1 introduction - lasers

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

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



Literature



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



Historical Background



Discovery of Stimulated Emission in 1917

Albert Einstein

* 14.3.1879 (Ulm, Germany) 18.4.1955, (Princeton, USA)



1960 First LASER Constructed

Theodore Harold Maiman

* 11.7.1927, Los Angeles, USA

5.5.2007, Vancouver, Canada



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



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



1960 First LASER Constructed

Theodore Harold Maiman



Physical Basics



Properties of Light



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



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



Electromagnetic Spectrum

visible spectrum: = 400 700 nm, = 7,5 4 1014 Hz

Geometric

Optics

(wave

character)

Quantum

optics

(particle

character)



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:



EM Wave

)t,r(E

electric field:

magnetic field: )t,r(H

wave vector: )t,r(k

E

H k

|k| = 2 /



Electromagnetic Fields in

Dielectric Media



Dielectric Media Non-Conducting

PED

0

electric field

electric displacement field:

polarization

E

MHB

0

magnetic field

magnetic induction:

magnetization

H

E

H k



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



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



Geometric Optics



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,



Reflection

'

angle of incidence = angle of reflection



Refraction



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/



Total Reflection

Fiber optic cable: total reflection important for signal

transmission!

Water tank: Reflected and refracted light

components!



Total Reflection

n

n

Normal

n > n

c

critical angle

'n

narcsinc

c )'sin('n)sin(n

Snells Law

sin() =1 !



Brewster Angle - Linear Polarisation

Brewster Angle: B

Reflected ray polarized due to radiation charachteristic of Hertzian Dipole!

B + =/2

Brewster Angle: B

Hertzian Dipole



Dispersion

dispersion = dependance between

frequency and wavelength: = ()

f = c / n()

f = c / (n() )

substitute = 2f and k = 2/

= kc / n(k)



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



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



Next Lecture

2. LASER Principle

1 introduction - lasers

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