Photonic Devices: Light Source

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Photonic DevicesLight Source
Photon Detector
Light -Tissue Interaction
Photon Source
Optical System for Light Delivery and Collection
Chapter 13 in Hecht
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WHAT GENERATES ELECTROMAGNETIC ENERGY?
HOW DOES THIS ENERGY PROPAGATE IN SPACE?
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All phenomena of light and electrons arise from
three basic actions
1. A photon goes from place to place
2. An electron goes from place to place
3. An electron emits or absorbs a photon
Richard Feynman ( in QED )
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What determines frequency and wavelength
Charge moves back and forth a certain number of
times per second, i.e., with a given frequency
or,
Charge moves from a higher to a lower energy
level frequency (energy difference)/(Plancks
constant)
Wavelength (velocity of
light)/frequency
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Light Source Generation of photons
Bohr Atom
excited state electronic cloud
nucleus

• Photon energy Eph? hc/?
• h Planck's constant

ground state electronic cloud
Energy
Photon Emission
Excited atom /molecules
h?
E
Electron
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Photon Sources
Conventional Source

• Thermal
• Globar
• Filament Lamp
• Gas Discharge
• Arc Lamp

Coherent Source

• Laser

Monochromatic Collimated High brightness
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Temporal Coherence
Eb
Ea
Correlation along the light propagation
direction. A high (good) temporal coherence
gives a narrow spectral bandwidth (pure light
of single wavelength/color)
E
Good temporal coherence
Poor temporal coherence
t
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Spatial Coherence
Ea
Eb
Correlation transverse to the light propagation
direction. A high (good) spatial coherence
gives small beam spreading
Good spatial coherence
Poor spatial coherence
Constant phase plane
Constant phase plane
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Coherent Sources

• High temporal coherence monochromatic
• Extremely small spectral bandwidth
• High spatial coherence well collimated beam
• Extremely small angular broadening

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Photon Sources
Coherent sources atoms or molecules radiate
coherently with identical phase, direction, and
photon energy
Incoherent source atoms or molecules radiate
incoherently or randomly
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Light sources
?c ?2/c?? lc??? ?2/n??
I
?

• Atoms or molecules radiate wavetrains of finite
  length
• More than one wavelength (spectral bandwidth)
• Fixed phase relation only within individual
  wavetrain

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Light Sources
Coherent light
Partial coherent light
Incoherent light
Phase break
t (or x)
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Temporal Coherence and Spectral Bandwidth
Coherent Light Monochromatic Light
n
FWHM
Partial Coherent Light Quasi -monochromatic Light
n
n
FWHM Full Width Half Maximum
Incoherent Light White Light
n
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Lasers

• Advantages
• monochromatic
• high spectral purity efficient pumping/probing
  of sample
• coherent (good for interferometry)
• constant phase relationship between different
  points on the beam
• low divergence relatively easy to couple into
  fibers (diodes can be difficult though)
• polarized
• fluorescence depolarization spectroscopy for
  kinetic studies
• Disadvantages
• monochromatic
• TiSaph, Dye lasers are tunable
• relatively expensive
• Sometimes require special power arrangements e.g.
  220 Volt, 3 phase

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Broad band sources

• Advantages
• Tunable
• Monochromator
• Filtered
• Stable
• Provide feedback loop photodetector/powersupply
• Disadvantages
• Coupling
• extended sources difficult to couple to fiber,
  through monochromator
• Typically lots of UV and IR light in spectral
  bands that arent of interest
• must manage (filters) to prevent sample and
  system degradation
• Often not compact

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Example Irradiance Hg-Xe arc Lamp
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Q-Th Lamp spectrum
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LASER Light Amplification by Stimulated
Emission of Radiation

• Albert Einstein (1917) Stimulated emission
• Nicolaas Bloembergen and Charles Townes
  (1956) Development of maser
• Arthur Schalow and Charles Townes (1958)
  Optical Maser Proposed
• Theodore Maiman (1960) First laser
  demonstrated

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LASER
Three Major Components
Pump
Gain Medium
Feedback
Feedback
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Gain Medium
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Photon Emission
Spontaneous Emission
Stimulated Emission

• Electrons in excited state have a finite average
  life time ?
• Photons emitted have random phase and direction

• Presence of an excited state atom in a radiation
  field increases the probability of emission of
  an identical photon
• Photons emitted have the same wavelength, phase,
  polarization, direction

E2
E2
h?
h?
h?
h?
E1
E1
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Population Inversion
E2
N2
E2
N2
N2
E2
h?
h?
h?
h?
h?
E1
N1
E1
N1
E1
N1
Stimulated Emission
Spontaneous Emission
Absorption
dN2/dt ???N1-N2)I(n)
dN2/dt ???N1-N2)I(n)
dN2/dt-N2/??
??N2-N1)lt 0
??N2-N1)gt 0
In thermal equilibrium, N2N1e-(E2-E1)/kbT ltlt N1
gt absorption
When N2 gt N1 (population inversion) gt
Stimulated emission
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Gain in Laser Medium
Amplifying medium when (N2gtN1)
Absorbing medium when (N2ltN1)
Gain ??????N2-N1 (population inversion)
????
?
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LASER
Three important processes in laser

• Pumping
• Stimulated emission
• Optical Feedback

E3
E2
Pump
h?
Laser Transition Stimulated Emission
h?
h?
E1
Ground State
Population Inversion ?N12N2-N1 gt 0
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Pumping
Electrical Pumping
Argon Ion Laser Excimer Laser Helium Neon
Laser Carbon Dioxide Laser Semiconductor Laser
E3
E2
h?
Pump
h?
h?
Optical Pumping
E1
Ground State
Nd YAG laser Ruby laser Dye Laser Holmium YAG
laser
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Optical Feedback
E field 0 at cavity wall, node
Laser cavity (mirrors)
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Laser Resonant Cavity
100 mirror
3099 mirror
m 2L/?m??gt?m???2L/m, or ?mmv/2L m is an
integer much larger than one
Broad atomic emission
c/2L
Resonant cavity modes
?
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LASER
Gain Stimulated emission from population
inversion due to the pumping
Loss Absorption of the laser medium, partial
reflection of the mirrors. Laser When Gain gt
Loss
????
Loss
Resonant modes
??
Laser frequency
?
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Three-Level Laser Energy Diagram
e.g. ruby laser
E3
fast decay
E2
h?
Pump
Laser Transition Stimulated Emission
h?
h?
E1
Ground State
Population Inversion ?N21N2-N1 gt 0
h?????????
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Four-Level Laser Energy Diagram
??
fast decay
??
h?
Laser Transition Stimulated Emission
h?
Pump
h?
??
Level 2
fast decay
??
Ground State
Population Inversion ?N32N3-N2 gt 0
h?????????
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Example NdYAG Laser
??
Radiationless decay
??
h?
Optical Pump
h?
??
Radiationless decay
??
Ground State
E1 0 eV E2 0.26 eV E31.43 eV E42.36 eV
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Characteristics of Lasers
Compare to conventional light source, laser
light has

• High temporal coherence monochromatic
• Extremely small spectral bandwidth
• High spatial coherence well collimated beam
• Extremely small angular broadening ?/d
• High Brightness
• Emitted power per unit area is very high

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Selection of Lasers

• Wavelength
• Energy
• Power
• Pulse Width
• Repetition Rate

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LASERS
(0.2-10 ps)
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Light Emitting Diodes (LEDs)
All pn junctions emit light on passage of forward
biased current free electrons in n-type
diffuse into p-type under forward bias in the
p-region they meet a majority of holes
recombine excess energy emitted as
light GaAs - 880 nm, GaP - 550 nm or 700 nm,
GaAsP - 580 nm or 660 nm
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Diode Lasers
Based on Heavily doped pn-junctions Modified
light-emitting diode structure, high
concentration of e-h pairs High doping
concentrations across junction Long
spontaneous lifetime materials enhance stimulated
emission Light emerges from ends rather than
through the wide gap Narrow active layer
contains holes across the whole length Ends
are cleaved, polished and made flat parallel
Sides are roughened to trap light inside crystal
High current densities are needed to produce
stimulated emission population inversion
Available across a wide range of wavelengths
633, 770, 809 nm, 850. 920, 980, 1.1, 1.3, 1.5 µm
AlGaInP, GaAlAs, InGaAsP Powers range from a
few mW's to several W's cw
p-region
n-region
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Mode Locking
lock cavity modes together, i.e. lock their
relative phases.  variable loss into the
cavity, such as an acousto-optic modulator, so
that the gain of the cavity is modulated at the
frequency c/2L (round trip transit time). 
interference causes the traveling light waves
inside the cavity to collapse into a very short
pulse.  every time this pulse reaches the
output coupler, the laser emits a part of this
mode-locked pulse.  pulse repetition rate is
determined by the time it takes for the pulse to
make one trip around the cavity.  more modes
interfere, shorter pulse duration. Titanium
Sapphire Laser 100 fs modelocked pulses 
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Lasers Examples

• Sources for fluorescence spectroscopy
• Argon (cw)
• 458 nm, 477 nm, 488 nm , 514 nm, etc.
• Nitrogen dye laser(pulsed)
• Fundamental -gt 337 nm
• Dyes Coumarins, Rhodamines, etc.
• 460 nm, 505 nm, etc.
• Sources for Raman and tissue spectroscopy
• NdYAG (pulsed)
• Fundamental -gt 1064 nm
• Doubling and Tripling crystals (KTP Potassium
  Titanyl Phosphate)
• 532 nm, 355nm, 266 nm
• Diode Lasers
• Ga0.5In0.5P/GaAs 670-686 nm
• GaAlAs/GaAs 750-870 nm (etc)

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Lasers Examples

• Sources for multiphoton fluorescence spectroscopy
  and near infrared spectroscopy
• Titanium Doped Sapphire Lasers (TiSaph)
• Tunable 700 - 900 nm
• mode-locked, ultra-short pulses (10-15 sec)