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 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)=I0ei Photons () I0 t · = c : dispersion in vacuum E = h∙ = p∙c p=h/λ : wave length E: energy : frequency p: momentum c: light velocity = 3108 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