1. Basics of LASER Physics

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
[email protected]
www.ma.uni-heidelberg.de/inst/cbtm/ckm
Outline: Biomedical Optics
1. Lecture - Basics of LASER Physics
•
Historical Background
•
Properties of Light
•
Maxwell´s 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 2/29 I 12/10/2015
Literature
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 3/29 I 12/10/2015
LASER
A LASER is a device that emits light through a process
of optical amplification based on the stimulated emission of
electromagnetic radiation
LASER
Light Amplification by Stimulated Emission of Radiation
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…
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 4/29 I 12/10/2015
Historical Background
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 5/29 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 6/29 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 7/29 I 12/10/2015
First LASER systems: 1960
Theodore H. Maiman (*1927, L.A./USA)
 Pulsed Solid-State
LASER
Hughes Research Laboratories (CA/USA)
Ali Javan (*1926, Teheran/Iran)
 Continuous-Wave (CW) Gas
LASER
Bell Telephone Laboratories (NJ/USA)
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 8/29 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
Nikolay Gennadiyevich Basow
Aleksandr Mikhailovich Prokhorov
* 28.7.1915, Greenville, USA
† 27.1.2015, Oakland, USA
* 14.12.1922, Usman, Russia
† 1.7.2001, Moscow, Russia
* 11.7.1916, Atherton, Australia
† 8.1.2002, Moscow, Russia
Theoreticl work: MASER
principle -> LASER
Concept of optical pumping
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 9/29 I 12/10/2015
1960 First LASER Constructed
Theodore Harold Maiman
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 10/29 I 12/10/2015
Physical Basics
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 11/29 I 12/10/2015
Properties of Light
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 12/29 I 12/10/2015
Wave – Particle Dualism of Light
Tissue
LASER
Matter
Light
Einstein (1905)
De Broglie (1924)
Wave-like behavior
of electrons
Particle:
Photoelectric effect
(Nobel Price 1921)
Geometric
Quantum
optics
particle
Optics
wave
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 13/29 I 12/10/2015
Properties of Light
Electromagnetic Wave
Light Quanta
(t)=I0ei
Photons ()

I0
t
 ·  = c : dispersion in vacuum
E = h∙ = p∙c
p=h/λ
: wave length
E: energy
: frequency
p: momentum
c: light velocity = 3108 m/s
h: Planck‘s constant
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 14/29 I 12/10/2015
Electromagnetic Spectrum
Geometric
Optics
Quantum
optics
(wave
character)
(particle
character)
visible spectrum:  = 400 – 700 nm,  = 7,5 – 4  1014 Hz
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 15/29 I 12/10/2015
Light - Electromagnetic (EM) Waves
EM Fields:
- defined by two vector fields:
 
• electric field: E( r , t )
 
• magnetic field: H( r , t )
- caused by
• electric charges
• electric currents
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 16/29 I 12/10/2015
EM Wave
 
• electric field:
E( r , t )
 
• magnetic field: H( r , t )

• wave vector:
k( r , t )

H

E
|k| = 2π / λ
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 17/29 I 12/10/2015

k
Electromagnetic Fields in
Dielectric Media
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 18/29 I 12/10/2015
Dielectric Media – Non-Conducting
electric displacement field:

 
D   0E  P
electric field
magnetic induction:

 
B   0H  M
magnetic field

H

E
polarization

k
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 19/29 I 12/10/2015
magnetization
Maxwell’s Equations (static fields)
1. Charges are the sources of electric fields
D  
Gauss´s Theorem

 D  dA  q( V )
Divergence of electric
field is created by charges
V
2. Magnetic monopoles do not exist
B  0
Gauss´s Theorem

 B  dA  0
In
the
absence
of
magnetic
monopoles,
divergence
of
the
magnetic field lines is
always zero.
V
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 20/29 I 12/10/2015
Maxwell’s Equations (dynamic fields)
3. A changing magnetic field creates an electric field


B
 E  
t
4. Magnetic fields are created by electrical current and by changing electric fields

  D
  H  Jf 
t
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 21/29 I 12/10/2015
Geometric Optics
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 22/29 I 12/10/2015
Geometric Optics
At a planar dielectric surface
• Reflection
• Refraction
• Transmission
media: air, water, glass, …
dielectric: electrical insulator (weak or non-conducting) that
can be polarized by an applied electric field
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 23/29 I 12/10/2015
Reflection
angle of incidence = angle of reflection

’
  '
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 24/29 I 12/10/2015
Refraction
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 25/29 I 12/10/2015
Refraction
A
refractive index n
Normal
n
vacuum: 1
air: 1.0003
water: 1.333
crown glass: 1.5
n’
c (medium)=c/η

’
B
Fermat´s Prinziple
Light minimizes the time the travel from
point A to B. Light velocity in media.
Snell´s Law
n  sin(  )  n ' sin(  ' )
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 26/29 I 12/10/2015
Total Reflection
Water tank: Reflected and refracted light
components!
Fiber optic cable: total reflection important for signal
transmission!
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 27/29 I 12/10/2015
Total Reflection
Snell´s Law
Normal
  c
n
n’
n  sin(  )  n ' sin(  ' )
n’ > n
sin(θ) =1 !
c
n
 c  arcsin 
 n' 
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 28/29 I 12/10/2015
critical angle
Brewster Angle - Linear Polarisation
Brewster Angle: θB
Hertzian Dipole
Brewster Angle: θB
α
α + θB=π/2
Reflected ray polarized due to radiation charachteristic of Hertzian Dipole!
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 29/29 I 12/10/2015
Dispersion
dispersion = dependance between
frequency and wavelength: ω = ω(λ)
λ∙f = c / n()
f = c / (n()∙ λ)
substitute ω = 2πf and k = 2π/ 
ω = k∙c / n(k)
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 30/29 I 12/10/2015
Dispersion – Group and Phase Velocity
wavepakage:   x, t    c j e
i ( j t  k j  x )
j
Gaussian Wavepakage
d  k /  (k ) 
d
 c
dk
dk

c
phase velocity: v phase  
k  (k )
group velocity: vgroup 
“= velocity of wave
package”
“= velocity of single
waves”
The refractive index is wavelength
dependent: n = n()
-> Speed of light in medium is
wavelength dependent: v = c/ n()
= v() !
-> A wave package disperses
If the refractive index (n) is not wavelength dependent 
v
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 31/29 I 12/10/2015
phase
= vGroup
 No dispersion!
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
•Maxwell´s 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(ω)
•Dielectric: η = η(k)
•Wavepackages disperse if group velocity ≠ phase velocity
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 32/29 I 12/10/2015
Next Lecture
2. LASER Principle
Biomedical Optics – „Basics of LASER Physics“
Dr. Sebastian Domsch I Slide 33/29 I 12/10/2015