Laser Physics
Content Laser physics Laser treatment parameters Laser safety 2
LASER PHYSICS 3
Light 4
Interaction of the light with tissue REFLECTION ABSORPTION TRANSMISSION 5
Laser light Monochromatic (only one wavelength) Coherent (waves are in phase) Collimated (beam is straight line) Non-monochromatic Non-coherent Monochromatic and coherent 6
Optical resonator Highly reflective mirror Flashlamp (pump source) Partially reflective mirror Nd:YAG crystal Q-switch Laser output Optical resonator 7
LASER TREATMENT PARAMETERS 8
Four different wavelengths 9
Four different wavelengths 10
Energy Energy is measured in Joule (J). The energy we need to lift, heat or evaporate whole cube of water (3x3x3 mm): Lift, 1 m high: 0,27 J potential Heat, from 0 C to 100 C: 10 J thermal Evaporate: 61.000 J thermal water 3 mm 3 mm 3 mm 11
Fluence (Surface density of Energy) E E E d d d E is energy of single laser pulse d is diameter of laser beam (spotsize) Fluence = Energy Surface 12
Laser beam profile Top-Hat profile Gaussian profile 13
Peak power power of single pulse Shorter pulse higher power for mechanical effect. Longer pulse lower power for thermal effects. Equal energies, but very different effects. 100 P (W) Power = Energy Time 10 10 100 t (msec) 14
Peak power power of single pulse Energy: 10 J & Pulse duration: 5 ns Peak power: 2 GW! 1 nanosecond vs. 1 second = 1 second vs. 31 years 100 P (W) Power = Energy Time 10 10 100 t (msec) 15
Controlled delivery of energy t 16
Repetition (ν) how many pulses are emitted per 1 second measured in Hertz (Hz) T is time between pulses: T = 1 / ν t is duration of laser pulse t T Time 17
Average power power of many pulses Peak power is power of single pulse: P peak = E p t. Average power is power averaged over many pulses: P ave = E p T = E p ν. Peak and average power can be different. t T Time 18
Articulated arm High reflectance mirrors reflect the laser beam Preserves the laser beam profile Durability and precision Vacuum Cell Technology OPTOflex 19
LASER SAFETY 20
Laser hazards Thermal effects (hyperthermia, coagulation) Mechanical effects (creation of plasma, vaporisation, cavitation) Electrical hazard (high voltage) Explosion and fire hazard (ignition) Chemical hazard (corrosion protective layers) Eye hazard 21
Standards To control the risk of injury, various specifications, for example IEC 60825 internationally, define "classes" of laser depending on their power and wavelength. These regulations impose upon manufacturers required safety measures, such as labeling lasers with specific warnings, and wearing laser safety goggles when operating lasers. Consensus standards, such as American National Standards Institute ANSI Z136, provide users with control measures for laser hazards, as well as various tables helpful in calculating maximum permissible exposure (MPE) limits and accessible exposures limits (AELs). Name the Laser Safety Officer to take care of laser safety issues. 22
Classification Class 1 safe under all conditions of normal use. (computer mouse, laser printers, CD player) Class 1M safe under all conditions except when passed through magnifying optics, such as microscopes and telescopes. (fiber communication systems) Class 2 safe because the blink reflex and applies only to visible light. Limited to power of 1 mw. (pointers, alignment lasers, barcode readers) Class 2M safe because the blink reflex and if not viewed through optical instruments. Class 3R safe if handled carefully. Limited to power of 5 mw. (pointers, laser scanners) Class 3B hazardous if eyes exposed dirrectly. Limited to power of 500 mw. (lasers in ophtalmology, spectrometry, stereolithography) Class 4 can burn skin, or cause devastating and permanent eye damage as a result of direct, diffuse or indirect beam viewing. (medical lasers) 23
Class 4 Doorswitch remote control Key physical access Sound when laser system turns on Sound when footswitch is pressed and laser light is emitted Atenuator (shutter) Labels: Caution Class 4 laser Laser aperture Laser characteristics Another caution on laser head 24
Maximum permissible exposure - MPE The MPE levels represent the maximum level to which the eye or skin can be exposed without consequential injury immediately or after a long time. Is related to the wavelength or radiation, the pulse duration or exposure time, the type of tissue. HANDPIECE MPE HANDPIECE MPE 25
Nominal Ocular Hazard Distance - NOHD Is the distance from the laser source at which the intensity or the energy per surface unit becomes equal to the MPE. The values of NOHD are usually bigger than operating room. HANDPIECE MPE NOHD MPE HANDPIECE NOHD 26
Cautions Carefully read warnings, cautions and notes in operator and applications manuals. The laser system should be isolated from areas where the uninformed and curious would be attracted by its operation. Warnings on the doors of the operating room should be present. Doors should be closed or locked to keep out unqualified personnel. To prevent unauthorised usage of the laser system, the key can be removed. 27
Cautions Windows in the operating room should be shuttered. No reflection surfaces in the operating room (mirrors, jewelry, ). Eye protection suitable to the laser must be provided and worn. 28
Cautions Before operating the laser, warn all personnel and visitors of the potential hazard, and ensure all safety measures are satisfied. Ablative procedures can create a plume of smoke that can potentially harm. Use specific masks and specific smoke evacuator. Always carefully read advisory messages on laser system display. 29
Eye hazard Always wear protective goggles Everybody in the room where procedure is done, should wear protective goggles Usually single protective goggles don t protect you from different laser devices. 30
Protective goggles Example of specification: 815-1100 IR LB7 Protected wavelengths (in nanometers) Operating modes: D continuous mode (>0.25 s) I pulsed mode (1 µs-0.25 s) R Q-switched mode (1 ns-1 µs) Optical density: 0 100 % transmission 1 10 % 2 1 % 3 0.1 % 4 0.01 % 5 0.001 % 6... 0.0001 % 7 0.00001 % 31