discharge current (????) EXCITE atoms to various
  energy states.

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selection rules of spectrum So 1s2s ? 1s2
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Helium atoms are excited to the 1s2s state which
is a metastable state (???). The helium atom can
lose energy by collisions with neon atoms
initially in the ground state.
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Helium (1s2s, 20.61eV) kinetic energy ? Neon
(5s, 20.66 eV)
? population inversion in neon, 5s ? 3p. ?
coherent light with wavelength 632.8nm,
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• Optical resonator

Semi-transparent
Mirror
Harmonic resonance
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11.2 The characteristics of laser
(1) Good directionality. Laser is almost
perfectly parallel. This means that it has good
directionality. For the same power, normal light
can travel very small distance before it is
diverted but laser could travel very large
distance almost without diversion, the distance
between the moon and the earth, for example.
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(2) High brightness and high intensity. Laser
is the brightest light source in the world now.
its brightness (40W for example) can be 1011
times higher than the normal light. (3) Good
monochromatic This factor is determined by the
width of spectrum (10-8nm) . This characteristic
is very useful in medical treatment because the
biological effect is strongly related to the
wavelength of light.
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• Good coherence (???)
• Time coherence Coherent light are emitted in
  the same place but in different time with
  interval ?c. ?c is called coherent time and
• Lc c ?c
• is called coherent length. It is known that from
  wave optics the coherent time is the continuous
  emitting time of the light (about 10-8s).

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For stimulated emission, it is the average
life-span of the atom in the metastable state.
For normal lights, the coherent length is less
100cm and for laser it can be tens or hundreds of
kilometers.
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Spatial coherence It is defined that the phase
relations of the two beams of lights emitted from
two different places do not vary with time. This
is called the spatial coherence of light. This
concept can be extended to the coherent area.
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To understand this characteristic, we recall the
simple double-slit interference experiment. A
mercury arc (??) placed directly behind the
double-slit would not give rise to interference
fringes because the light issuing from the two
slits would come from different parts of arc and
would not retain a constant phase relationship.
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In the use of the usual laboratory arc-lamp
sources, it is necessary to use the light from a
very small portion of the source to illuminate
(not eliminate) the double slit. The slightly
diverging beam from a laser, however, may be
allowed to fall directly on a double slit because
the light rays from any two points of a cross
section are in phase, and are said to exhibit
spatial coherence.
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• (5) Good polarization (??).
• As different material allows different light
  with specially polarized light to penetrate, the
  polarization of light is also quite useful in
  experiments and applications to human life.
• Comparison Laser and normal light.

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11.3 Laser applications to medicine
1. The biological functions of laser The
biological functions (????) of laser contain a
lot of useful aspects in medical treatment. These
functions can be summarized as heating,
mechanical, actinic (??), electromagnetic field
and biological stimulation.
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• Heating or thermal function Biological
  tissues can be heated under the irradiation of
  laser. Its temperature will get higher. These
  functions can be controlled by doctors in order
  to do some treatments to patients.
• Mechanical function Biological system can be
  evaporated and produces mechanical waves by the
  laser energy.

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• Actinic (??) function Big biological molecules
  absorbing laser photon could be stimulated and
  cause a series of chemical reactions together
  with biological tissues. This phenomenon is
  called actinic reaction it has two laws
  (absorption law and quantum law) .
• Electromagnetic function
• biological stimulating function

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2. Laser applications to the fundamental studies
of medicine On the other hand, laser is also
useful for the fundamental studies of medicine.
As laser has some special functions to the
biological molecules, cells and tissues, you
could use it to do a lot of studies about the
treatment to patients.
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3. Clinical applications of Laser (1) Laser
diagnosis methods laser spectroscopic analysis
method laser interference analysis method
Laser scattering analysis method Laser
diffraction analysis method laser transmission
analysis method laser polarization method.
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(2) Laser treatment methods laser
operation, weak laser treatment which
includes laser physical therapy (??), laser
acupuncture (??), laser photon dynamical
treatment and laser endoscope (??) treatment.
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4. Medical laser lots of lasers have been
obtained and they are used in different purpose
according to their properties. Most of these
lasers and their functions are given in your text
book.
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• Safety protection against laser
• laser can cause harmful effect to human beings.
  It can damage eyes, skin, nerve system and
  internal organs. The protection method can be
  obtained from your book and proper training
  should be given before hands.

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Example 1 (a) if a laser emits 10 Joules of
energy in a pulse lasting 5 ? 10-11 s, what power
is emitted? () (b) What is the intensity of the
beam if it is 2 ? 10-6 m2 in area?
Solution (a) The power is defined as the amount
of energy emitted per unit time. So, we can
obtain that
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(b) The intensity is defined as the amount of
energies transmitted per unit time per unit area
()
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Example 2 A ruby laser emits light at 693.4 nm.
If the energy released in each 10-11 second pulse
is 0.1 J, how many photons are there in the
pulse? ()
Solution (1) calculate one photon energy
(2) total energy / one photon energy
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),???????????,????????????????X????????,??????????
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Example 3. Calculate the minimum wavelength x-ray
that can be produced when a target is struck by
an electron that has been accelerated through a
potential difference of 15.0 kV ()