University:
Boston University
Course:
CAS PY 105 | Physics 1
Academic year:

2022
Views:

467

Pages:

40
Author:

Josephine Dorsey

FUNDAMENTALS OF LASER OPERATION Laser Basic Laser Safety What is a Laser? Light Amplification by Stimulated Emission of Radiation  The energy generated by a laser is in or near the optical portion of the electromagnetic spectrum Definitions Properties of Laser Radiation  Monochromatic - all light produced by the laser is of one wavelength or color. Exceptions include multiline gas lasers and new white-light lasers. ex. Nd:YAG (532,1064nm)  Directional - collimated (divergence~1mRad) photons traveling in the same direction  Coherent - all peaks of sinusoidal waves (photon) are in phase with each other *Lasers pose more hazard than ordinary light because energy is focused into a small area Types of Laser  Continuous Wave (CW) – operates as a continuous output for a period > 0.25 seconds  Pulsed Laser – delivers energy in single pulses with a duration of less than 0.25 seconds  Q-switched laser – emits short (10 ns – 250 ns) highpower pulses by means of a Q-switch Laser Systems Laser Components Active Medium  Solid State-operate in pulsed or continuous wave (CW) mode  Semiconductor  Gas - CW CO2, Argon, HeNe, Excimer  Liquid (dye) (CW or pulsed) Excitation Mechanism (Pump)  Optical  Electrical  Chemical Optical Resonator  Mirror  Partially reflective mirror Semiconductor (Diode)  GaAlAs- 750-950mm range  InGaAsP (Indium Phosphate 1100-1650 nm Laser Basic Laser Safety - EM Spectrum The optical spectrum Laser light is nonionizing and ranges from the:  ultra-violet (100 - 400nm)  visible (400 - 700nm), and  infrared (700nm - 1mm). BIOEFFECTS OF LASER RADIATION ON THE EYE AND SKIN Eye EYE  Light rays enters the eye through the cornea (the clear front “window” of the eye). Light is refracted by the cornea and the bent rays pass through the pupil, the opening in the center of the iris, through which light enters the eye.  Light rays pass through the eye and come to a sharp focusing point on the retina. Eye Anatomy Retinal Injury     Photoretinitis: Photochemical effect from duration of greater than 10 seconds and intense exposure to laser wavelengths from 400 – 500 nm Retinal burns: Photocoagulation of retina by brief (pulsed) and intense exposure to laser radiation between 400 – 1400 nm Photo disruption of Retina: Retinal hemorrhage from q-switched laser pulse Visual Effect: Scotoma – blind spot in the visual field RETINAL DAMAGE CAN CAUSE PERMANENT LOSS OF VISION Corneal Injury     Photo keratitis: Welder’s Flash or snow blindness – photochemical effect on corneal epithelium by UVB/UVC laser radiation Corneal Burns: thermal effect on corneal epithelium by IRB or IRC laser radiation Superficial Injury: lesion clears in 24 – 48 hours Deep Burns: penetrating burns that produce permanent damage – corneal transplant for repair may be required 4 5 3 2 1 Each number represents a laser hit to the vitreous and in close proximity to the retina 8 6 7 Skin burns Skin Photochemical and Thermal Burns  Ultraviolet (UV) Sunburn: erythema: skin reddening caused by photochemical effect on skin epithelium by UVB or UVC laser radiation  UV Delayed Effects: accelerated skin aging/skin cancer  Thermal Skin Burns: typically from IR laser radiation, thermal effect on skin epithelium.  High powered (Class 4) lasers, can burn the skin and set clothes or flammable objects in the optical path on fire. Potential Biological Laser Effects* Band Wavelength Eye (nm) UV-C 200-280 Corneal burn UV-B 280-315 Corneal burn UV-A 315-400 Photochemical cataract Visible 400-780 IR-A 780-1400 IR-B 1400-3000 IR-C 3000-1 mm Photochemical and thermal retinal injury Cataract, retinal burn Corneal burn, aqueous flare, possibly cataract Corneal Burn Skin Erythema (sunburn), skin cancer Accelerated skin aging, pigmentation Pigmentation darkening, photsensitive reaction, skin burn Photosensitive reaction, skin burn Skin burn Skin burn Skin burn SIGNIFICANCE OF SPECULAR AND DIFFUSE REFLECTIONS Lasers Physics Diffuse or Specular? Specular – Smooth surfaces irregularities or roughness are < the incident light wavelength. Light reflect at a definite angle Diffuse – Surface irregularities or roughness are > the incident light wavelength Light is scattered in all directions Lasers Physics Not all viewing conditions are the same Specular reflection Convex reflector Diffuse reflection Concave reflector Commonly Used Laser Operating Parameters  Power* – the rate at which energy emitted from a laser (expressed in units of Watts)  Beam diameter** – distance across the center of the beam such that the irradiance (intensity) is 1/e2 of the maximum irradiance  Spot size** – the radial distance from the center point of maximum irradiance to the 1/e2 point.  Power Density (irradiance)* - The amount of incident laser power on a unit surface area (expressed as Watts/cm2) Lasers Physics Light Interaction Objects tend to selectively absorb, reflect, or transmit light of certain frequencies  Absorption* – transformation of radiant power to another type of energy, typically heat  Reflection* -those wavelengths not absorbed or transmitted  Transmission* – passage of electromagnetic radiation through a medium  Diffusion (scattering)* – deflecting unidirectional beams into many directions  Laser-induced fluorescence (LIF)** – optical emission from molecules excited to higher energy levels by the absorption of electromagnetic radiation NON-BEAM HAZARDS OF LASERS Hazards Associated Potential Hazards Recognize hazard potential to eliminate potential disasters:     Blindness Burns Fire Death Non -Beam Hazard Electrical Hazards-Non Beam Hazards  Inspect cords, cables, and plugs  Improperly insulated electrical terminals  Excessive wires and cables on floor (trip hazard)  Equipment inspection certification Non -Beam Hazard Basic Laser Safety - Non-Beam Hazards Chemical Dye lasers Gases from laser operations Optical UV from laser welding UV from discharge tubes and pumping Fire/Explosion Ignition of gases and/or vapors Electrical wiring and capacitor banks Non-beam hazard Laser Gas Air Contaminates (LGAC) Laser plume may contain biological entities, carcinogens, fine dust mutagen, irritants, metal oxides, etc. Effective Controls  Safe work practice  Ventilation system with exhaust to the outside air (HEPA filter) LASER AND LASER SYSTEM CLASSIFICATIONS Lasers Laser Classification • Class 1 - is safe under all conditions of normal use. This means the maximum permissible exposure (MPE) cannot be exceeded • Class 1M - is safe for all conditions of use except when passed through magnifying optics such as microscopes and telescopes. Class 1M lasers have a total output power is below class 3B but the power that can pass through the pupil of the eye is within Class 1. Class 1M lasers produce large-diameter beams, or beams that are divergent. NOTE: The MPE for a Class 1M laser cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. If the beam is refocused, the hazard of Class 1M lasers may be increased and the product class may be changed. Please verify with the laser safety officer if your application narrows a Class 1M beam. Lasers Laser Classification • Class 2 - is safe because the blink reflex will limit the exposure to no more than 0.25 seconds. It only applies to visible-light lasers (400– 700 nm). Class 2 lasers are limited to 1 mW continuous wave, or more if the emission time is less than 0.25 seconds or if the light is not spatially coherent. NOTE: Intentional suppression of the blink reflex could lead to eye injury. • Class 2M - A Class 2M laser is safe because of the blink reflex if not viewed through optical instruments. As with class 1M, this applies to laser beams with a large diameter or large divergence, for which the amount of light passing through the pupil cannot exceed the limits for Class 2. Lasers Lasers Laser Classification • Class 3R - is considered safe if handled carefully, with restricted beam viewing. With a class 3R laser, the MPE can be exceeded, but with a low risk of injury. Visible continuous lasers in Class 3R are limited to 5 mW. For other wavelengths and for pulsed lasers, other limits apply. • Class 3B - is hazardous if the eye is exposed directly, or reflected but diffuse reflections such as from paper or other matte surfaces are not harmful. Continuous lasers in the wavelength range from 315 nm to far infrared are limited to 0.5 W. For pulsed lasers between 400 and 700 nm, the limit is 30 mW. Other limits apply to other wavelengths and to ultra short lasers. Protective eyewear is required where direct viewing of a class 3B laser beam may occur. Class-3B lasers must be equipped with a safety interlock. Lasers Laser Classification Class 4 - can burn the skin, in addition to potentially devastating and permanent eye damage as a result of direct or diffuse beam viewing. These lasers may ignite combustible materials, and thus may represent a fire risk. Class 4 lasers include all lasers with beam power greater than class 3B. They must be equipped with a safety interlock and require administrative and engineering controls. Most industrial, scientific, military, and medical lasers are in this category. CONTROL MEASURES Exposure Defined Basic Laser Safety - Exposure Limits  MPE (Maximum Permissible Exposure): the highest laser energy exposure for eye or skin for a given laser that will not cause injury  NHZ (Nominal Hazard Zone): Area within which the MPE can be met or exceeded Eye Safety Laser Operation and Eye Exposure  Always think before doing, when aligning laser/optical systems  Follow the SOP and beam alignment procedures (class 3B and 4)  Only approved users after specific training from the Permit Holder may align lasers.  KEEP THE LIGHTS ON  Smaller iris lets-through less light  Don’t do involved alignment in an overly-fatigued state  Beam block when inserting new optical elements  Be aware of lens and mirror surface reflections Eye Protection Laser Eye Wear       Optical Density (OD) is the transmission of light through a highly blocking optical filter. Only use laser eye protection specifically labeled for the type of laser used Lens color is not an indication of wavelength or OD protection OD and specific wavelengths is usually on the front of goggles Use viewing devices (IR viewer) that will allow visualization of the beam without being exposed to any laser. Do not mix non-laser eyewear in the laser storage rack Administrative Basic Laser Safety - Safety Controls Administrative and Procedural: organizational and procedural aspects of laser safety Administrative Controls  Warning Signs  Labels  SOPs- Class 3B and 4 laser systems  Training  Security  SOP’s  Standard Operating Procedures (SOP)        Training Credentialing requirements Procurement and Registration of Class 3B and 4 lasers Incident reports Inspections Identify the NHZ Servicing Laser Laboratory Engineering Basic Laser Safety - Safety Controls Engineering Controls        Beam Housings Shutters Beam Blocks Attenuators Remote viewing devices Interlocks/switch lock receptacles Emergency Disconnects Operational Optical protection: practical tips for laser operators  REMOVE jewelry, especially watches and rings to avoid reflection and other reflective material from your body  Most incidents occur during alignment Avoid using reflective tools in alignment  Monitor all reflection from optics in the setup  Close eyes when bowing through plane of laser table – i.e. picking up a dropped tool  Communicate with other lab operators, when performing high‐risk tasks  Wear laser safety glasses or goggles when appropriate Eye Safety EYE SAFETY  Avoid “eye level” beams  Avoid situations where the beam is, or might be deflected upward i.e. beam steering, “flippers,” periscopes  Use beam blocks  Exercise caution when leaning down to beam-level  Always look away from table area when bending-down  Think twice before leaning to table level to get a better look at your experiment  Be VERY CAUTIOUS using “IR cards” and Ultraviolet (UV) Sensor Cards Alignment Beam Alignments The majority of accidents occur during beam alignments. Common scenario: unanticipated reflection from an optic when you are • • • • • Not wearing any protective eyewear „ Wearing inappropriate eyewear „ Using the wrong OD „ Wearing low-OD alignment eyewear with operational power levels „ Wearing high-OD operational eyewear with low-power (alignment) power levels„ • Using multiple wavelengths and there is a wavelength compatibility problem with your eye wear Beam alignment procedures are REQUIRED for all class 3B & 4 laser setups and must be submitted to LSSC for approvals Laser Safety Basic Laser Safety - Accidents Common Causes of Accidents        Accidental energization or firing of laser Bypass of Interlocks Beam Alignment Grounding No laser eye wear Movement of beam path Reflective objects in beam path (clutter)

Fundamental Of Laser Operation

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