2.717j/mas.857j optical engineering– transmission and reflection coefficients for a dielectric...
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Today’s summary• Multiple beam interferometers: Fabry-Perot resonators
– Stokes relationships– Transmission and reflection coefficients for a dielectric slab– Optical resonance
• Principles of lasers• Coherence: spatial / temporal
MIT 2.71/2.710 Optics10/20/04 wk7-b-1
Fabry-Perot interferometers
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Relation between r, r’ and t, t’
a
arat
air
a’
a’t’
a’r’
glass airglass
Proof: algebraic from the Fresnel coefficientsor using the property of preservation of thepreservation of thefield properties upon time reversal12 =′+
−=′
ttrrr
field properties upon time reversal
Stokes relationships
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Proof using time reversal
a
arat
air glassatt’
arat
air
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glass
atr’art
ar2
⇔
( )( ) 1
0 22 =′+⇒=′+
′−=⇒=′+
ttrattrarrtrra
Fabry-Perot Interferometer
incident
reflected
transmitted
L
Resonance condition: reflected wave = 0⇔ all reflected waves interfere destructively
nmL2λ
=wavelength in free space
refractive index
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Calculation of the reflected wave
incoming a
reflected ar
transmitted ateiδ
reflected atei2δr’transmitted ateiδt’
reflected atei3δ(r’)2
transmitted atei2δr’t’
reflected atei4δ(r’)3
transmitted atei3δ(r’t’)2
reflected atei5δ(r’)4
transmitted atei4δ(r’)3t’
λπδ nL2=air, n=1 glass, n air, n=1
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Calculation of the reflected wave
( ){ }
⎭⎬⎫
⎩⎨⎧
′−′′+=
+′+′+′′+=
δδ
δδδ
222
44222reflected
e11e
ee1e
ii
iii
rrttra
rrrttraa
12 =′+
−=′
ttrrr
Use Stokes relationships
( )δ
δ
22
2
reflected e1e1
i
i
rraa−−
=
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Transmission & reflection coefficients
( )δ
δ
22
2
reflected e1 e1
i
i
rraa
−−
= δ22dtransmitte e1 irttaa
−′
=
2rR ≡
( )( )
( ) δ
δδ
22
22dtransmitte
22
22reflected
sin411
tcoefficienontransmissi
sin41sin4
tcoefficienreflection
RRR
aa
RRR
aa
+−−
==⎟⎟⎠
⎞⎜⎜⎝
⎛
+−==⎟⎟
⎠
⎞⎜⎜⎝
⎛
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Transmission & reflection vs pathR=0.95
R=0.5
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Fabry-Perot terminology
nLmc2
( )nL
cm2
1+ ( )nL
cm2
2+ frequency ν
bandwidth
freespectralrange
resonancefrequencies
Fabry-Perot terminology
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∆νFWHM
FWHM Bandwidth is inversely proportionalto the finessefinesse F (or quality factorquality factor)of the cavity
∆νFSR
RRF
−≡
1π
nLFc
2FWHM =∆ν
FFSR
FWHMνν ∆
=∆ ( ) ( )( )finesse
range spectral freebandwidth =
Spectroscopy using Fabry-Perot cavityGoal: Goal: to measure the specimen’s absorption as function offrequency ω
Experimental measurement principle:Experimental measurement principle:
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container with specimen to be measured
transparentwindows
light beam ofknown spectrum
spectrum of light beamis modified by substance
scanningstage
(controls cavitylength L)
partially–reflectingmirrors (FP cavity)
powermeter
Spectroscopy using Fabry-Perot cavityGoal: Goal: to measure the specimen’s absorption as function offrequency ω
Experimental measurement principle:Experimental measurement principle:
container with specimen to be measured
transparentwindows
light beam ofknown spectrum
spectrum of light beamis modified by substance
MIT 2.71/2.710 Optics10/20/04 wk7-b-13
partially–reflectingmirrors (FP cavity)
powermeter
electro-optic(EO) modulator
(controls refr.index n)
Spectroscopy using Fabry-Perot cavity
ω
I(ω)
unknownspectrum
Fabry–Perottransmissivity
ω1sample measured:
I(ω1)MIT 2.71/2.710 Optics10/20/04 wk7-b-14
Spectroscopy using Fabry-Perot cavity
ω
I(ω)
unknownspectrum
Fabry–Perottransmissivity
ω2sample measured:
I(ω2)MIT 2.71/2.710 Optics10/20/04 wk7-b-15
Spectroscopy using Fabry-Perot cavity
ω
I(ω)
unknownspectrum
Fabry–Perottransmissivity
ω3sample measured:
I(ω3)MIT 2.71/2.710 Optics10/20/04 wk7-b-16
Spectroscopy using Fabry-Perot cavity
ω
I(ω)
unknownspectrum
Fabry–Perottransmissivity
ω3sample measured:
I(ω3)
unknown spectrumwidth should not exceed the FSR
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Spectroscopy using Fabry-Perot cavity
ω
I(ω)
unknownspectrum
Fabry–Perottransmissivity
ω3sample measured:
I(ω3)
spectral resolutionspectral resolutionis determined by thecavity bandwidth
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Lasers
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Absorption spectra
λ (µm)
Atm
osph
eric
tran
smis
sion
human visionMIT 2.71/2.710 Optics10/20/04 wk7-b-20
Semi-classical view of atom excitationsEnergy
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Ze+e-
Atom in ground stateAtom in ground state
Energy
Atom in excited stateAtom in excited state
Light generation
Energy
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ground state
excited state
equilibrium: most atomsin ground state
Light generation
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ground state
Energyexcited state
A pump mechanism (e.g. thermal excitation or gas discharge) ejects some atoms to the excited state
Light generation
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ground state
Energyexcited state
hν
hν
The excited atoms radiativelydecay, emitting one photon each
Light amplification: 3-level systemEnergy
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ground state
super-excited state
excited state
equilibrium: most atomsin ground state; note the existenceof a third, “super-excited” state
Light amplification: 3-level systemEnergy
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ground state
super-excited state
excited state
Utilizing the super-excited stateas a short-lived “pivot point,” thepump creates a population inversion
Light amplification: 3-level systemEnergy
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ground state
super-excited state
excited state
hν
When a photon enters, ...
Light amplification: 3-level system
ground state
Energy super-excited state
excited state
hνhν
hν
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When a photon enters, it “knocks” an electron from the inverted population down to the ground state, thus creatinga new photon. This amplification process is called stimulated emission
Light amplifier
Gain medium(e.g. 3-level system
w population inversion)
Pin
Pout =gPin
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Light amplifier w positive feedback
Gain medium(e.g. 3-level system
w population inversion)
Pin
Pout =gPin
When the gain exceeds the roundtrip losses, the system goes into oscillation
gΣ+
+
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Laserinitial photon
Gain medium(e.g. 3-level system
w population inversion)
Partiallyreflecting
mirrorLightAmplification throughStimulatedEmission ofRadiation
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Laser
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Gain medium(e.g. 3-level system
w population inversion)
amplified once initial photon
Partiallyreflecting
mirrorLightAmplification throughStimulatedEmission ofRadiation
Laser
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Gain medium(e.g. 3-level system
w population inversion)
amplified once
reflected
initial photon
Partiallyreflecting
mirrorLightAmplification throughStimulatedEmission ofRadiation
Laser
Gain medium(e.g. 3-level system
w population inversion)
initial photonamplified once
reflected
amplified twice
Partiallyreflecting
mirrorLightAmplification throughStimulatedEmission ofRadiation
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Laser
Gain medium(e.g. 3-level system
w population inversion)
initial photonamplified once
reflected
amplified twice
reflected
output
Partiallyreflecting
mirrorLightAmplification throughStimulatedEmission ofRadiation
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Laser
Gain medium(e.g. 3-level system
w population inversion)
initial photonamplified once
reflected
amplified twice
Partiallyreflecting
mirror
reflected
output
amplified againetc.
LightAmplification throughStimulatedEmission ofRadiation
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Confocal laser cavities
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waist w0
diffractionangle
0
tannwπλθ =
Beam profile: 2D Gaussian function
“TE00 mode”
Other “transverse modes”
TE10 TE11
(usually undesirable)
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Types of lasers
• Continuous wave (cw)• Pulsed
– Q-switched– mode-locked
• Gas (Ar-ion, HeNe, CO2)• Solid state (Ruby, Nd:YAG, Ti:Sa)• Diode (semiconductor)• Vertical cavity surface-emitting lasers –VCSEL– (also sc)• Excimer (usually ultra-violet)
MIT 2.71/2.710 Optics10/20/04 wk7-b-39
CW (continuous wave lasers)
Laser oscillation well approximated by a sinusoid
1/ν
t
Typical sources: • Argon-ion: 488nm (blue) or 514nm (green); power ~1-20W• Helium-Neon (HeNe): 633nm (red), also in green and yellow; ~1-100mW• doubled Nd:YaG: 532nm (green); ~1-10W
Quality of sinusoid maintained over a time duration known as“coherence time” tc
Typical coherence times ~20nsec (HeNe), ~10µsec (doubled Nd:YAG)MIT 2.71/2.710 Optics10/20/04 wk7-b-40
Two types of incoherencetemporaltemporal
incoherencespatialspatial
incoherenceincoherence incoherence
1r
2r1rmatched
pathspointsource
d2d1
Michelson interferometer Young interferometer
poly-chromatic light(=multi-color, broadband)
mono-chromatic light(= single color, narrowband)
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Two types of incoherencetemporaltemporal
incoherencespatialspatial
incoherenceincoherence incoherence
1r
2r1rmatched
pathspointsource
d2d1
waves from unequal pathswaves from unequal pathsdo not interfere
waves with equal pathswaves with equal pathsbut from different pointsbut from different points
on the wavefronton the wavefrontdo not interfere
do not interfere
do not interfereMIT 2.71/2.710 Optics10/20/04 wk7-b-42
Coherent vs incoherent beams
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1e11φiaa =
2e22φiaa =
Mutually coherent: superposition field amplitudeamplitudeis described by sum of complex amplitudessum of complex amplitudes
221
2
212121 ee
aaaI
aaaaa ii
+==
+=+= φφ
Mutually incoherent: superposition field intensityintensityis described by sum of intensities1I sum of intensities
21 III +=(the phases of the individual beams vary randomly with respect to each other; hence,we would need statistical formulation todescribe them properly — statistical optics)
2I
Coherence time and coherence length
Intensity
l1
• l1-l2 much shortershorter than “coherence length” ctc
sharp interference fringes
l1
l2
0
2I0
• l1-l2 much longerlonger than “coherence length” ctc
incominglaserbeam no interference
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Michelson interferometer Intensity
I0
l1
Coherent vs incoherent beams
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1e11φiaa =
2e22φiaa =
Coherent: superposition field amplitudeamplitudeis described by sum of complex amplitudessum of complex amplitudes
221
2
212121 ee
aaaI
aaaaa ii
+==
+=+= φφ
Incoherent: superposition field intensityintensityis described by sum of intensities1I sum of intensities
21 III +=(the phases of the individual beams vary randomly with respect to each other; hence,we would need statistical formulation todescribe them properly — statistical optics)
2I
Mode-locked lasers
Typical sources: Ti:Sa lasers (major vendors: Coherent, Spectra Phys.)Typical mean wavelengths: 700nm – 1.4µm (near IR)
can be doubled to visible wavelengthsor split to visible + mid IR wavelengths using OPOs or OPAs(OPO=optical parametric oscillator;OPA=optical parametric amplifier)
Typical pulse durations: ~psec to few fsec(just a few optical cycles)
Typical pulse repetition rates (“rep rates”): 80-100MHzTypical average power: 1-2W; peak power ~MW-GW
MIT 2.71/2.710 Optics10/20/04 wk7-b-46
Overview of light sourcesLasernon-Laser
Thermal: polychromatic,spatially incoherent(e.g. light bulb)
Gas discharge: monochromatic,spatially incoherent(e.g. Na lamp)
Light emitting diodes (LEDs):monochromatic, spatially incoherent
Continuous wave (or cw):strictly monochromatic,spatially coherent(e.g. HeNe, Ar+, laser diodes)
Pulsed: quasi-monochromatic,spatially coherent(e.g. Q-switched, mode-locked)
~nsec ~psec to few fsec
pulse duration
mono/poly-chromatic = single/multi colorMIT 2.71/2.710 Optics10/20/04 wk7-b-47