coherence factors that compromise coherence: 1. thermal fluctuations 2. vibrational fluctuations 3....

CoherenceCoherence

Factors that compromise coherence:Factors that compromise coherence:1. thermal fluctuations1. thermal fluctuations2. vibrational fluctuations2. vibrational fluctuations3. emission of multiple wavelengths3. emission of multiple wavelengths4. multiple longitudinal modes4. multiple longitudinal modes

Temporal Coherence – How long do the light waves remainTemporal Coherence – How long do the light waves remainin phase as they travel?in phase as they travel?

Coherence Length = Coherence Length = 22/n/n

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CoherenceCoherence

Spatial Coherence – Over what area does the light remainSpatial Coherence – Over what area does the light remainin phase?in phase?

www.wikipedia.org

Are you getting the conceptAre you getting the concept

Calculate the coherence length for the sources belowCalculate the coherence length for the sources belowusing nusing nairair = 1.00: = 1.00:

(a)(a)light bulb emitting from 400-1000 nmlight bulb emitting from 400-1000 nm(b)(b)semiconductor laser emitting from 799.5 – 800.5 nmsemiconductor laser emitting from 799.5 – 800.5 nm(c)(c)He-Ne laser emitting from 632.799 – 632.801 nmHe-Ne laser emitting from 632.799 – 632.801 nm

Laser WavelengthsLaser Wavelengths

Factors influencing monochromaticity of laser light:Factors influencing monochromaticity of laser light:1. transitions responsible for emission1. transitions responsible for emission2. nature of transition determines bandwidth2. nature of transition determines bandwidth3. resonance cavity characteristics3. resonance cavity characteristics

Doppler bandwidth:Doppler bandwidth:

= [5.545 kT/Mc= [5.545 kT/Mc22]]½½ where M is the mass of the atom/moleculewhere M is the mass of the atom/molecule

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Limiting Emitted Limiting Emitted s with a Fabry-Perot Etalons with a Fabry-Perot Etalon

Insert a pair of reflective surfaces that form a resonant cavityInsert a pair of reflective surfaces that form a resonant cavitytilted at an angle to the axis of the laser medium.tilted at an angle to the axis of the laser medium.

www.wikipedia.org

TransmittedTransmitted depends on: depends on:1.1. the angle the light travels through the etalon (the angle the light travels through the etalon ())2.2. the thickness of the etalon (l)the thickness of the etalon (l)3.3. the refractive index of the material between the 2 surfaces (n)the refractive index of the material between the 2 surfaces (n)

Emission ModeEmission Mode

Lasers can emit light in Lasers can emit light in continuous wave (cw)continuous wave (cw) mode or they can mode or they canproduce produce pulsespulses..

Heisenberg’s Uncertainty Principle places the limitations:Heisenberg’s Uncertainty Principle places the limitations:

Bandwidth (Hz) = 0.441/Pulse Length (s)Bandwidth (Hz) = 0.441/Pulse Length (s)

EEt t ≥≥ ħħ/2/2Consequences:Consequences:Long pulse – narrow bandwidthLong pulse – narrow bandwidthShort pulse – broad bandwidthShort pulse – broad bandwidth

Long pulse – high resolutionLong pulse – high resolutionShort pulse – low resolutionShort pulse – low resolution

Are you getting the concept?Are you getting the concept?

Calculate the minimum pulse length for a laser with a 1-nmCalculate the minimum pulse length for a laser with a 1-nmemission bandwidth at a center wavelength of 500 nm.emission bandwidth at a center wavelength of 500 nm.

Are you getting the concept?Are you getting the concept?

Calculate the best spectral resolution (in cmCalculate the best spectral resolution (in cm-1-1) that can be) that can beachieved with a pulse length of 368 fsec.achieved with a pulse length of 368 fsec.

Output PowerOutput Power

Output power will depend on:Output power will depend on:1.1. variations in power level with timevariations in power level with time2.2. efficiency of converting excitation energy into laser energyefficiency of converting excitation energy into laser energy3.3. excitation methodexcitation method4.4. laser sizelaser size

What is wall-plug efficiency?What is wall-plug efficiency?A practical measurement of how much energy put into theA practical measurement of how much energy put into thelaser system (from the wall plug) comes out in the laser beam.laser system (from the wall plug) comes out in the laser beam.

Active Mediumpowersupply

Pulsed Laser Power ConsiderationsPulsed Laser Power Considerations

Consider a Gaussian beam profile:Consider a Gaussian beam profile:

Pea

k P

ower

FWHM

RiseTime

FallTime

Pow

er

Time

If power was constant: E = PtIf power was constant: E = PtIn this case, E = In this case, E = ∫P(t)dt∫P(t)dt

Average Power = Average Power = ΣΣE/t or Peak Power x Duty CycleE/t or Peak Power x Duty CycleDuty cycle = Pulse Length x Repetition RateDuty cycle = Pulse Length x Repetition Rate

Controlling Laser Pulse CharacteristicsControlling Laser Pulse Characteristics

There are 3 primary methods to control laser pulse time:There are 3 primary methods to control laser pulse time:

Q Switched Lasers – cavity mirrors are temporarily unavailableQ Switched Lasers – cavity mirrors are temporarily unavailableso the laser medium stores energy rather than releasing it. so the laser medium stores energy rather than releasing it. When the mirror is made available, a high energy pulse isWhen the mirror is made available, a high energy pulse isreleased. released.

Cavity dumped lasers – an extra cavity mirror momentarilyCavity dumped lasers – an extra cavity mirror momentarilydiverts photons from a fully reflective cavity after photon energydiverts photons from a fully reflective cavity after photon energyhas accumulated for awhilehas accumulated for awhile

Modelocked lasers – “lock” together multiple longitudinal modesModelocked lasers – “lock” together multiple longitudinal modesso that a laser simultaneously oscillates on all of them to emit so that a laser simultaneously oscillates on all of them to emit very short pulsesvery short pulses

Q-SwitchingQ-Switching

Build up population inversion by preventing lasing while pumping.Build up population inversion by preventing lasing while pumping.

SystemSystem is momentarily realigned to allow lasing.is momentarily realigned to allow lasing.

Results in short (~10-200 nsec), high-intensity (up to MW) pulse.Results in short (~10-200 nsec), high-intensity (up to MW) pulse.

Only possible if the laser can store energy in the excited state Only possible if the laser can store energy in the excited state longer than the Q-switched pulse.longer than the Q-switched pulse.

Demtröder, W. Demtröder, W. Laser Spectroscopy, Laser Spectroscopy, Springer, Berlin: 1996.Springer, Berlin: 1996.

switch )1(

2L Length Pulse

c

n)1(

2L Length Pulse

c

n

Cavity DumpingCavity Dumping

Laser cavity has two “fully” reflective mirrors.Laser cavity has two “fully” reflective mirrors.

A steady power grows inside the cavity during normal operation.A steady power grows inside the cavity during normal operation.

Momentarily, a third mirror enters the light path and directs the Momentarily, a third mirror enters the light path and directs the beam out of the cavity.beam out of the cavity.

All energy is dumped in one pulse lasting as long as it takes the All energy is dumped in one pulse lasting as long as it takes the light to make a round trip in the laser cavity.light to make a round trip in the laser cavity.


c

n2L Length Pulse c

n2L Length Pulse

Mode - LockingMode - Locking

Edward Piepmeier, Edward Piepmeier, Analytical Applications of LasersAnalytical Applications of Lasers, , John Wiley & Sons, New York, 1986.John Wiley & Sons, New York, 1986.

Method for producing very short pulse widths (~10Method for producing very short pulse widths (~10-12-12 s). s).

Synchronize longitudinal modes.Synchronize longitudinal modes.

44.0 Duration Pulse Minimum

44.0 Duration Pulse Minimum

Are you getting the concept?Are you getting the concept?A laser has a bandwidth of 4.4 GHz (4.4 x 10A laser has a bandwidth of 4.4 GHz (4.4 x 1099 Hz). What is the Hz). What is theshortest modelocked pulse it can generate according to theshortest modelocked pulse it can generate according to thetransform limit? transform limit?

Accessible WavelengthsAccessible Wavelengths

Lasers have also been prepared for the vacuum UV (VUV, 100-200 nm) and XUV Lasers have also been prepared for the vacuum UV (VUV, 100-200 nm) and XUV (eXtreme UltraViolet; also called the ultrasoft X-ray region; <100 nm).(eXtreme UltraViolet; also called the ultrasoft X-ray region; <100 nm).

The shortest wavelength laser produced so far emits at 3.5 nm. Projects to extend The shortest wavelength laser produced so far emits at 3.5 nm. Projects to extend this range to 0.1 nm by 2011 are in progress.this range to 0.1 nm by 2011 are in progress.

Why x-ray lasers are so difficult to build: Why x-ray lasers are so difficult to build: AAjiji/B/Bijij = 8 = 8 h h 33 / c / c33

http://www.cvimellesgriot.com/Products/Documents/Capabilities/CVIMG_Laser_Capabilities.pdf

http://www.cvimellesgriot.com/Products/Documents/Capabilities/CVIMG_Laser_Capabilities.pdf

Diode LASERs

McCreery, R. L., McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000, Wiley, New York: 2000

• Conversion of electrical to optical power up to 30%.Conversion of electrical to optical power up to 30%.

• Polished faces of semiconductor act as mirrors and reflect ≈95% of photons Polished faces of semiconductor act as mirrors and reflect ≈95% of photons from leaving resonance cavity.from leaving resonance cavity.

Stimulated Emission

Agrawal, G.P.; Dutta, N.K. Agrawal, G.P.; Dutta, N.K. Semiconductor LasersSemiconductor Lasers, Van Nostrand Reinhold, New York: 1993., Van Nostrand Reinhold, New York: 1993.

Semiconductor Semiconductor (Diode) Laser(Diode) Laser

Used in Used in telecommunications, CD telecommunications, CD players, laser pointers players, laser pointers etc.etc.

Blue and UV (375 – 400 Blue and UV (375 – 400 nm) diode lasers have nm) diode lasers have recently been recently been developed.developed.

Eli Kapon, Eli Kapon, Semiconductor Lasers ISemiconductor Lasers I, , Academic Press, San Diego, 1999.Academic Press, San Diego, 1999.

Semiconductor (Diode) LaserSemiconductor (Diode) Laser

Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.

Neodymium:YAG LaserNeodymium:YAG Laser

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

• NdNd3+3+ in yttrium-aluminum- in yttrium-aluminum-garnet (Ygarnet (Y33AlAl55OO1212))

• Four level laserFour level laser

• Powerful line @ 1064 nm; often Powerful line @ 1064 nm; often doubled or tripleddoubled or tripled

• Pump: Kr/Ar arc lamp or flash Pump: Kr/Ar arc lamp or flash lamplamp

• CW or pulsed operationCW or pulsed operation

Ion Lasers (ArIon Lasers (Ar++ and Kr and Kr++))CW – pumped using an electrical discharge.CW – pumped using an electrical discharge.

Very reliable.Very reliable.

Inefficient because energy is required to ionize gas.Inefficient because energy is required to ionize gas.

Power up to ~40 W (distributed over many lines).Power up to ~40 W (distributed over many lines).

Argon ion is most common.Argon ion is most common.

488 nm and 514 nm are most powerful lines.488 nm and 514 nm are most powerful lines.

Cluster of ~10 lines in 454 – 529 nm.Cluster of ~10 lines in 454 – 529 nm.

UV: 334, 352, 364 nm (need several W in visible to get UV: 334, 352, 364 nm (need several W in visible to get ~50 mW in UV)~50 mW in UV)

Deep UV: 275 nm (need 20-30 W in visible to get ~10mW Deep UV: 275 nm (need 20-30 W in visible to get ~10mW @ 275 nm)@ 275 nm)

Excimer LasersExcimer Lasers

Excimer is a dimer that is only stable in the excited state.Excimer is a dimer that is only stable in the excited state.

e.g. ArFe.g. ArF++, KrF, KrF++, XeF, XeF++

Pass current through noble gas / FPass current through noble gas / F22 mix. mix.

Lasing occurs as excimer returns to the ground state.Lasing occurs as excimer returns to the ground state.


Dye LasersDye Lasers


Molecular transitions in the solution Molecular transitions in the solution phase.phase.

Active species is an organic dye (e.g. Active species is an organic dye (e.g. rhodamines, coumarins, fluoresceins). rhodamines, coumarins, fluoresceins).

To prevent overheating, a jet of the dye To prevent overheating, a jet of the dye solution is pumped through focal point solution is pumped through focal point of optical system.of optical system.

Broad transitions. Can be tuned over Broad transitions. Can be tuned over ~50 nm.~50 nm.

Lases in UV-Vis-IRLases in UV-Vis-IR

Difficult and expensive to operate.Difficult and expensive to operate.

Optically pumped with flashlamp or Optically pumped with flashlamp or another laser.another laser.

Dye LasersDye Lasers
