2. LASER Physics and Systems

Transcription

1 2. LASER Physics and Systems Simon Hubertus, M.Sc. Computer Assisted Clinical Medicine Medical Faculty Mannheim Heidelberg University Theodor-Kutzer-Ufer 1-3 D Mannheim, Germany Outline: Biomedical Optics 1. Lecture Basic Optics 2. Lecture LASER Physics and Systems Light Amplification and Inversion LASER Systems Gas LASER Solid-State LASER Tunable LASER 3. Lecture LASER Resonators 4. Lecture Tissue Interactions I 5. Lecture Tissue Interactions II 6. Lecture Biomedical Applications Wednesday, , 1-3pm House 1, Level 0, Lecture Hall 09 Simon HubertusI Slide 2/63 I 10/24/2017 1

2 LASER Light Amplification by Stimulated Emission of Radiation 100 % mirror 90 % mirror active medium mirrors: optical resonator LASER light high power flash lamp light source: energy pump Simon HubertusI Slide 3/63 I 10/24/2017 Light Amplification and Inversion Simon HubertusI Slide 4/63 I 10/24/2017 2

3 Light Amplification light absorption depends on: electronic energy states of the atoms band structure of the molecules (rotation / vibration) source: P.W. Milonni, J.H. Elberly. Lasers. Wiley 1988 Beer-Lambert-Law 0 e α: absorption coefficient g: optical gain coefficient (g = -α) absorption: g < 0 amplification: g > 0 Simon HubertusI Slide 5/63 I 10/24/2017 Atomic Energy States: 2 Level System Boltzmann distribution in thermal equilibrium E E e, N e E g, N g occupation number: e Boltzmann factor: e 1 (relative) occupation number difference: at thermal equilibrium: the higher the energy gap, the lower the occupation number thermal equilibrium: w < 0 Simon HubertusI Slide 6/63 I 10/24/2017 3

4 Amplifying Medium in thermal equilibrium: in the LASER medium: E E e, N e E E e, N e E g, N g E g, N g N e < N g Inversion: N e > N g light amplifying media are NOT in thermal equilibrium e >1 T < 0 Simon HubertusI Slide 7/63 I 10/24/2017 Stimulated and Spontaneous Processes I, ω B ge B eg A eg E e, N e E g, N g coupling of an atom to a monochromatic light wave coupling to the driving field: stimulated absorption coupling to the driving field: stimulated emission (coherent in phase, frequency and polarisation) incoherent process: spontaneous emission (independent of driving field) transition probabilities Einstein coefficients ) ( & + *! 3 ħ! # +.. /( & 3!ħ & spontaneous emission stimulated absorption/emission equal probabilities!!: electric permittivity #: speed of light ħ % : reduced Planck s constant &' ( : matrix element of dipole operator (QM) Simon HubertusI Slide 8/63 I 10/24/2017 4

5 Driving Light Field I, ω E e, N e 0 ħ1 2 E g, N g high pumping efficiency: high on-resonant driving field high absorption cross-section Stationary condition 2-level system d d ) d d ) 9 : < ) 8 0 d d7 0 no stationary inversion possible in 2-level system Simon HubertusI Slide 9/63 I 10/24/2017 Occupation Number Difference Thermodynamic Equilibrium I, ω Not in Thermodynamic Equilibrium E E e, N e E E e, N e E g, N g E g, N g N e < N g N e = N g more stimulated absorption Equal rate of stimulated absorption and emission! stimulated absorption / emission No amplification! Inversion NOT possible in a 2-level system Simon HubertusI Slide 10/63 I 10/24/2017 5

6 3 Level System = > 0 9 pump E p, N p E g, N g Fast non-radiative transition E e, N e Laser transition d d7 85 = 9 ) d d7 5 = : B CD B C BD stationary inversion possible in 3-level system when 5 = E A But: Inefficient! Simon HubertusI Slide 11/63 I 10/24/ Level System = > 0! 9 pump E p, N p E 0, N 0 Fast non-radiative transition E e, N e Laser transition E g, N g Fast non-radiative transition d d7 0 d d7 5 =! ) B C 0 : E 0 B C BD D G stationary inversion is guaranteed in 4-level system and efficient Simon HubertusI Slide 12/63 I 10/24/2017 6

7 Optical Gain Beer Lambert-Law 0 e H: absorption coefficient I 8H: optical gain coefficient H J K 1 8 / L 8I, I H cm Q 1 9 N/N! Resonant absorption cross section: K L 3#& 2/* & Simon HubertusI Slide 13/63 I 10/24/2017 Optical Gain Simon HubertusI Slide 14/63 I 10/24/2017 7

8 LASER Systems Simon HubertusI Slide 15/63 I 10/24/2017 Natural Abundance of LASER/MASER Cosmic LASERs: source of intense, coherent light fields hot-star : radiation spectrum from IR to UV (black-body) abundance of hydrogen atoms UV light leads to sustained inversion in H 2 molecules IR light leads to stimulated emission in the microwave range (MASER) MWC349 discovered in 1988 λ = 169 µm Simon HubertusI Slide 16/63 I 10/24/2017 8

9 LASER Types source: P.W. Milonni, J.H. Elberly. Lasers. Wiley 1988 gas LASER (HeNe, CO 2, excimer, N2) solid-state LASER: ruby, Nd:YAG (neodymium-doped yttriumaluminium-garnet) LASER diode dye LASER (dye: coloured substance in aqueous solution) Simon HubertusI Slide 17/63 I 10/24/2017 Gas LASER Simon HubertusI Slide 18/63 I 10/24/2017 9

10 Properties of Gas LASER active medium: substances in gaseous phase at room temperature noble gases, e.g. argon, krypton good beam quality high frequency stability low energy consumption high output power in continuous wave operation Simon HubertusI Slide 19/63 I 10/24/2017 HeNe-LASER: Setup capillary tube: S 1 mm tube length: l S 1 m U V ~182 kv He-discharge: several ma power: mw mixing ratio: He/Ne=10/1 He pressure: p 10 mbar Simon HubertusI Slide 20/63 I 10/24/

11 HeNe-LASER: Amplifier He: Pumping Ne: LASER Transition no photonic transition! e-2.png#/media/file:hene-2.png red line: nm line used as length standard meteorology: precision measurements interfereometric and reading devices Simon HubertusI Slide 21/63 I 10/24/2017 HeNe-LASER Why not simply increase the tube length or voltage to increase the LASER power? increasing tube length: decrease of resonant frequency increase of beam width increasing voltage: increase of electron collisions, i.e. deexcitation superradiance between 3s and 3p level (λ 3.39 ]m) withdraws energy Simon HubertusI Slide 22/63 I 10/24/

12 Ar-Ion-LASER: Setup tube with argon plasma, i.e. ionised gas gas pressure: ^ mbar tube length: l m Ar + ions diffuse to cathode: extra gas reservoir high plasma temperature: erosion of walls copper or BeO elements for fast heat conduction magnetic field to focus plasma on axis Simon HubertusI Slide 23/63 I 10/24/2017 Ar-Ion-LASER: Amplifier excitation of Ar + states by step-wise e - -impact highest output power in optical range LASER lines: blue, green, yellow-green optical transitions up to UV (λ~100 nm) applications in entertainment, holography, medicine pumping LASER for other tunable LASER, e.g. dye and titan-sapphire LASER Simon HubertusI Slide 24/63 I 10/24/

13 Metal-Vapour-LASER Example: copper-vapour amplifier excitation by discharge (e - -impact) 2 3-level system pumping: 1 2 laser transition: 2 3_ and 2 3` important LASER lines: yellow, green 1 3a 3b high operation temperature (1500 a) high power (~100 W) quasi CW: pulsed (10 khz, 10 ns) Large excitation probability; strong coupling of dipole-allowed transitions Simon HubertusI Slide 25/63 I 10/24/2017 Kinetic Energy Levels molecules: additional kinetic energy level rotational vibrational bcd efg hihj complex spectrum of transition frequencies LASER lines rotational and vibrational before electronic excitation microwave range: MASER Simon HubertusI Slide 26/63 I 10/24/

14 CO 2 -LASER: Setup infrared LASER: λ 9.4 ]m and 10.6 ]m (kinetic energy level transition) high power output: ^ 80 kw noiph ~100 kj high efficiency ~ 20% low production costs industrial material processing Simon HubertusI Slide 27/63 I 10/24/2017 CO 2 -LASER: Amplifier anti-symmetric stretching bending symmetric stretching discharge excitation of metastable N 2 energy transfer to CO 2 molecules level system pumping: 1 2 LASER transition: 2 3 deexcitation: 3 1 (collision processes, vibrational relaxation) 1 vibrational energy states: v 1, v 2, v 3 vibrational quantum numbers CO 2 gas heating add He to increase thermal conductivity (cooling) Simon HubertusI Slide 28/63 I 10/24/

15 CO 2 -LASER: Configurations conventional longitudinal gas flow: transversly excited: more power highest power LASER operation disturbed by discharge: CO 2 CO + O regeneration of CO 2 necessary continuous gas flow through LASER tube catalyst: H 2 O + CO CO 2 + H 2 Simon HubertusI Slide 29/63 I 10/24/2017 Eximer-LASER eximer = excited dimer diatomic molecules that exist only in an excited state lifetime > 10 ns pulsed LASER ground state: dissociation into two unbound atoms dissociation time > one vibrational period (10-13 s) inversion: no lower level population Simon HubertusI Slide 30/63 I 10/24/

16 Excimer-LASER: Amplifier noble gas: Ar, Kr, Xe e.g. ArF*, KrF*, XeF*, XeCl* excitation: electric discharge wavelength KrF: λ 248 nm ArF: λ 193 nm F 2 : λ 157 nm Simon HubertusI Slide 31/63 I 10/24/2017 Excimer-LASER Applications medicine: cutting with minimal heating of surrounding tissue lithography LASIK surgery (Laser-assisted in-situ Keratomileusis) IBM logo on human hair Simon HubertusI Slide 32/63 I 10/24/

17 Gas LASER: Overview Simon HubertusI Slide 33/63 I 10/24/2017 LASER Applications LASIK surgery (Laser-assisted in-situ Keratomileusis) excimer LASER: ~ 50 mw CO 2 LASER: 250 W solid-state LASER: ~ 500 kw Simon HubertusI Slide 34/63 I 10/24/

18 Solid-State LASER Simon HubertusI Slide 35/63 I 10/24/2017 Properties of Solid-State LASER amplifier media: rare earth metals (lanthanides + yttrium + scandium) yttrium: higher abundance than chemically similar lanthanides non-tunable Simon HubertusI Slide 36/63 I 10/24/

19 Properties of Solid-State LASER: Crystals host lattice: doped with optically active ions ion concentrations of a few % impurity ion density particle density in gas LASER higher gain density inhomogeneous temperature pattern thermal lensing : refraction index depends on temperature Active research: LASER crystals with..reduced thermal lensing..higher gain densities..more LASER frequencies Simon HubertusI Slide 37/63 I 10/24/2017 Properties of Solid-State LASER compact design and robust construction low production costs; economical efficient excitation by diode LASER power conversion: electrical light efficiency: ~ 20% applications: pump LASER for excitation of tunable LASER material processing demanding high intensity LASER todays workhorses Maiman's chromium-doped ruby LASER (Cr: Al 2 O 3 ) in 1960 Simon HubertusI Slide 38/63 I 10/24/

20 Properties of Solid-State LASER: Dopants rare earth ions: triply ionised with e - configuration 4f n (1 S n S 13) optical properties of host crystal determined by 4f electrons broadening of electronic states due to weak phonon coupling wealth of spectrum due to high electron number rare earth ion electron configuration: 4f n energy levels of rare earth ions Simon HubertusI Slide 39/63 I 10/24/2017 Nd:YAG-LASER: Amplifier 2 3 neodymium-doped yttrium-aluminiumgarnet (Nd:Y 3 Al 5 O 12 ) level system Nd pumping with diode LASER: 1 2 fast phonon relaxation: 2 3 radiation-free electron-phonon interaction LASER transition: 3 4 fast phonon relaxation: 4 1 Simon HubertusI Slide 40/63 I 10/24/

21 Nd:YAG-LASER: Configurations Classically pumped with Xe-lamp End-pumped LASER Neodymium mirror drive current noble gas Xe mirror pump lamp and LASER bar located at the two foci of an elliptical resonator Nd LASER longitudinally pumped with a diode LASER until the late 1980s: high-pressure noble gas lamps, e.g. Xe significant heat production today: high-power LASER diodes higher efficiency Simon HubertusI Slide 41/63 I 10/24/2017 Er:YAG-LASER: Amplifier 2 erbium-doped yttrium-aluminium-garnet (Er:Y 3 Al 5 O 12 ) 3 3-level system Er pumping with diode LASER: 1 2 LASER transition: 2 3 medical applications, eye-safe operation wavelength LASER transition: 3 1 infrared 1 2S+1 L J Simon HubertusI Slide 42/63 I 10/24/

22 Erbium-Doped Fiber Amplifier (EDFA) Er-doped optical fibers D. Payne, E. Desurvire 1989 amplification at λ 1.55 ]m breakthrough for long distance data transmission Simon HubertusI Slide 43/63 I 10/24/2017 Yb:YAG-LASER: Amplifier ytterbium-doped yttrium-aluminium-garnet (Yb:Y 3 Al 5 O 12 ) advantages over Nd high gain density due to strong Ybdoping up to 25 % (Nd: 1-2 %) 940 nm (Nd: 808) LASER nm (Nd: 1064) less excitation into non-lasing states less reabsorption of fluorescent light less heat generation thin-disc technology: Improved heat removal Simon HubertusI Slide 44/63 I 10/24/

23 Frequency-Doubled Nd-LASER has replaced expensive Ar-LASER pump energy applied through fiber bundles (high power) LBO crystal lithium triborate (LiB 3 O 5 ) non-linear frequency doubling wavelength: λ 1064/2 nm 532 nm (visible light) non-linear crystal properties quantum mechanics: high-intensity light causes frequency doubling in a crystal; two photons are absorbed, only one is emitted electrodynamics: light forces atomic dipoles to oscillate at the same and higher frequencies Simon HubertusI Slide 45/63 I 10/24/2017 Excursus: Non-Linear Optics electric displacement field polarisation: linear in E only for low intensities; higher order terms become important at high intensities wave equation from Maxwell's equations consideration of second order term ansatz: wave with two distinct frequencies * Q, * & solution: polarisation with 5 frequency components 2* Q, 2* &, * Q 9* &, * Q 8* & and 0 Simon HubertusI Slide 46/63 I 10/24/

24 LASER Diodes most common type of LASER..fiber optic communications..barcode reader..laser pointer..cd/dvd..laser printing..laser scanning pumping: electrical active medium: p-n junction Simon HubertusI Slide 47/63 I 10/24/2017 LASER Diodes: p-n Junction semiconductor small energy gap between valence and conduction band non-conducting at low temperature strongly conducting with increasing temperature (threshold) e.g. silicon (Si), germanium (Ge) doping: impurities with different number of valence electrons n-type: higher number of valence electrons (donator) p-type: lower number of valence electrons (acceptor) donator energy level n-doped acceptor energy level p-doped conduction band valence band p-n junction: combination of p- and n-doped semiconductors Simon HubertusI Slide 48/63 I 10/24/

25 LASER Diodes: p-n Junction diffusion driven charges anode conducting direction I DC cathode Simon HubertusI Slide 49/63 I 10/24/2017 LASER Diodes Mirror 90% Photon Photon + Phonon Mirror 100% Anode I DC Cathode electric current forces recombination of electrons and holes annihilation leads to photon emission light emitting diode (LED) high current and optical resonator LASER Simon HubertusI Slide 50/63 I 10/24/

26 Tunable LASER Simon HubertusI Slide 51/63 I 10/24/2017 Tunable LASER: Materials transition metal ions in host crystal colour centres (crystal defects absorbing light) dyes: organic molecules with a C double bond (C=C) Simon HubertusI Slide 52/63 I 10/24/

27 Tunable LASER: Materials dyes transition metal ions optical range transition metal ions wide frequency range Simon HubertusI Slide 53/63 I 10/24/2017 Transition-Metal-Ion-LASER electron configuration: 3d n strong coupling of the ions to the lattice b/c the 3d-electrons form the outermost shell Simon HubertusI Slide 54/63 I 10/24/

28 Vibronic-LASER Vibronic States of Solid-State Ions 2 tunable over large range different positions of ground and excited state in thermal equilibrium large separation of absorption and emission frequency 4-level system pumping: 1 2 v-v relaxation: 2 3 LASER transition: 3 4 v-v relaxation (10 Q& ): thermal distribution in the configuration coordinate Q Spectra of Ti-Sapphire Crystal spontaneous emmison of photons after excitation of atoms Simon HubertusI Slide 55/63 I 10/24/2017 Colour Centres no impurities but rather vacant lattice sites (holes) vacancy: effective charge relative to crystal binding of e - and holes electronic states LASER wavelength: near-infrared (λ 183 ]m) costly operation temperature 77K (liquid nitrogen) additional light source my be required to prevent parasitic states (spontaneous emission) Simon HubertusI Slide 56/63 I 10/24/

29 Dye-LASER..LASER using an organic dye, e.g. dissolved in alcohol, as medium Dyes: organic molecules with C=C double bond gold standard of tunable LASERs: nm Singlet state Triplet state band structure: rotation-vibration fine structure of dye broadened to continuous bands because of interaction with the solvent (similar to vibronic ions) Simon HubertusI Slide 57/63 I 10/24/2017 Ring-LASER narrowing of spectral width from to 10 Q& to 10 s Hz range by adding optical components travelling wave propagation standing-wave LASER spectroscopic applications with high spectral resolution possible SA: servo-amplifier optical diode: allows only one polarisation; no reflected light piezo drive and Glavo plates: varies path length for tuning Lyot filter: crystal where only certain frequencies can pass in certain directions; spectral narrowing Etalon: select a single mode Simon HubertusI Slide 58/63 I 10/24/

30 Air-LASER??? Simon HubertusI Slide 59/63 I 10/24/2017 Air-LASER air: 78% N 2, 21% O 2, 0.4% CO 2 nitrogen amplifier: λ 337nm (UV) high voltage: breakdown of air at sharp tip spark discharge along knife edges knife edges Air aluminum foil ~25kV Simon HubertusI Slide 60/63 I 10/24/

31 Repetition Light Amplification and Inversion Beer-Lambert Law Boltzmann factor in equilibrium Stationary inversion not possible in 2-level, possible in 3-level (W p > A) and guaranteed in 4-level system Cosmic Laser/Maser Solid State Laser: transition metal amplifier: host doped with ions nearly continuous electronic state due to strong coupling of electrons to lattice Dye LASER broadened electronic states due to coupling to solvent tunable frequency Simon HubertusI Slide 61/63 I 10/24/2017 Repetition Solid State Laser: rare earth ion amplifier: host crystal doped with ions ion concentration few % broadened electronic states due to weak coupling of electronic to lattice Neodymium LASER & Erbium LASER pumped with LASER Diodes Erbium Doped Fiber Amplifier long distance signaling Ytterbium LASER: higher ion concentration higher gain density Frequency Doubled Neodymium LASER Nonlinear Crystal fixed frequency LASER Diodes most common type of LASER pn-junctions operated with direct current in conduction direction light intensity increased by optical resonator Simon HubertusI Slide 62/63 I 10/24/

32 Next Lecture 3. LASER Resonators Simon HubertusI Slide 63/63 I 10/24/