1. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE a level 1 training course: basic competency (for new users) 30th October 2010Version 3.1

2. KEYSTONE 2

3. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE 3

4. What is a laser? Let’s answer this question by considering the schematic drawing of a helium-neon (HeNe) gas-filled laser, a familiar laser in teaching and research labs.helium-neon helium-neon gas filled glass envelope anodecathode 100% reflective mirror 95% reflective mirror / output coupler laser beam; λ = 632 nm The He-Ne comprises 1.An optical resonator  a sealed glass envelope with a fully reflective mirror at one end and a 95% reflective mirror at the aperture. 2.A gain medium  a mix of helium and neon gases. 3.A power source  a ~1000V electrical discharge that causes electrons to flow through the gain medium from the cathode to the anode. 4

5. KEYSTONE So how does it work? 1.Electrons emitted by electric discharge collide with and excite He atoms.electric discharge 2.Energy is transferred from He to Ne atoms, raising (pumping) the Ne electrons from their ground state (L1) to a high energy state (L3).pumping 3.A population inversion is created at L3 by further pumping of ground state Ne atoms. 4.Spontaneous + stimulated emission of electrons from L3 to L2 releases 632 nm photon. 5.Electrons decay rapidly from L2 to L1. 6.Because the stage from the release of electrons through to Ne-L2 is faster than the stage from L2 to L1 the gain medium remains saturated, the population inversion is maintained, and the laser continually emits 632 nm photons. cathode anode Ne-L1 L2 L3 632 nm heat He 5

6. KEYSTONE More on population inversions  Many substances will support electron transitions such as described for helium-neon.  However, for a substance to be suitable for use as a gain medium it has to be able to support a population inversion, i.e. if a medium can support more electrons in excited states than in ground states then it will be possible to pump electrons into higher states faster than the rate of spontaneous decay, and thereby achieve a continuous emission of photons.population inversion  Other examples of complex gain media include those used to fill carbon dioxide lasers (~20% CO 2 + 15% N 2 + ~3% H 2 / He 2 ) and the solid state YAG lasers (yttrium aluminium garnet host doped with ‘impurities’ such as neodymium, chromium, titanium).carbon dioxide lasersYAG lasers  If a population inversion can be supported, the process of stimulated emission becomes possible.stimulated emission  Stimulated emission is ‘why lasers are’. Laser beams are monochromatic wavelengths which oscillate between two mirrors at either end of the laser cavity. When the mirrors are set to the correct harmonic the reflected ‘waves’ become constructive, i.e. they are in phase (coherent) and the energy of the beam is summed.monochromaticharmonic constructivecoherent 6

7. KEYSTONE...and more on stimulated emission (1)(2)(3) (4) (5) (1)An excited orbital electron spontaneously decays and emits a characteristic photon [L3  L2]. (2)The passage of a photon corresponding to the energy gap L3  L2 induces the emission of a photon of the same frequency as the passing photon. (3)The emitted photons are in phase, and a constructive standing wave develops. (4)The wave is reflected into the gain media and stimulates further emission. (5)In a continuous wave laser ‘light’ is emitted through the partially reflective mirror. 7

8. KEYSTONE Continuous wave or pulsed?  Thus far we have looked at continuous wave (CW) lasers, where the power output is continuous and is directly related to the steady pumping of the laser. Typically, power outputs from CW lasers were in the region of miliwatts to tens of watts, although technological developments are now realising kilowatts of power.  Pulsed lasers, however, deliver peak outputs of hundreds to thousands of watts in ‘trains’ of pulses where each pulse lasts a fraction of a second. Pulsing finds particular use in laser drilling and ablation, where there is sufficient energy deposition to vaporise shallow depths / small volumes of materials. By contrast, low energy femtosecond pulsing allows biochemical reactions or physical changes to be followed. Pulsed lasers  Q-switching - crudely, an attenuator is fitted inside the optical cavity that enables the power of a CW laser and releasing it in short gigawatt pulses. [High pulse energies, long pulse duration]. Q-switching  Modelocking - uses the time-bandwidth between oscillating standing waves to modulate the production of constructive waves at pico / femtosecond intervals. Modelocking  Gain switching – electrons are pulsed into the active lasing medium, causing electron gates cycle between open & closed states stimulating photon emissions. Gain switching 8

9. KEYSTONE Resumé  Excepting diode lasers, which work by ‘gating’ an electron flow, lasers are very simply an optical or resonant cavity with highly polished mirrors at either end,  one fully reflective  the other partially so (the aperture).  The cavity is filled with a lasing medium (gas, liquid / dye, or a solid matrix) that is capable of existing in a predominantly excited state when pumped.  The length of the cavity is related to the wavelength or harmonics of the emitted photons (laser ‘light’)...key is the creation of a constructive coherent standing wave.  The laser medium is pumped (energised) by a power source, which may be electric discharge, a flashlight, another laser...anything that can supply a constant energy flux.  Lasers may be pulsed in order to increase peak power or achieve short pulse duration.  Laser beams are monochromatic (or comprise a few related λ), are coherent and collimated (low beam divergence). 9

10. KEYSTONE 10 Classic Spectacular Concert, photograph by Fir0002/Flagstaffotos, reproduced under GFDL licence.GFDL licence

11. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE 11...coming up next...

12. KEYSTONE Laser types Laser types are classed by their gain media and broadly fall into four categories. Gas lasers: carbon dioxide, excimer, HeNe,carbon dioxideexcimerHeNe Solid lasers: Nd:YAG, ruby, Ti-saphire,Nd:YAGrubyTi-saphire Liquid / dye lasers: tunable lasers using chemical dyes to select the wavelength of interestlasers Laser diodes: semi-conductor lasers.lasers 12

13. KEYSTONE Laser applications  Applications include Applications  Raman spectrometry: monochromatic photons excite chemical bonds or orbital electrons to induce characteristic atomic / molecular vibration effects that can be used to fingerprint molecules, investigate chemical bonding and composition, study temperature effects, and characterise materials. In addition, polarised light can be used to probe crystalline structures. Thus applications are found in the physical sciences, forensics, archaeology, process monitoring. Raman spectrometry  LIBS: another spectroscopy tool, where samples of ablated materials are formed into a plasma the plume subject to spectrometry. LIBS  Materials processing: ablation, cutting, engraving, drilling.ablationcuttingengravingdrilling  Medicine & healthcare: surgery, dentistry, ophthalmic surgery, cosmetic surgery, hair removal.surgerydentistryophthalmic surgerycosmetic surgeryhair removal  Product development: printers, measurement, optical discs, pointers, holography.printersoptical discspointersholography  Recreational: laser light shows.laser light shows  Military / police: dazzlers, weapons systems, range finding, speed cameras. 13

14. KEYSTONE 14 Laser harp used by Jean Michel Jarre, photograph by Wikipedysta:Maksymus007, reproduced under GFDL licence.GFDL licence

15. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE 15 The measure of danger

16. KEYSTONE Classification schemes 16 Lasers are classified under BS EN 60825-1:1994 or its replacement BS EN 60825-1:2007. Both standards are presented during the course of the next few slides because lasers labs are likely to have lasers and laser products that have been classified under both schemes. The standards are available free from the British Standards Institute: enter their site through the institutional login page.British Standards Instituteinstitutional login page Two things to remember:  laser class and,  laser product classification. The laser class is the classification of the actual laser itself, whereas the laser product is the classification of the device or instrument:  example 1) a CDROM drive is a class 1 product that contains a class 3B laser  that the laser has been rendered inaccessible renders the product as class 1,  example 2) a confocal microscope may be fitted with a class 3B laser that is inaccessible under normal operating conditions  product class 1.

17. KEYSTONE The ‘old’ laser classification scheme ClassReasonPower limits Class 1 Safe Lasers are safe under reasonably foreseeable conditions. <0.98 mW Class 2 (visible) Safe - low power For CW lasers protection is afforded by the blink reflex (0.25 secs). <1.0 mW Class 3A Usually safe - low power An extension of Class 2, where the blink reflex protects. Using optical aids may be hazardous. <5 mW (irradiance <25 mW -2 ) Class 3B Caution - medium power Direct intrabeam viewing is always hazardous. Spectral reflections may be hazardous. The viewing of diffuse reflections is usually safe. <0.5 W Class 4 Warning - high power Diffuse reflections are hazardous and may cause skin burns. Fire hazard; laser beams can drill through metal. >0.5 W 17

18. KEYSTONE The revised classification scheme: low risk Laser classHazardControl measure Class 1 (all wavelengths) Minimal risk Safe.No protective measures are necessary. Class 1M (all wavelengths) Low risk Beam divergence ensures safe to eyes. Do not stare into the beam. Do not re-focus the beam. Prevent viewing through binoculars, optical sights etc. Prevent beam being directed towards people. Class 2 (visible λ only) Low risk Protection afforded by the blink (aversion) reflex. Class 2M (visible λ only) Low risk Safe under normal operational conditions. May be unsafe if magnified viewing instruments used. Prevent direct viewing of the beam. Use a ‘beam stop’ to terminate the beam. Class 3R (all wavelengths) Low risk Safe under normal operational conditions. Prevent direct eye exposure to the beam. 18

19. KEYSTONE The revised classification scheme: high risk Laser classHazardControl measures Class 3B (all wavelengths) Caution Moderate risk Laser beams have a level of emission that is harmful to the eye and potentially harmful to the skin. Spectral reflections may be harmful to the eye. Prevent intrabeam viewing. Prevent eye and skin exposure to spectral reflections. Prevent laser beams from leaving the optical bench. Terminate all beams. Class 4 (all wavelengths) Warning High risk Laser beams have a level of emission that is always harmful to the eyes and skin. Spectral and diffuse reflections are always harmful to the eye. Diffuse reflection may be harmful to the skin. Risk of fire or fumes. Prevent eye and skin exposure to primary laser emissions and to diffuse or scattered radiation. Prevent against laser interaction hazards. Ensure barriers and screens likely to be struck by beams will not ignite or melt. 19

20. KEYSTONE Warning signs 20 CAUTION – CLASS 3B LASER RADIATION WHEN OPEN AVOID EXPOSURE TO THE BEAM CAUTION – CLASS 4 LASER RADIATION WHEN OPEN AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION Areas and equipment should be designated according to the risk they pose... CAUTION means low risk which if not avoided could result in minor / moderate injury. WARNING  serious injury / death.

21. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE 21 Laser-human interactions

22. KEYSTONE Tissue damage threshold – wavelength (1/2) 22 The extent of tissue damage is strongly related to the energy deposited in a volume of tissue, although wavelength is important in that ultraviolet, visible and infra red photons penetrate to different depths of tissue. UVCUVAVisibleIR This means that  Ultraviolet radiations effect surface tissues, such as the epidermis or cornea of the eye.  Infra red radiations penetrate into deeper tissue, e.g. Structures in the subcutaneous layer or the retina of the eye.  UV may burn the hairs of the skin, but IR will damage the root.

23. KEYSTONE Tissue damage threshold – intensity (2/2) 23 Green lasers of different intensities, photograph by Gonioul, reproduced under GFDL licence.GFDL licence However, the energy deposited will determine the extent or amount of damage; a 50 mW Class 3B UVC laser beam may cause the skin to tingle, but a 500mW beam will burn.

24. KEYSTONE Damage mechanisms (1/2) 24 Photochemical damage The deposition of relatively low energies by absorbed photons causes the chemical excitation of irradiated molecules. Damage is usually reversible. e.g. Exposure to diffuse reflections from a Class 4 UV excimer laser may cause skin erythema or photokeratitis (inflammation of the superficial cells of the cornea). Thermal damage The absorption of radiant energy causes the vibration of molecules and localised heating, possibly leading to the coagulation of proteins. Damage at higher intensities is irreversible. e.g. Exposure to specular reflections from a Class 3B diode IR laser may cause deep skin burns, hair loss – follicle damage or enzyme denaturation in skin glands. Thermo-acoustic damage High irradiances delivered over short time periods, such as from pulsed lasers, cause rapid thermal expansion of tissue and vapourisation of cellular components...explosively! e.g. Exposure to specular reflections from a Class 4 visible laser may cause deep tissue burns or puncture the retina

25. KEYSTONE Damage mechanisms (2/2) 25 Damage to the retina of the eye following exposure to a 40 mW HeNe laser Thermal damage: localised coagulation marks on the retina Thermo-acoustic damage: internal bleeding from the retina

26. KEYSTONE Absorption of light by the eye: ultraviolet 26 ArF excimer (193 nm) XeCl excimer (308 nm) Cadmium vapour (325 nm)

27. KEYSTONE Absorption of light by the eye: visible 27

28. KEYSTONE Absorption of light by the eye: infrared 28 Nd:YAG (1.06 μm) HF (2.9 μm) CO 2 (10.6 μm)

29. KEYSTONE 29 Beams in fog + car windshield, photograph by Jeff Keyzer, reproduced under GFDL licence.GFDL licence

30. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE 30 Should I wear laser goggles?

31. KEYSTONE PPE, a few don’ts and a do 31  What protective equipment, and when to wear it, should be determined by risk assessment.  If the laser process includes hot work, chemicals, dirt you may need to wear thermal gloves, latex / nitryl gloves, lab coats, etc.  Aligning lasers and setting optics, basically most optical bench activities, are impossible with gloves. You will need instruction from a competent person on how to carry out these activities safely.  Don’t  Wear jewellery, watches, bangles, dangling neck chains etc. Laser beams can be reflected off the laser bench by shiny objects.  Wear loose clothing or ties when leaning over lasers. They can catch and misalign optics or catch fire if beam energies are high enough.  Do  Protect the beam path and optics at all times.

32. KEYSTONE PPE: eyewear – the problem 32  Reliance on laser goggles is dangerous!  Laser goggles only work for specific wavelengths and for specific energies.  Assume a lab has several lasers operating. Any scattered beams from one laser e.g. an IR laser, could cross the lab and the goggles worn by the UV laser user would not protect them.  Also, a laserist may wear goggles that protect them. But a colleague nearby without goggles will not be protected.

33. KEYSTONE PPE: eyewear – the solution 33  Work MUST be carried out in a such a way that it is not possible for stray beams or reflections to leave the optical bench. This is the only way to maintain a safe environment.  Only wear eye protection if there is a non-trivial risk of injury from accidental exposure.  If goggles are to be worn then everyone in the lab must wear a pair. Check that:  the optical density (OD) will reduce laser energy below the MPE (safe level),  the goggles are CE marked and are marked with wavelength(s) they will absorb,  the goggles fully enclose the eyes,  the goggles are in good condition, clean and fit properly. Read PD IEC TR 60825-14:2004 pp44-45

34. KEYSTONE Beam alignment 34 Most optical related injuries occur during beam alignment, and the groups at most risk are novices and the very experienced. This is a simple checklist to help you make the right decisions and stay safe.  Can you use a low power laser mounted in tandem? If not,  can you align with the laser turned to low power, e.g. <25 mW? If not,  is it practicable to use a camera, remote tool or viewing aid? If not,  is it practicable to wear goggles and will you be able to see (some goggles cut out visible light.  Are you going to use burn card, phosphorescent card, or black card?  Have you been shown how to ‘lead’ laser beams through optical arrays? And,  have you practised with low power lasers?  Remember that optics can create multiple reflections. Do you know where the paths of all reflections will lie? Be aware of prisms moving beams in unexpected directions.  Be wary of reflective and shiny metal surfaces.  Always terminate beams at the end of each alignment phase.

35. KEYSTONE 35 Laser tuning, reproduced by permission of the University of Leeds.

36. Radiation Protection Service Department of Wellbeing, Safety & Health KEYSTONE 36 Safety management and organisation

37. KEYSTONE The statutory framework! 37 Health and Safety at Work etc Act 1974  Section 2  UoL has a duty to look after you, make sure equipment is safe, risks are assessed and provide information and training.  Section 7  You have a duty to co-operate, not to misuse or interfere with anything, and to look after yourself and not endanger your colleagues. Management of Health and Safety at Work Regulations 1999  Reg 3  Employer must make risk assessments and identify control measures.  Reg 5  Employer must plan, organize, control, monitor and review.  Reg 7  Appoint competent persons and give them time and the means to assist.  Regs 8 & 9  Emergency plans and medical arrangements.  Reg 10  Give information  Reg 13  Ensure staff are capable of performing tasks and given training.  Reg 14  Employees must follow procedures.  Regs 16-18  Arrangements for pregnant workers. Control of Artificial Optical Radiation at Work Regulations 2010  Same as the above

38. KEYSTONE What UoL has done 38 Competent persons Faculties / Schools have appointed Laser Safety Officers who are competent and trained to a high standard (Health Protection Agency / Loughborough University Laser safety Management Course). Your LSO is there to help you. Revitalized its safe organization Health & Safety Policy identifies duties associated with roles e.g. Dean, HoS, Managers, Competent Persons. Read the policy here.Read the policy here Risk assessment An Excel based risk assessment procedure. Information Local rules (instructions on how to work safely with lasers) and guidance documents are hosted on the VLE. To obtain access you must register for a permit.VLE Permit system To work with lasers you must have a valid reason and apply for a permit. LSOs will be able to download you an application form from the VLE. Design assessment & critical examination procedure All equipment should be examined by the Radiation Protection Service before being commissioned for use. Laser safety will be reviewed during the academic year 2010-2011.

39. KEYSTONE Safety management at UoL 39

40. KEYSTONE How to make a risk assessment 40 Firstly, who should make the RA. Legislation says the employer. We say yes, but it is in your own interests to do this with your LSO. 1.Consider normal operation and reasonably foreseeable conditions – no need to look at the weird and whacky. 2.Summarise : process, optical, installations (gases, coolants, electrical supply, etc.). 3.Do you have documentation, manuals data on the laser (Class, power, wavelengths, pulse duration, peak energy, repetition rate). 4.Assess risk to those exposed (laserists, cleaners, visitors). 5.Identify the MAJOR hazards...not every tiddly trifling risk. 6.Determine the risks to those who may be exposed. 7.Write it down and say when control measures will be put in place.

41. KEYSTONE Please... 41 common sense, common safety Lord Young of Graffham is your friend!