Principles of Lasers

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Principles of Lasers
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The Nobel prize in physics 1964
Nicolay G. Basov (1922 - 2001)
Aleksandr M. Prokhorov (1916 2002)
Charles H. Townes (b. 1915)
For fundamental work in the field of quantum
electronics, which has led to the construction of
oscillators and amplifiers based on the
maser-laser principle."
http//www.nobel.se/physics/laureates/1964/index.h
tml
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Schawlow and Townes

• Arthur L. Schawlow and Charles H. Townes invent
  the laser, then publish "Infrared and Optical
  Masers" in the American Physical Society's
  Physical Review. The paper describes the basic
  principles of the LASER (Light Amplification by
  Stimulated Emission of Radiation), initiating
  this new scientific field.

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1960 The first laser
In 1960, the race to build the first laser was
red hot. Bell Lab's Arthur L. Schawlow and
Charles H. Townes had, two years previously,
published their theoretical paper, "Infrared and
optical masers," but no one had yet built a
working model. Theodore Ted Maimans laserthe
worlds first working laserrepresented a major
breakthrough in the field of applied physics.
Today, the amplification of light resulting from
a laser is used for a variety of activities
ranging from surgery to weaponry.
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Historical horizon

• 1917 stimulated emission has been postulated
• 1928 stimulated emission has been experimentally
  verified
• 1950's work on light generation and amplification
• 1954 first Maser (C. H. Townes, J. P. Gordon, and
  H. J. Zeiger) based on NH3 (ammonia molecule)
• 1958 Ideas have been applied to the optical
  spectral region
• 1960 first laser (Ruby)
• 1961 first gas laser (Helium Neon laser)
• Why maser before laser?
• microwave technology was well developed (II. WW)
• investigated optical systems had only small
  amplification

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Basic picture of a laser

• A laser is an optical oscillator. It consists
  essentially of an amplifying medium placed inside
  a suitable optical resonator or cavity that
  provides positive feedback.
• The laser oscillation can be described as a
  standing wave in a cavity. The output consists of
  an intense beam of highly monochromatic radiation.

Amplifying medium
Output
Mirror
Mirror
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Small signal gain G0
z
from last time

• For a gain medium of length l this equation
  becomes

need population inversion
stimulated emission cross section
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Population inversion

• Can be achieved using
• Optical pumping or photon excitation
• Ruby laser (3 level system)
• dye laser
• Inelastic atom-atom collisions
• A B ? A B
• HeNe laser (4 level system)

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Population inversion

• Direct electron excitation
• Argon ion laser
• Injection of carriers in pn junction
• diode laser
• Chemical reactions
• Excimer laser

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Threshold requirements for a laser

• To increase the gain, it is not practical to make
  long lasers but rather use a pair of mirrors
  located at the ends of the gain medium
• At threshold, the pump power is sufficiently high
  that a population inversion is achieved and the
  net round trip gain is equal to the net round
  trip loss

pumping process
I0
I1
I3
I2
Laser gain medium
R ? 100
R lt 100
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Example Ruby laser
Population inversion through optical pumping 3
level system
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Laser operation
When the pumping is increased, a population
inversion occurs The first spontaneously emitted
photons stimulate a chain reaction One photon
triggers the rapid, in phase emission of another,
dumping energy from the metastable state into the
evolving lightwave The wave grows as it goes back
and forth across the active medium Light is
coupled out through the partial mirror
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Optical output of a laser as a function of
pumping power
Further increase in pump power provides
proportionally more atoms in upper laser level to
participate in stimulated emission and output
power increases linearly with pump power
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Erbium doped fiber amplifier (EDFA)

• Amplifier material traditional silicon fiber
  doped with Er3 rare earth ions to make an
  active medium for laser pumped gain
• Quasi three level system
• Pump energy provided by 980nm or 1480nm radiation

If we provide positive feedback, we can build a
laser using the EDFA as the gain medium!
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A ring laser
Losses due to transmission through output
coupler, absorption, scattering and reflection in
cavity
In steady state, the total loss associated with
one round trip is exactly compensated by the
single pass gain
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A ring laser
Mathematical treatment
straight line!
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Laser output power vs. pump power
straight line!
slope (SE)
intersect
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The Erbium fiber ring laser
use T0 20, 40, 60, 80 coupling ratios
go in steps of 3, 6, 9, 12 dB excess loss
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1. Small signal gain of gain medium
from this plot you can read off the small signal
gain for a certain pump power
You did this in the EDFA experiment, if not you
need to redo it, use -30dBm for signal laser
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2. Laser output vs. pump power
this is what you dial in with the attenuator
40 coupling ratio
determine slope efficiency SE
threshold, read off pump power 12mW and find
small signal gain from previous measurement
do this for T0 20, 40, 60, 80 output
coupling ratios
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3. Threshold gain vs. excess loss
read off y-intersect to give you the total round
trip intra cavity losses including Li and (1-T0),
use this to find Li
pump threshold increases with excess loss, such
that the small signal gain coefficient at
threshold exactly offsets that loss
Li intrinsic intra cavity loss T0 output
coupling ratio
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4. Slope efficiency vs. excess loss
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5. Laser output power vs. pump power
intra cavity loss 10dB
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Mirror cavity and resonant modes

• In most lasers, a mirror cavity is used to
  provide feedback. For low gain lasers, this
  feedback is critical to achieving the threshold
  pumping condition.
• The cavity resonates at discrete wavelengths.
  Standing waves are set up when an integral number
  of half-wavelengths occur between the mirrors
  (you need a node at each mirror)

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Mirror cavity and resonant modes
It is possible for light waves at these
frequencies to become strong enough to give
stimulated emission.
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Longitudinal laser cavity modes
Fabry-Perot supports large number of discrete
wavelengths called longitudinal or axial modes.
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Longitudinal laser cavity modes
This is the familiar Airy function we encountered
earlier.
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Laser operation above threshold

• The gain makes available a broad range of
  frequencies out of which the cavity selects and
  amplifies only certain narrow bands (modes)
• Only modes which experience a small signal gain
  greater than the threshold gain can lase

multimode operation
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Laser output and gain
In continuous wave (cw) operation, the gain for
these laser modes will saturate to equal the
threshold gain. Because this happens only at the
wavelength of the laser modes, we can observe
spectral hole burning. This phenomenon is not
observable in homogeneously broadened gain media.
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Single mode operation
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Transverse laser modes
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Laser spatial modes
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Laser spatial modes

• Also referred to as Transverse Electro-magnetic
  (TEM) modes, labeling according to the number of
  nodal lines in the x and y directions across the
  emerging beam.

Electric field
The 00 mode is the Gaussian beam no phase
shifts in E across the beam, spatially coherent,
angular divergence is smallest, can be focused to
the smallest spot size. Higher-order modes
involve multiplication of a Gaussian by a
Hermite polynomial.
Irradiance
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Mode patterns
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Mode patterns
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Suppression of transverse modes
Introduce an aperture into the cavity, to cut off
higher order transverse modes
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Properties of laser light

• Highly directional
• Light quanta that leave the active region are
  getting absorbed, whereas light quanta along the
  resonator axis are getting amplified
• Monochromatic
• Due to the mirror cavity resonance and the
  quantum condition on photon energy, the light is
  composed of only a very narrow range of
  wavelengths
• We can easily achieve Dllt10-1nm-10-6nm, stable
  lasers are specified in Hz, world record is
  0.16Hz (Jim Berquists group - NIST-Boulder)
• Coherent
• Due to the stimulated emission process, the light
  waves are all emitted in phase or in step rather
  than being emitted randomly
• Interferometric distance measurements over 50km
  and more are possible

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Spatial characteristics and focusing

• As a result of the high directionality (they are
  so called diffraction limited), they can be
  focused to a spot of dimension l ? ?
• ? incredible power densities can be reached
• By comparison, conventional sources, even when
  focused cannot yield a higher effective
  temperature that that of the source one cannot
  achieve a temperature higher than 6000K in
  focused sunlight regardless of the means of
  focusing
• For a 1MW laser, it is possible to obtain a power
  density of 1012W/cm2 for visible wavelengths
• Much higher densities have been achieved in
  projects to trigger nuclear reactions (NOVA
  laser, LLNL)