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8/11/2019 Modes of Laser Operation
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Laser Engineering
A.K. NathDate: 10-08-2011
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Modes of Operation of Laser
* Continuous Wave (CW)
* Pulsed Mode
*
* Mode-locked + Pulse Compression
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Time
Free Running Laser
Relaxation Oscillation
Pump duration is longerthan the Cavity Decay Time
Laser pulses= 10s s spikeson ms pulse envelop
Pump Pulse
Gain
LoseLine
LaserPulse
Total pulseduration= 0.1-20ms
dN/dt = .N0 I . .N /h N/t 2
dI/dt = I. .N .c I/t c
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Methods of Pulsed Laser OperationQ-Switched Laser
Q- Quality Factor of Optical Resonator High-Q: Low Cavity Loss Low Q: High Cavity Loss Switching from Low Q (High Cavity
Loss) to High-Q( Low Cavity Loss)
Q-switch involves* Preventing the laser from lasing until
pumping is over, and* Abruptly allowing the laser to lasewhen the population inversion is max.
Laser pulse rise time is limited by theswitching time and the peak powerdepends on the initial populationinversion N max .
Laser pulse fall time depends upon thecavity photon decay time, t c
Time
Q-SwitchedLaser Pulse
CavityLoss-
Curve
Q-Switched Laser Pulse: 1-100ns
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Electro-Optic Effect
Change in the optical properties of amaterial in response to an electric field
Refractive Index = f (E)
V /4
PlanePolarizedBeam
PlanePolarizedBeam
PlanePolarizedBeam
CircularlyPolarizedBeam
Two orthogonal polarized rays havedifferent velocities.Emerging rays have
phase difference Applied Voltage
V /4 /2 Phase differencePlane Polarized Beam Circularly Polarized Beam
V /2 Phase differencePlane Polarization rotates by 90 0
Round trip with V /4 VoltagePlane Polarization rotates by 90 0
Two Types of Electro-Optic Effects:
1. Pockels Effect: Change in refractiveindex n = n e no E Electric Field
Electro-optic MaterialsAmmonium dihydrogen phosphate (ADP)Potassium dihydrogen phosphate (KDP)Potassium dideuterium phosphate (KD *P)Lithium niobate (LN)
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2. Kerr effect : Change in refractive index n E2
Refractive Index in presence of Intense Laser Beam n = n 0 +n 2.I
where n 2 - the second-order nonlinear refractive index,
I Laser Beam Intensity
The refractive index change is proportional to the intensity of the light travelingthrough the medium.
I n FocusingEffect
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Acousto- Optic Effect
Sound wave (series ofcompressions and
rarefactions ) travelingthrough a transparentmaterial, causes
periodic variations ofthe index of refraction.
Light beam travelingthrough the periodicvarying refractiveindex gets diffracted:Acousto- Optic Effect
Piezoelectric
Intensity of undeflected beamreduces i.e. loss is introduced
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Cavity Dumping:Laser Pulse Rise time in Q-Switched Laser:Switching time of Q-Switch device &Initial Population Inversion
Fall Time ~ 5-6 t c ; Several 10s nstc =Cavity Photon Decay Time= 1/c[a-(1/2L)lnR 1.R 2]
Q-SwitchedLaser pulse
Time
L a s e r
P o w e r
A-OSwitch
Cavity Dumping Process:With high Q, Laser Power isallowed to build to the maximum.At the maximum laser power Q isreduced to almost zero bydeflecting the beam either by E-Oor A-O Switch out of the lasercavity.Entire laser energy inside cavitycomes out in nearly single round
trip time rt , a few ns.
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Laser Action on MultipleLongitudinal Modes:
Longitudinal Modes inrandom phases
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Out of phase Out of phaseIn phase
LOCKED phases for all the laser modes
Out of phase
RANDOM phase for all the laser modesIrradiance vs. Time
Mode Locking:
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M ode L ocking
( 1) /2 ( 1) /2
( 1) /2 ( 1) /2exp ( ) ( )
N N
n m ax n mn N m N A A j n m t j t t
( 1) /2 ( 1) /2 ( 1)/2
2
( 1)/2 ( 1)/2exp exp ( ) ( )
N N N
n n m ax n mn N m n n N
A A A j n m t j t t
Random phases
M ul timode lasing
( 1)/ 2
0( 1)/ 2
( ) exp ( ) N
n ax n N
E t A j n j t
2
2ax
rt FSR
A
2
( 1) /220
( 1) /2( ) ~ ( ) exp ( )
N
n ax n N
S t E t A j n t j t
0
2
nnS A S
( ) 0n m j t e S
t
S
=2 .f = 2 . c
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L ocked phases0 0
( 1)/ 2
( 1) /2( ) ( )n ax
N j t j t j jn t
n e N
E t e A e e E t e
( 1) /2
( 1) / 2( ) n ax
N
j jn t e n N
E t A e e Equal ampli tudes and phases , 0n n A A
( 1) / 2( 1) / 2
( 1) / 2
sin( / 2)1( )
1 sin( / 2)
ax
ax ax
ax
jN t N jn t j N t ax
e j t N ax
N t e E t A e Ae A
e t
222
sin ( / 2)( ) ~sin ( / 2)
ax
ax
N t S t At
,min12
1~ p
M inimum pul se length p
trt = 2L/c
M ax. no, of modes Nmax ~ 12. rt
Peak Power = N 2.A2
Average Power = N.A 2
N- No. of L asing modes
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Mode-Locked Laser Several Longitudinal modes each
separated by c= c/2L can lase Usually they are in random-phases Laser output is incoherent
superimposition of intensities ofseveral lasing modes.
Locking the phases of all laserlongitudinal modes yield a train ofUltra-short laser pulses
Laser pulse duration is limited bylaser emission / gain bandwidth,
p 1/ s Laser pulses are separation the
round-trip time of the cavity
Random Phases
Phase LockedRandomPhase
RandomPhase
Round-trip timert= 2nL/c
Laser PulseDuration =
p ~ 1/ s
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Requirement for generating Short-duration Laser Pulse:
Large Gain Bandwidth
Mechanism for Mode-locking
Gain bandwidth of Different Lasers:
CO 2 Laser: 50MHz
He-Ne Laser : 1.5GHz
Nd:YAG Laser: 1200GHz p~ ps
Dye Laser: 40THz p~ 25fs
Ti-Sapphire Laser: 100THz p~ 10fs
Typical Cavity mode separationc = c/2L =3x10 10/ 30 = 10 9Hz = 1GHz
Mode-locking Techniques :
1. Saturable Absorber
2. Electro-optic Modulator
3. Acousto-optic Modulator
Modulators provide low-loss ata frequency = 1/ rt
M 1
M 2Laser
SaturableAbsorber
Polarizer EOM/AOM
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Active mode-locking: the electro-optic modulator
V
If V = 0, the pulse polarizationdoesnt change.
Laser pulse builds up andoutput pulse comes out
If V = V /4, the pulse plane polarization switches to circular polarization & inreturn pass it becomes plane polarized, but 90 rotated.
Polarizer deflects the pulse out of optic axis- thus introduces loss
Applying a sinusoidal voltage yields sinusoidal modulation to cavity loss and whenvoltage is zero, loss is minimum and laser pulse comes out.
Modulation frequency, fm = 1/t rt , trt = Round trip time = 2nL/c, n = Av. refractive index
Pockels cell
Polarizer
, trt = Round trip
time = 2nL/c Lasermedium
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Active mode-locking: the acousto-optic modulator
Sinusoidally modulating the acoustic wave amplitude at round-tripfrequency yields mode-locked laser pulses.
Quartz DiffractedBeam (Loss)
Acoustictransducer
Pressure, density, and refractive-index variations due to acoustic wave
Output
beamLasermedium
, t rt = Round trip
time = 2nL/c
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Laser Mode Locking with Kerr Lens
Kerr Lens
Mode-locking
HR
PR
Laser Medium
Pump Aperture
WithoutMode-locking
With
Mode-locking
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GVD Compensation
GVD can be compensated if optical pathlength is different for blue and red components of the pulse.
0
R B
R b
If OR + RR > OB, GVD < 0
Diffraction grating compensator Prism compensator
Wavelengthtuning mask
Red component of the pulse propagates in glass more than theblue one and has longer optical
path (n x L).
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Pump
HR
GainOC
Mode-locking
Mechanism
Dispersion
Compensation
Components of ultrafast laser system
Kerr LensMode-locking
Ti-Sapphire:Kerr Medium
v
v
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Typical fs Oscillator
GVDC
Active medium(Also Kerr medium) From the pump
laser
Wavelengthtuning mask
Typical Ti: Sapphire fs Oscillator Layout Tuning range 690-1050 nm Pulse duration > 5 fs (typically
50 -100 fs) Pulse energy < 10 nJ Repetition rate 40 1000 MHz
(determined by the cavity length) Pump source:
Ar-ion laser (488+514 nm)DPSS CW YAG laser (532 nm)
Typical applications:
time-resolved emission studies,multi-photon absorption spectroscopyand imaging
O. Zvelto, Principles of lasers, Plenum, NY (2004)
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Amplification of fs Pulses
Oscillator Stretcher Amplifier Compressor
Stretch femtosecond oscillator pulse by ~100 times Pulse stretched exploiting frequency dispersion is called Chirped Pulse Amplify Recompress amplified pulse
Concept:
Issue-2: Shorter pulse High Peak power Damage of Laser Medium
Due to high intensity, fs pulses can not be amplified as is.
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Femtosceond Laser: Chirped pulse amplification
High Precision Alignment ofoptical componentsHigh cost
Expert for maintenance
Short Laserpulse: Oscillator
Energy=mJ/pulse10-100kHzA few Watts
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Summary:
*
* Mode-locking + Pulse Compression and Amplification