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8/2/2019 11623 Dye Lasers
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Dyelasers
Dye solutions/dyes doped in solid matrix(polymers / organic molecules)Liquid lasers/dye doped solid state lasers
Large size organic molecules
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Energy levels (simplified model)
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Absorption and
emissionDye absorptionDye emission
Green dye absorption
Yellow dye emission
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Pumps
1. Flash lamp2. Lasers( N2, excimer, Nd, ion
lasers, Cu vapour etc)
Pulsed systems/ CW systemsPhoto-degradation; dye
“degrades” on absorption of
light;limits the life of the dye
Continuous circulation for CW
lasers
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Flash lamp pumped dyelaser
Linear/coaxial flash lamp pumped laser, dye solution is made to flow; in low
power dye circulation is under gravity while high power sources it is pumps
are usedAverage power ~ several KW
Pulse energy ~ 100J/10Hz
UV-IR, 1GHz line-width
1-5000µs pulse widths, 1ps pulses have been produced
1-2% wall-plug efficiency
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Schematics of a flash lamp pumped dyelaser
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Laser output of flash lamp dye lasers
Linear/coaxial flash lamp pumped
laser, dye solution is made to flow; inlow power dye circulation is undergravity while high power sources it is
pumps are usedAverage power ~ several KW
Pulse energy ~ 100J/10Hz
UV-IR, 1GHz line-width
1-5000µs pulse widths, 1ps pulseshave been produced
1-2% wall-plug efficiency
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Typical pulsed laser pumped dye laser
Tuningelement
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Schematics of a laser pumped pulseddye laser
Withmagnetic
stirrer
Tuningelement
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N2 laser pumped dye laser
N2 LASER
CYLINDRICAL
LENS
DYE
CELL
OUTPUT
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N2 laser pumped dye laser; ANNIMATION
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Schematics of laser pumped CW dyelaser (linear cavity)
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Dye doped solid statelasers
Rd6G doped LAP
developed at IIT Kanpur
PMMA,MPG (polymers), silicate glass,
phosphate glass, silica gel etc
available commercially
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Q-switching
Quality factor “Q” of a resonator is
the measure of the amount of energythat can be stored in the resonator. Ahigh Q factor corresponds to lowresonator losses per roundtrip, andvice versa.
Q=-ωε/(dε/dt), where ε is the storedenergy; in case of a laser resonator
energy is stored in the form of “population inversion density”.
Q=ν/δν(1/2)
In lasers we have Q-switching; a
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Q-switching (Contd)
Initially the laser medium is pumped while the Q-switchprevents feedback of light into the gain medium
(producing an optical resonator with low Q). Thisproduces a population inversion, but laser operationcannot yet occur since there is no feedback from theresonator. Since the rate of stimulated emission isdependent on the amount of light entering the medium,
the amount of energy stored in the gain mediumincreases as the medium is pumped. Due to losses fromspontaneous emission and other processes, after acertain time the stored energy will reach somemaximum level; the medium is said to be gain
saturated . At this point, the Q-switch device is quicklychanged from low to high Q, allowing feedback and theprocess of optical amplification by stimulated emissionto begin. Because of the large amount of energy alreadystored in the gain medium, the intensity of light in thelaser resonator builds up very quickly; this also causesthe energy stored in the medium to be depleted almost
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Q-switching techniques
1. Mechanical methods
a.Rotating resonator mirror
b. Mechanical chopper inside lasercavity
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Q-switching techniques (contd.)
Saturable absorber (absorption
changes with intensity)
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Q-switching techniques (contd.)
Electro-optic/magneto-optic/acousto-optic switches
Refractive index is modulated byapplication of voltage / acoustic wave
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Acousto-optic Q-switch
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Electro-optic Q-switch
Polarization is changed onapplication of voltage, so the lasercavity has high loss(low Q). Suddenlyvoltage is switched-off and Q of
cavity is high and a laser output inthe form of pulse is observed
Q-switching increases peak power.What about the average power?
Does it also increase? Think
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Mode-locking
Output of a windows
The output is viewed in a planperpendicular to laser-axis (line joiningthe resonator mirrors through the gain
medium); transverse laser modes
Output of a laserwhich hascylindricalsymmetry
Out-put of a laser
with Brewsterwindows