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 sebastian.domsch@medma.uni-heidelberg.de 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 Dr. Sebastian Domsch I Slide 2/29 I 12/10/2015
Literature 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 Dr. Sebastian Domsch I Slide 4/29 I 12/10/2015
Historical Background 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) 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 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 Dr. Sebastian Domsch I Slide 8/29 I 12/10/2015 Bell Telephone Laboratories (NJ/USA)
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 27.1.2015, Oakland, USA Nikolay Gennadiyevich Basow * 14.12.1922, Usman, Russia 1.7.2001, Moscow, Russia Aleksandr Mikhailovich Prokhorov * 11.7.1916, Atherton, Australia 8.1.2002, Moscow, Russia Theoreticl work: MASER principle -> LASER Concept of optical pumping Dr. Sebastian Domsch I Slide 9/29 I 12/10/2015
1960 First LASER Constructed Theodore Harold Maiman Dr. Sebastian Domsch I Slide 10/29 I 12/10/2015
Physical Basics Dr. Sebastian Domsch I Slide 11/29 I 12/10/2015
Properties of Light Dr. Sebastian Domsch I Slide 12/29 I 12/10/2015
Wave Particle Dualism of Light Tissue Matter LASER Light De Broglie (1924) Wave-like behavior of electrons Einstein (1905) Particle: Photoelectric effect (Nobel Price 1921) Quantum optics Geometric Optics particle wave Dr. Sebastian Domsch I Slide 13/29 I 12/10/2015
Properties of Light Electromagnetic Wave Light Quanta (t)=i 0 e i Photons () I 0 t = c : dispersion in vacuum E = h = p c p = h / λ : wave length : frequency c: light velocity = 310 8 m/s E: energy p: momentum h: Planck s constant Dr. Sebastian Domsch I Slide 14/29 I 12/10/2015
Electromagnetic Spectrum Geometric Optics (wave character) Quantum optics (particle character) visible spectrum: = 400 700 nm, = 7,5 4 10 14 Hz Dr. Sebastian Domsch I Slide 15/29 I 12/10/2015
Light - Electromagnetic (EM) Waves EM Fields: - defined by two vector fields: electric field: magnetic field: E(r,t) H(r,t) - caused by electric charges electric currents Dr. Sebastian Domsch I Slide 16/29 I 12/10/2015
EM Wave electric field: E(r,t) H(r,t k(r,t magnetic field: ) wave vector: ) H E k k = 2π / λ Dr. Sebastian Domsch I Slide 17/29 I 12/10/2015
Electromagnetic Fields in Dielectric Media Dr. Sebastian Domsch I Slide 18/29 I 12/10/2015
Dielectric Media Non-Conducting electric displacement field: D E 0 P electric field polarization magnetic induction: B H 0 M H E k magnetic field magnetization Dr. Sebastian Domsch I Slide 19/29 I 12/10/2015
Maxwell s Equations (static fields) 1. Charges are the sources of electric fields V D D Gauss s Theorem da q(v) 2. Magnetic monopoles do not exist Divergence of electric field is created by charges V B B 0 Gauss s Theorem da 0 In the absence of magnetic monopoles, divergence of the magnetic field lines is always zero. 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 E B t 4. Magnetic fields are created by electrical current and by changing electric fields H J f D t Dr. Sebastian Domsch I Slide 21/29 I 12/10/2015
Geometric Optics 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 Dr. Sebastian Domsch I Slide 23/29 I 12/10/2015
Reflection angle of incidence = angle of reflection ' Dr. Sebastian Domsch I Slide 24/29 I 12/10/2015
Refraction Dr. Sebastian Domsch I Slide 25/29 I 12/10/2015
n A Refraction Normal refractive index n vacuum: 1 air: 1.0003 water: 1.333 crown glass: 1.5 n B c (medium)=c/η 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( ' ) 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! Dr. Sebastian Domsch I Slide 27/29 I 12/10/2015
Total Reflection n n c Normal n sin( ) n' sin( ' ) c Snell s Law n > n sin(θ) =1! c arcsin n n' critical angle Dr. Sebastian Domsch I Slide 28/29 I 12/10/2015
Brewster Angle - Linear Polarisation Brewster Angle: θ B Hertzian Dipole Brewster Angle: θ B α α + θ B =π/2 Reflected ray polarized due to radiation charachteristic of Hertzian Dipole! 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) Dr. Sebastian Domsch I Slide 30/29 I 12/10/2015
Dispersion Group and Phase Velocity Gaussian Wavepakage wavepakage: group velocity: v phase velocity: v group phase x, t j c e j i( t k x) d d k / ( k) c dk dk c k ( k) j j = 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 phase = v No dispersion! Group Dr. Sebastian Domsch I Slide 31/29 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 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 Dr. Sebastian Domsch I Slide 32/29 I 12/10/2015
Next Lecture 2. LASER Principle Dr. Sebastian Domsch I Slide 33/29 I 12/10/2015