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Introduction to Lasers Lecture 2
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INTRODUCTION LASER FUNDAMENTALS AND LASER BEAM
PROPERTIES Pumping Schemes
Two-Level Laser
2E
1E Suppose we try to increase N2 with strong light at h to
create a population inversion. This wont work!
. . . Pump Power
Net absorption, small here
2 12inv
1
g NN N -g
=
xx x2 1hh E -Ep = =
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Introduction to Lasers Lecture 2
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Three-Level Laser (Good Can Create Population Inversion)
3 Fast Decay
2 x x x
1 Example: Ruby Laser
Four-Level Laser (Better Easier to get a large inversion)
3 Fast Decay
2 x
1 Fast Decay 0 Example: Nd:YAG Laser
x
x
3 1
h= E -E
p2 1
h=E -E
x x03
h= E -E
p2 1h E E =
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Introduction to Lasers Lecture 2
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Quasi-Three-Level Laser (Also called a quasi-four level
laser)
~ kBT Example: Yb:YAG The lower lasing level is partially
occupied in thermal equilibrium
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Introduction to Lasers Lecture 2
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Properties of Laser Beams
Monochromaticity:
E2 h x h x x h
E1 Note: Cavity resonance further narrows the laser line
width
Example: Nd:YAG Laser 14=1.064 m, =2.810 Hz
11FWHM~3kHz (1 part in 10 !)
[100 msec window] [10 second window]
We see drift when we observe for a longer
time
Laser amplifies at: 2 1-E Eh
= but there is a finite spread for because of:
1) Finite upper-state lifetime 2) Interaction with the
surrounding environment
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Introduction to Lasers Lecture 2
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Temporal Coherence:
Spatial Coherence:
) ) ) ) ) ) 1 2 . . .
Causes Laser Speckle
Directionality: A result of the laser cavity.
We can define a phase front for a laser beam.
Mirror Mirror Diameter = D
2
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Introduction to Lasers Lecture 2
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The optical mode has finite extent to fit the mirrors.
Diffraction theory tells us that the beam diverges with an
angle
, 1D
= Brightness:
Brightness B cosdP
d dS
In words Power emitted in direction 00 per unit solid angle, per
unit area, taking into account the effective reduction in the
emitting surface due to tilt
Normal to Surface
dS
O
O
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Introduction to Lasers Lecture 2
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For a diffraction limited laser beam:
2
2B P
Very bright ! (Because of beam directionality) See homework
Short Pulses: Using techniques called Q-Switching and Mode Locking
we can make optical pulses of duration ~ 1-nsec 5-fsec (5 x 10-15
sec) Note: Emission of short pulses is a less general property of
lasers. All lasers can be made monochromatic (in principle),
but
1~pulsepulse
we can only make very short pulses with a broad gain
spectrum.
Pulse with only a few optical cycles
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Introduction to Lasers Lecture 2
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Laser Types:
Gas (e.g. HeNe, CO2), Liquid (dye), Solid-State (e.g. Nd:YAG,
Yb:YAG, Ruby, Ti:Sapphire), Fiber (a special case of solid-state
lasers), Semiconductor, Chemical (HF), Free-Electron, X-Ray
X-Ray ( ~1 ) to far infrared ( ~1 )nm mm
CW power ~ 1-mW (communications, data storage, laser
pointers) to ~ 100-kW (machining) to ~ 5MW (military)
Pulsed Power to ~ 1015 W
Pulse Length as short as ~ 5-fsec
Cavity Length ~ 1m (VCSEL) to 6.5-km
