Transcript Level 1: for new users

Radiation Protection Service
Department of Wellbeing, Safety & Health
a level 1 training course: basic competency (for new users)
LASER SAFETY
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Version 3.1
30th October 2010
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Radiation Protection Service
Department of Wellbeing, Safety & Health
Overview: how lasers work
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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.
cathode
helium-neon gas filled glass envelope
anode
laser beam;
λ = 632 nm
95% reflective mirror /
output coupler
100% reflective
mirror
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.
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So how does it work?
1. Electrons emitted by electric discharge collide with and excite He atoms.
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).
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.
L3
cathode
anode
632 nm
L2
He
Ne-L1
heat
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.
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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.

Other examples of complex gain media include those used to fill carbon dioxide lasers
(~20% CO2 + 15% N2 + ~3% H2 / He2) and the solid state YAG lasers (yttrium aluminium
garnet host doped with ‘impurities’ such as neodymium, chromium, titanium).

If a population inversion can be supported, the process of stimulated emission
becomes possible.

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.
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...and more on stimulated emission
(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.
(1)
(5)
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(2)
(3)
(4)
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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.
 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].
 Modelocking - uses the time-bandwidth between oscillating standing waves to
modulate the production of constructive waves at pico / femtosecond intervals.
 Gain switching – electrons are pulsed into the active lasing medium, causing
electron gates cycle between open & closed states stimulating photon emissions.
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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).
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Classic Spectacular Concert, photograph by Fir0002/Flagstaffotos,
reproduced under GFDL licence.
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Radiation Protection Service
Department of Wellbeing, Safety & Health
...coming up next...
Types & applications
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Laser types
Laser types are classed by their gain media and broadly fall into four categories.
Gas lasers: carbon dioxide, excimer, HeNe,
Solid lasers: Nd:YAG, ruby, Ti-saphire,
Liquid / dye lasers: tunable lasers using chemical dyes to select the wavelength of interest
Laser diodes: semi-conductor lasers.
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Laser applications

Applications include

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.

LIBS: another spectroscopy tool, where samples of ablated materials are formed into
a plasma the plume subject to spectrometry.

Materials processing: ablation, cutting, engraving, drilling.

Medicine & healthcare: surgery, dentistry, ophthalmic surgery, cosmetic surgery, hair
removal.

Product development: printers, measurement, optical discs, pointers, holography.

Recreational: laser light shows.

Military / police: dazzlers, weapons systems, range finding, speed cameras.
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Laser harp used by Jean Michel Jarre, photograph by Wikipedysta:Maksymus007,
reproduced under GFDL licence.
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Radiation Protection Service
Department of Wellbeing, Safety & Health
The measure of danger
Laser classification
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Classification schemes
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.
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.
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The ‘old’ laser classification scheme
Class
Reason
Power 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
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The revised classification scheme: low risk
Laser class
Hazard
Control measure
Class 1 (all wavelengths)
Minimal risk
Safe.
No protective measures are
necessary.
Class 1M (all wavelengths) Beam divergence ensures
Low risk
safe to eyes.
Class 2 (visible λ only)
Low risk
Protection afforded by the
blink (aversion) reflex.
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 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.
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The revised classification scheme: high risk
Laser class
Hazard
Control 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.
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Warning signs
Areas and equipment
should be designated
according to the risk
they pose...
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
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CAUTION means low
risk which if not
avoided could result in
minor / moderate
injury.
WARNING  serious
injury / death.
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Radiation Protection Service
Department of Wellbeing, Safety & Health
Laser-human interactions
Biological Effects
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Tissue damage threshold – wavelength (1/2)
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.
UVC
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UVA
Visible
IR
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.
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Tissue damage threshold – intensity (2/2)
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.
Green lasers of different intensities, photograph by Gonioul,
reproduced under GFDL licence.
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Damage mechanisms (1/2)
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
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Damage mechanisms (2/2)
Thermal damage:
localised coagulation
marks on the retina
Thermo-acoustic
damage: internal
bleeding from the
retina
Damage to the retina of the eye following
exposure to a 40 mW HeNe laser
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Absorption of light by the eye: ultraviolet
ArF excimer
(193 nm)
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XeCl excimer
(308 nm)
Cadmium
vapour
(325 nm)
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Absorption of light by the eye: visible
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Absorption of light by the eye: infrared
Nd:YAG
(1.06 μm)
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HF
(2.9 μm)
CO2
(10.6 μm)
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Beams in fog + car windshield, photograph by Jeff Keyzer,
reproduced under GFDL licence.
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Radiation Protection Service
Department of Wellbeing, Safety & Health
Should I wear laser goggles?
PPE & safe beam alignment
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PPE, a few don’ts and a do

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.
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PPE: eyewear – the problem

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.
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PPE: eyewear – the solution

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
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Beam alignment
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.
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Laser tuning, reproduced by permission of
the University of Leeds.
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Radiation Protection Service
Department of Wellbeing, Safety & Health
Safety management and organisation
Legislation, rules & risk
assessment
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The statutory framework!
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
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What UoL has done
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.
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.
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.
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Safety management at UoL
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How to make a risk assessment
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.
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Please...
common sense, common safety
Lord Young of Graffham is your friend!
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