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Introduction to Biomedical Optics
Lecture 3
Laser Princi le and Technolo
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LASER: Light Amplification by Stimulated
Spontaneous emission
is incoherent.
and has the same frequency as
the incoming light.
Stimulated emission is negligible
compared to stimulated absorption.
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Light Amplification by Population Inversion
Energy levelsF F+dF
)NF(Ndz
dF
mnnm
=
m
dz
)NF(Ndz
dFnmnm =
n m,
Nn< Nm, stimulated emissionAn active medium of length
dzamplifies the incoming
photon flux from F to F + dF
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Lasing Medium: Three-level System
Three-level energyNon-radiative
4F14F2
ecay
2E
Pump
Laser at
694.3 nm
A three-level system in a Ruby laser
ro ems o t ree- eve systems:
Requiring a significant external energy because the lower level is the ground levelMust be operated in pulsed mode because the population inversion is difficult to sustain
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Lasing Medium: Four-level System
Four-level energy
diagram
Non-radiative
decay
4
Pump Laser at
1064 nm
4I
A four-level system in a
neod mium Nd:YAG laser
Lower excitation power Can be operated in a continuous mode
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Basic Properties of Lasers
Coherent
Can be Monochromic
Directional
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Lasin medium
Gas, Dye, Semiconductor
Pumping source
Optical pumping, Electric pumping
Cavity
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Schematic of a Laser Cavit
High reflection mirror Output coupler
Output
Lasing medium
beam
Pumping source
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Pumping Source: Optical Pumping
TsunamiTi:sapphire Laser
MillenniaNd: YVO4 laser
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Pumping Source: Optical Pumping
Excitation at 532 nm from a
4
Emission 700 nm to 1000 nm
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Pumping Source: Electrical Pumping
LBO
doubling
crystalOutput
beamTi:sapphire
Nd: YVO4Diode Diode
mediumpump pump
-
or Verdi (Coherent Inc)
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Pumping Source: Electrical Pumping in a
Output
Note: Electrical pumping is also used in gas lasers.
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Laser Cavity: Longitudinal Mode
=
N: an integer
: cav y eng
: wavelength
f= c / 2L
~ z or = cm
f ~ 500 MHz for L = 30 cm
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Selecting a Single Longitudinal Mode with
ong u na mo e
selection using a
transmission Fabr -
Perot etalon.
Configuration ofetalon
longitudinal modeselection
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Laser Cavity: Transverse Mode
TEM (Transfer, Electric,
and Magnetic modes)
TEM (mn): m and nindicate the number of
minima in two
perpendicular direction
A beam with a TEM(00)
mode is called a Gaussian
eam
A TEM(00) beam can be
focused to the diffraction
limits
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Specifications of Some Lasers
Wavelength Laser medium Excitation
CO2 laser 10.6 m
400 nm to 1.9 m
CO2 gas
Semiconductors
Electrical
Nd:YAG laser 1064 nm Nd ions in a crystal
of YAG
Diode laser
Dye laser
Ar on-Ion laser
400 to 800 nm
488 nm / 514.5 nm
Dyes
Argon gas at 1 torr
Ar-ion laser
Electrical
Ti:sapphire laser
/ 453 nm
690 to 1000 nm
pressure
Ti3+ ions in a Frequency
2 3
Nd:YAG laser
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Paraxial Description of a Gaussian Beam
zw0
( ) ( ) ( )
+ zRkzkzizww /2
2/20
( )zw0,
( ) ( ) waistBeam,/1/
rangeRayleigh,2/z
ocusa ewa seam,. ..
2
00
00
0
zzwzw
kw
w
+=
=
=
( )
( ) ( ) shiftphaseGouy,/tan
radiusWavefront,/
0
1
2
0
zzz
zzzzR
=
+=
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Continuous Wave (CW) and Pulse
CW operation
Single or multi longitudinal mode
Pulse operation
Q-switch: intense nanosecond pulsescous o-op c -sw c
Electro-optic Q-switch
Mode-locking: femtosecond (10
15 s) orpicosecond (1012s ) pulses
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Q (quality)-Switch Operation
Electro-o tic Q-switch 5-20 ns
Pockels Cell
Acoustic-optic Q-switch 100 ns
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Mode-locking of Longitudinal Modes
If we lock thelongitudinal modes
0 + n oge er in phase, we have
0
1/ 2( )
0
1 / 2
( )N
i n t
N
E t I e
+
=
0
0
sin( / 2)
sin( / 2)
i t N tI e
t
=
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Mode-locking Generate Ultrashort Pulse
2
0 2
sin ( / 2)( )
N tI t I
Pulse width
~ 2 / (N )
N2I0
Average Intensity
N I0
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Repetition Rate: Number of
The time separation ( t) between adjacent
.
t = 2 n
=
t = 2 / = 2L /c
Repetition rate = 1/ t
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Compensation of Group Velocity
Dis ersion Gires-Tournois Interferometer (GTI) for picosecond
Prisms for femtosecond lasers
Prism 2 GTI
Mira 900
em osecon
Coherent Inc Prism 1
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Mode-locking Using a Acoustic-Optic
Acoustic optic
mo u a or
operating at afrequency of c / 2L
Lok-to-clock locks
80 MHzGTI
PZT feedbackadjusts the cavity
length
Product of Spectra-Physics
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Measurement of fs or ps Pulse Widths by an
-
measured by
Nonlinear crystal
x or
1 m 3.33 fs
x
Intensity autocorrelation: =>