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1. Basics of LASER Physics Dr. Sebastian Domsch (Dipl.-Phys.)
Computer Assisted Clinical Medicine
Medical Faculty Mannheim
Heidelberg University
Theodor-Kutzer-Ufer 1-3
D-68167 Mannheim, Germany
www.ma.uni-heidelberg.de/inst/cbtm/ckm
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 2/29 I 12/10/2015
Outline: Biomedical Optics
1. Lecture - Basics of LASER Physics
Historical Background
Properties of Light
Maxwells Equations
Wave Particle Dualism
Geometric Optics
2. Lecture - LASER Principle
3. Lecture - LASER Systems
4. Lecture - LASER Resonators
5. Lecture - LASER Tissue Interactions 1
6. Lecture - LASER Tissue Interactions 2
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 3/29 I 12/10/2015
Literature
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 4/29 I 12/10/2015
LASER
LASER Light
short light pulses, spatial coherence focusing to a tight spot over long distances
Laser Applications
Laser Cutting Laser Printers Optical Disc Drives Barcode Scanners Laser Pointer Laser Surgery Fiber Optic Free-Space Communication Distance measurements (LUNAR LASER Ranging Experiment: precision < 4cm!!) many more
LASER
Light Amplification by Stimulated Emission of Radiation
A LASER is a device that emits light through a process
of optical amplification based on the stimulated emission of
electromagnetic radiation
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 5/29 I 12/10/2015
Historical Background
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 6/29 I 12/10/2015
Discovery of Stimulated Emission in 1917
Albert Einstein
* 14.3.1879 (Ulm, Germany) 18.4.1955, (Princeton, USA)
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 7/29 I 12/10/2015
1960 First LASER Constructed
Theodore Harold Maiman
* 11.7.1927, Los Angeles, USA
5.5.2007, Vancouver, Canada
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 8/29 I 12/10/2015
First LASER systems: 1960
Ali Javan (*1926, Teheran/Iran)
Continuous-Wave (CW) Gas
LASER
Bell Telephone Laboratories (NJ/USA)
Hughes Research Laboratories (CA/USA)
Theodore H. Maiman (*1927, L.A./USA)
Pulsed Solid-State
LASER
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 9/29 I 12/10/2015
Nobel Prize in Physics in 1964
for fundamental work in the field of quantum electronics, which has led to the
construction of oscillators and amplifiers based on the maser-laser principle
Charles Hard Townes
* 28.7.1915, Greenville, USA
Aleksandr Mikhailovich Prokhorov
* 11.7.1916, Atherton, Australia
8.1.2002, Moscow, Russia
Nikolay Gennadiyevich Basow
* 14.12.1922, Usman, Russia
1.7.2001, Moscow, Russia 27.1.2015, Oakland, USA
Theoreticl work: MASER
principle -> LASER Concept of optical pumping
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 10/29 I 12/10/2015
1960 First LASER Constructed
Theodore Harold Maiman
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 11/29 I 12/10/2015
Physical Basics
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 12/29 I 12/10/2015
Properties of Light
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 13/29 I 12/10/2015
Wave Particle Dualism of Light
Matter Light
particle wave
Einstein (1905)
Particle:
Photoelectric effect
(Nobel Price 1921)
De Broglie (1924)
Wave-like behavior
of electrons
Tissue LASER
Geometric
Optics Quantum
optics
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 14/29 I 12/10/2015
Properties of Light
E = h = pc p = h /
: frequency
E: energy
h: Plancks constant
Light Quanta
Photons ()
= c : dispersion in vacuum
c: light velocity = 3108 m/s
: wave length
Electromagnetic Wave
(t)=I0ei
t
I0
p: momentum
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 15/29 I 12/10/2015
Electromagnetic Spectrum
visible spectrum: = 400 700 nm, = 7,5 4 1014 Hz
Geometric
Optics
(wave
character)
Quantum
optics
(particle
character)
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 16/29 I 12/10/2015
Light - Electromagnetic (EM) Waves
)t,r(E
electric field:
magnetic field:
EM Fields:
)t,r(H
- caused by
electric charges
electric currents
- defined by two vector fields:
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 17/29 I 12/10/2015
EM Wave
)t,r(E
electric field:
magnetic field: )t,r(H
wave vector: )t,r(k
E
H k
|k| = 2 /
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 18/29 I 12/10/2015
Electromagnetic Fields in
Dielectric Media
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 19/29 I 12/10/2015
Dielectric Media Non-Conducting
PED
0
electric field
electric displacement field:
polarization
E
MHB
0
magnetic field
magnetic induction:
magnetization
H
E
H k
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 20/29 I 12/10/2015
Maxwells Equations (static fields) 1. Charges are the sources of electric fields
Divergence of electric
field is created by charges
D
)V(qdADV
0V
dAB
0B
2. Magnetic monopoles do not exist
In the absence of
magnetic monopoles,
divergence of the
magnetic field lines is
always zero.
Gausss Theorem
Gausss Theorem
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 21/29 I 12/10/2015
Maxwells Equations (dynamic fields)
3. A changing magnetic field creates an electric field
t
BE
t
DJH f
4. Magnetic fields are created by electrical current and by changing electric fields
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 22/29 I 12/10/2015
Geometric Optics
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 23/29 I 12/10/2015
Geometric Optics
Reflection
Refraction
Transmission
At a planar dielectric surface
dielectric: electrical insulator (weak or non-conducting) that
can be polarized by an applied electric field
media: air, water, glass,
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 24/29 I 12/10/2015
Reflection
'
angle of incidence = angle of reflection
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 25/29 I 12/10/2015
Refraction
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 26/29 I 12/10/2015
Refraction
n
n
Normal
)'sin('n)sin(n
refractive index n
vacuum: 1
air: 1.0003
water: 1.333
crown glass: 1.5
Snells Law
Light minimizes the time the travel from
point A to B. Light velocity in media.
Fermats Prinziple
A
B
c (medium)=c/
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 27/29 I 12/10/2015
Total Reflection
Fiber optic cable: total reflection important for signal
transmission!
Water tank: Reflected and refracted light
components!
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 28/29 I 12/10/2015
Total Reflection
n
n
Normal
n > n
c
critical angle
'n
narcsinc
c )'sin('n)sin(n
Snells Law
sin() =1 !
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 29/29 I 12/10/2015
Brewster Angle - Linear Polarisation
Brewster Angle: B
Reflected ray polarized due to radiation charachteristic of Hertzian Dipole!
B + =/2
Brewster Angle: B
Hertzian Dipole
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 30/29 I 12/10/2015
Dispersion
dispersion = dependance between
frequency and wavelength: = ()
f = c / n()
f = c / (n() )
substitute = 2f and k = 2/
= kc / n(k)
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 31/29 I 12/10/2015
Dispersion Group and Phase Velocity
wavepakage:
= velocity of wave package
phase v
Group v
group velocity:
phase velocity:
If the refractive index (n) is not wavelength dependent
Gaussian Wavepakage
= No dispersion!
The refractive index is wavelength
dependent: n = n()
-> Speed of light in medium is
wavelength dependent: v = c/ n()
= v() !
-> A wave package disperses
( ),
/ ( )
( )
j ji t k x
j
j
group
phase
x t c e
d k kdv c
dk dk
cv
k k
= velocity of single waves
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 32/29 I 12/10/2015
Repetition
Einstein: Discovery of stimulated emission 1917 First pulsed ruby LASER by Maiman in 1960 Nobel prices for Townes, Basow and Prokhorov in 1964: fundamental work in quantum electronics) fascilitating LASERs/MASERs
Light, both wave and particle character Electromagnetic wave: B- and E fields Maxwells Equation: the cause and the relation of and between B(t)- and E(t)
Geometric optics: reflection, refraction, transmission Reflection: angle of incident = angle of reflection Total Refraction: angle of reflection > 90 Brewester Angle: linearly reflected light if refracted and reflected light 90
Dispersion relation: k = k() Dielectric: = (k) Wavepackages disperse if group velocity phase velocity
Biomedical Optics Basics of LASER Physics
Dr. Sebastian Domsch I Slide 33/29 I 12/10/2015
Next Lecture
2. LASER Principle