Laser Physics and Safety PDF

LASER PHYSICS AND SAFETY NICHOLAS R. GREEN, OD, FAAO  H O W D I D W E G E T T O H AV E L A S E R S ? Came from our evolving understanding of light, its properties, & how to manipulate it Socrates observed that light from a solar eclipse was thermally damaging to the eye Max Planck determin...

LASER PHYSICS AND SAFETY NICHOLAS R. GREEN, OD, FAAO  H O W D I D W E G E T T O H AV E L A S E R S ? Came from our evolving understanding of light, its properties, & how to manipulate it Socrates observed that light from a solar eclipse was thermally damaging to the eye Max Planck determined that energy released from a black body was related to its frequency times a constant (Planck’s Constant) In the 1940s, German ophthalmologist Gerhard Meyer-Schwickerath used sunlight directed by mirrors on the clinic roof to create precision burns on the retina In 1960, Thomas Maiman developed the first working laser (Ruby Laser) LASER PHYSICS L A S E R S TA N D S F O R … LIGHT AMPLIFICATION by SIMULATED EMISSION of RADIATION W H AT I S L I G H T ? Spectrum of electromagnetic radiation Can be considered either as a wave or a particle Described and classified by either frequency or wavelength Frequency is inversely proportional to wavelength For our purposes, we will use wavelength L I G H T A S A W AV E Describes how light moves through space Expressed mathematically as: Light sources oscillate which creates v = c/λ wavelengths v = frequency Two types of wavelengths: c = velocity Longitudinal or compression waves λ = wavelength in nm Moves in the same plane as the direction of travel Transverse waves Moves perpendicular to the direction of travel INTERFERENCE AND COHERENCE Light waves behave like sine waves which means they have an amplitude, frequency, and phase When two waves come together, they can combine (called interference) Constructive Interference When two waves in the same phase combine The amplitudes add and make the wave stronger This is what we want from lasers Destructive Interference When two waves whose phases are 180 degrees different combine The amplitudes cancel each other out When different waves have the same amplitude, frequency, and phase, they are said to be coherent All light sources are incoherent, except for lasers L I G H T A S A PA R T I C L E Describes light as a packet of electromagnetic radiation (called a quantum) Divided into units called photons Describes the transfer of energy between particles or atoms Described mathematically with the following equation: E = hv E = energy of the photon in Joules h = Planck’s constant (6.626x10^-34 Joule seconds) v = frequency of a particular photon in second^-1 AT O M A N AT O M Y ELECTRON ENERGY LEVELS Electrons occupy distinct energy levels called orbitals Typically, they will occupy the lowest orbital due to stability (i.e., ground state) The farther away the orbital is from the nucleus, the higher the energy level They can move to further orbitals (i.e., excited state)  Electrons can move to a further orbitals if it absorbs energy from an external source (i.e., a photon) The photon must be exactly equal to the change in energy (ΔE) between the orbitals ABSORPTION EMISSION The production of light by electrons changing from higher to lower orbitals Classified based on how the light is produced: Incandescence Light produced by heating a solid (i.e., a filament in a light bulb) Usually produce multiple wavelengths of light Inefficient Luminescence Light produced by direct excitation of electrons Usually produces photons of specific wavelengths Lasers emit in this way  Electrons will only stay in the excited state for a short period of time It will move spontaneously to the ground state and emit a photon of light S P O N TA N E O U S E M I S S I O N  When an incident photon equal to ΔE passes by an electron in the excited state, it induces the electron to drop to the ground state and release a photon of the same energy level The released photon will have the same energy, phase, and direction as the incident photon Electrons must in an excited state for a longer time for simulated emission to occur S IM U L AT E D E M IS S IO N  Population Inversion Most electrons in an atom must be in an excited state Metastable State Most electrons must be in an excited state, but not in the furthest possible orbital Electrons are more stable and therefore, less likely to spontaneously emit Leads to amplification of photons of the same wavelength WHAT CONDITIONS ARE NEEDED FOR S I M U L AT E D E M I S S I O N ? L A S E R C H A R AC T E R I S T I C S A N D COMPONENTS LASER LIGHT PROPERTIES Monochromatic A single wavelength that is amplified through simulated emission Temporally Coherent Has a nearly infinite length Collimated Goes in the same direction with minimal divergence Creates a highly concentrated beam of light Powerful More photons emitted per second per radiating surface area POWER DENSITY J = Joules or number of photons emitted W = Watts or Joules/second Cm^2 = given area where the photons are transferred to (spot size) Think about this relationship as we discuss how a laser operates LASER COMPONENTS Output Coupler Lasing Medium Provides atoms for simulated emission Can be a gas, solid crystal, dye, or semiconductor (diode) Excitation Mechanism or Pump Provides energy to induce simulated emission Done with electricity or light (either a flash bulb or another laser) Optical Resonator or Resonance Cavity Stores lasing medium using precisely aligned mirrors Keeps most photons inside laser to amplify the signal Length determines the wavelength of light emitted HOW DOES A LASER WORK? Energy from the excitation mechanism is pumped into lasing medium Electrons within the lasing medium are brought to an excited and metastable state Electrons in the most excited state spontaneously emit and bounce around the resonance cavity This light then stimulates emission of electrons in metastable states This emission creates monochromatic light which bounces within the resonance cavity, amplifying and harmonizing the direction of the light A small portion of the light escapes through the output coupler, creating a laser beam LASER PULSE TYPES Continuous Quotient (Q)-Switching Emits laser light as long as its Emits laser light in nanosecond pulses activated Achieved by adding a shutter which Typically causes thermal effects to allows for a build-up of energy target tissues Does not cause thermal effects Long Pulse Mode Locking Emits laser light in millisecond pulses Emits laser light in femtosecond Also typically causes thermal effects pulses Achieved by adding an absorber which only allows for release of high frequency laser light  SPOT SIZE CAN ALSO BE MODIFIED L A S E R - T I S S U E I N T E R AC T I O N S THE BASICS Can occur in several ways, with overlaps Depends on: Power Pulse-duration Wavelength indices Interactions between the laser beam and tissue consists of three components Laser Transmission Absorption Degradation LASER TRANSMISSION Depends on ocular media transparency Transmits light in the ultraviolet, visible, and infrared spectrums ABSORPTION Dependent on the wavelength of emitted light and the absorbing ocular tissue Absorbing Tissues: Melanin Found in the RPE, iris, and trabecular meshwork Absorbs a wide range of wavelengths Xantophyll (ie. Macular pigment) Absorbs light in visible spectrum Hemoglobin Absorbs light at approximately 532 nm the best W AV E L E N G T H A B S O R P T I O N O F T I S S U E S 1 = Melanin 2 = Reduced hemoglobin (no O2 attached) 3 = Oxyhemoglobin 4 = Xanthophyll  What happens when the laser light is absorbed by a tissue Photocoagulation Absorbed light increases temperature of tissues and denatures the proteins Causes scarring and contraction of surrounding tissues Pigment dependent Seen in retinal laser procedures and anterior segment procedures involving argon lasers Photovaperization Infrared light is absorbed by the tissue, vaporizing intra- and extracellular water D E G R A DAT I O N Pigment dependent Excellent for cauterization and bloodless procedures Used by carbon dioxide lasers Photoablation UV light used to break up long chained polymers Used in corneal refractive surgery Pigment independent  Photoradiation or Photostimulation Light induces production of cytotoxic free radicals within targeted tissues Requires use of photosensitizing agent and long exposure times Used in photodynamic therapy (ie. dye laser) Pigment independent Selective Photothermolysis Like Photostimulation excepts it targets specific pigments Triggers macrophages to digest targeted pigments Applied in short pulse to prevent heat production D E G R A DAT I O N Pigment dependent Photodisruption Pulses of high energy light strips electrons from atoms Creates a plasma which rapidly expands and produces a shock wave, disrupting the targeted tissue Pulse so short (nanoseconds) that it does not cause any thermal damage Pigment independent TA B L E O F L A S E R - T I S S U E I N T E R AC T I O N S Laser Tissue Reaction Excimer Photoablation Argon Photocoagulation Krypton Photocoagulation Dye Photoradiation ND:YAG Photodisruption Frequency Doubled ND:YAG Photocoagulation Carbon Dioxide Photovaperization S A F E T Y C O N S I D E R AT I O N S WHO GOVERNS LASER SAFETY? American National Standards Institute (ANSI) Develops laser classification and safety standards Responsible for developing laser safety standards in US Occupational Health and Safety Administration (OSHA) Develops classification system and workplace safety standards International Electrotechnical Institute (IEC) Develops classification system Food and Drug Administration (FDA) Develops classification system and safety standards  L A S E R C L A S S I F I C AT I O N S Y S T E M Class FDA Class IEC Laser Product Hazard Product Examples I 1, 1M Considered non-hazardous. Hazard increases if viewed with optical laser printers aids, including magnifiers, binoculars, or telescopes. CD players DVD players IIa, II 2, 2M Hazard increases when viewed directly for long periods of time. bar code scanners Hazard increases if viewed with optical aids. IIIa 3R Depending on power and beam area, can be momentarily hazardous laser pointers when directly viewed or when staring directly at the beam with an unaided eye. Risk of injury increases when viewed with optical aids. L A S E R C L A S S I F I C AT I O N S Y S T E M Class FDA Class IEC Laser Product Hazard Product Examples IIIb 3B Immediate skin hazard laser light show from direct beam and projectors immediate eye hazard industrial lasers when viewed directly. research lasers IV 4 Immediate skin hazard laser light show and eye hazard from projectors exposure to either the industrial lasers direct or reflected beam; research lasers may also present a fire medical device lasers for hazard. eye surgery or skin treatments REQUIRED SAFETY FEATURES ON CLASS IIIB & IV LASERS Protective Housing Safety Interlocks Key Activation Emission Indicator Warning Labels OTHER SAFETY MEASURES Protective eyewear Designed to block specific wavelengths Different eyewear for different types of lasers Nominal Hazard Zone Area where risk of eye damage is high Posted Warning Signs Safety Manual Minimizing people near a laser when in use LASER-INDUCED OCULAR DISEASE N E G AT I V E H E A LT H E F F E C T S O F L A S E R S WAVELENGTH OCULAR EFFECT SKIN EFFECT UV C (100-280 nm) Photokeratitis Sunburn, skin cancer UV B (280-315 nm) Photokeratitis Accelerated skin aging, increased pigmentation UV A (315-400 nm) Photochemical UV cataract Pigment darkening, skin burn Visible light (400-780 nm) Photochemical and thermal Photosensitivity, skin burn retinal injury Infrared A (780-1400 nm) Cataract, retinal burns Skin burn Infrared B (1400-3000 nm) Corneal burn, aqueous flare, Skin burn infrared cataract Infrared C (3000-10000 nm) Corneal burn Skin burn P H O T O K E R AT I T I S Acute and painful, but self-limiting condition due to excessive exposure to UV light Typically, UV-B and UV-C (