Laser
Content
Laser Radiation and the Electromagnetic Spectrum
Electromagnetic radiation is a natural phenomenon found in almost all
areas of daily life, from radio waves to sunlight to x-rays. Laser radiation –
like all light – is also a form of electromagnetic radiation. Electromagnetic
radiation that has a wavelength between 380nm and 780nm is visible to
the human eye and is commonly referred to as light. At wavelengths
longer than 780nm, optical radiation is termed infrared (IR) and is invisible
to the eye. At wavelengths shorter than 380nm, optical radiation is
termed ultraviolet (UV) and is also invisible to the eye. The term "laser
light" refers to a much broader range of the electromagnetic spectrum
that just the visible spectrum, anything between 150nm up to 11000nm
(i.e. from the UV up to the far IR).
The Fundamentals of Laser Operation
The word ‘laser’ is an acronym for Light Amplification by the Stimulated
Emission of Radiation. A laser was first demonstrated in 1960 by Theodore
H Maiman working at the Hughes Corporation, although the term ‘laser’
was first coined by Gordon Gould of Columbia University.
To understand how laser action occurs, one must first consider the atomic
nature of matter. An atom consists of a central nucleus surrounded by a
cloud of electrons. Quantum theory explains that the electrons in atoms
exist in discrete energy states and at thermal equilibrium they are
maintained in the so-called ‘ground state’. The energy state of an atom
can be altered by the emission or absorption of a photon of
electromagnetic radiation (light). The electron cloud in atoms which have
absorbed light is referred to as existing in an ‘excited’ or ‘higher level’
energy state.
In ordinary light sources such as a filament bulb, electrical energy passing
through the tungsten ribbon raises the electrons in the metal atoms into
excited energy states. The electrons return to their ground state
spontaneously and in so doing release packets, or quanta, of optical
energy called photons. Atoms in ordinary light sources radiate photons
independently of each other and no phase relationship exists between
them. In other words, light generated by spontaneous emission is
incoherent. This lack of coherence is one important characteristic that
distinguishes ordinary light sources from lasers.
Einstein was the first scientist to propose that an excited atom can return
to its ground state in either of two processes, which he referred to as
‘spontaneous’ and ‘stimulated’ emission. The conditions for stimulated
emission, and hence laser operation, require that the excited atom is
prompted into emission of a photon by the application of light of the same
frequency (wavelength) as that which the electron decay itself will
produce. Unlike spontaneous emission, the photon emitted as a result of
this electromagnetic stimulation carries the same phase, frequency,
polarisation and direction as that of the stimulating radiation. If one were
to consider the case where all of the atoms have been excited into a
higher energy state (known as a population inversion), the stimulating
radiation would trigger an avalanche of stimulated photons that are all
emitted in phase and in the same direction.
The stimulating radiation in a laser is provided as a result of feedback
within a resonant optical cavity. In practice, a laser cavity is normally
comprised of a 100% reflecting rear mirror and a partially reflecting front
mirror (the partial reflectance is necessary to let some light out of the
laser whilst still providing the feedback required for laser action). When
illuminated (or "pumped") by a radiation source such as a flashlamp or
diode laser, the broadband emission that results from the spontaneous
photon emission travels back and forth and experiences the highest gain
(amplification) at a frequency determined by the configuration of the
resonator and the optical characteristics of the laser medium. That gain
leads to all of the photon emissions occurring on the same, chosen,
transition in the same direction, and all with the same phase
relationship. This amplification of the stimulated emission of coherent,
near-monochromatic, unidirectional photons is what defines laser action.
Laser action can occur in many materials that exist in solid (crystaline or
semiconductor), liquid or gaseous states. An example of a solid state laser
is the Nd:YAG, while semiconductor lasers are commonly referred to as
diode lasers. The dye laser is an example of a liquid laser, while probably
the most common gas laser is the CO2.
Laser Operating Modes
It is often said that almost any material can be made to lase. Lasers differ
not only with respect to the wavelength of the light they produce but also
the optical power and the manner in which the power is emitted. Lasers
which emit a continuous beam of light are termed "cw" (after continous
wave operation), while other lasers produce an output in the form of
pulses of light. For the purpose of classifying their hazards within the laser
safety standards, pulsed lasers are placed into different groups depending
upon the length of their optical pulses. The table below lists the four
groups.
Operating
Mode
Designatio Description
n
Pulse
Length
Continuous
Wave (CW)
Pulsed
D
> 200ms
Giant Pulsed
R
Modelocked
M
I
A laser that produces a continous
output
A laser that produces a single or
sequence of periodically repeated
pulses
A laser that produces very short
pulses (e.g. Q-switched)
A laser that produces ultrashort
pulses (i.e. picosecond or
femtosecond)
> 1µs to
200ms
1ns to 1µs
< 1ns
In the case of pulsed operation with a low pulse repetition frequency (the
number of pulses emitted per second), the critcal parameter from a laser
safety point of view is the peak power of each pulse. If the repetition rate
inceases, the average power becomes the more dominant
parameter. Please note that certain lasers can be operated in more than
one mode.
The Four Laser Hazard Classes
Lasers are categorised into four, general hazard classes based
upon accessible emission limits or AELs. These limits indicate the class of
the laser and are listed in EN 60825-1 and the American National Standard
ANSI Z136.1 for the safe use of lasers. The AEL values for the laser classes
are derived from the medical MPE (Maximum Permissible Exposure)
values. The MPE values specify the danger level for the eye or the skin
with respect to the laser radiation. Since November 2001, the laser
classes are summarised per the table below.
Laser
Class
Description
1
This class of laser does not produce dangerous radiation. No
need for protective equipment.
1M
This class of laser is eye safe when used without optical
instruments but may not be safe if optical instruments are
used. No need for protective equipment if used without optical
instruments.
2
This class of laser is eye safe as a result of normal human
aversion responses, including the blink reflex. No need for
protective equipment.
2M
This class of laser has the same powers as a class 2 beam but
the beam divergence and/or diameter may render it unsafe if
optical instruments are used. No need for protective equipment
if used without optical instruments.
3R
This class of laser emits radiation that exceeds the maximum
permissible exposure (MPE). The radiation is a maximum of 5 x
AEL of class 1 (invisible) or 5 x AEL of class 2 (visible). The risk
is slightly lower than that of class 3B. Dangerous to the
eyes, laser safety glasses are recommended.
3B
For this class of laser, the view into the laser is dangerous.
Diffuse reflections are not considered as dangerous. This is the
old class 3B without 3R. Dangerous to the eyes, laser safety
glasses are obligatory.
4
This class of laser is inherent unsafe. Even scattered radiation
can be dangerous. There is also a danger of fire and a danger
to the skin.Personal safety equipment is necessary (glasses,
screens).
http://www.pro-lite.co.uk/File/laser_safety_laser_basics.php
Laser Safety - Biological Effects
Why is Laser Radiation Dangerous?
The ‘light’ from powerful lasers can be concentrated to power densities
(power per area or Watts/m2) that are high enough to evaporate human
tissue and even metal or ceramics. In the medical field, laser radiation is
used to remove tattoos or to cut human tissue. These are examples of
applications which require high power lasers and as such there is a high
potential risk of accidental exposure of the laser beam to the user.
Because the eyes are much more sensitive to light, they are at increased
risk. In fact, it is possible to cause irreversible ocular injury with just one
glance into a direct or reflected laser beam even at relatively low
power levels. Laser eye protection must be taken extremely seriously.
Laser light can travel over great distances as a nearly parallel, collimated
beam. This means that the beam can travel a considerable distance and
still represent a significant ocular hazard. Compare an ordinary light bulb
with a laser and you will note an important difference. The light bulb emits
light more or less equally in all directions and in accordance with the
inverse squared rule, the irradiance decreases with the square of the
increase in distance travelled. The power received through a 7mm limiting
aperture at a distance of 1m from the lamp would be 100,000 times lower
than for a laser that emits the same optical power. The 7mm aperture is
not just a random area - this is the diameter of the dilated pupil in a dark
adapted eye.
In addition to the quantity of light that can hit the eye, the coherent
radiation produced by a laser can be focussed to extremely small
sizes. While the light bulb creates an image on the retina of approximately
100μm, the laser light is reduced to a spot of just a few micrometers (~
10μm) in diameter. Therefore, the power from the laser that hits the eye
which is already 100,000 higher than for the lamp is further concentrated
into a much smaller focussed spot on the back of the eye. The power
density resulting from this concentration may be sufficient to cause
irreversible damage to the retina. In the worst case, a single laser pulse
can cause a total loss of sight in that eye.
Different Laser Wavelengths Present Different Hazards
The risk of losing your eyesight from an accidental exposure to laser
radiation is due to the special optical properties of the human eye. The
type of ocular hazard depends on the wavelength of laser light you are
exposed to. The eye is only transparent in the wavelength range between
370 and 1400nm.
UV Radiation Below 350nm
Ultra violet light at wavelengths shorter than 350nm either penetrates as
far as the lens or is absorbed on the surface of the eye. A consequence of
exposure to high power laser light at these wavelengths is damage to the
cornea by ablation, or by the creation of a cataract.
Visible Radiation from 380-780nm
Light in the visible wavelength region (380-780nm) penetrates through
the cornea and lens and onto the retina. The eye sees light in this wave
band and has developed natural protective mechanisms against bright
lights. When the light appears too bright, we automatically turn away and
close our eyes. This aversion response is called the blink reflex. This
automatic reaction is effective for radiation up to 1mW power. With higher
power levels, too much energy reaches the retina before the blink reflex
can respond, which can result in irreversible thermal damage and loss of
vision.
IR Radiation Above 780nm
The near infrared wavelengths (780-1400nm) are a type of radiation that
is especially dangerous to the human eye because there is no natural
protection mechanism. The radiation in this band again penetrates to the
retina, but the light is invisible and a dangerous exposure is only noticed
after the damage has been done. Infrared radiation at wavelengths above
1400nm is absorbed at the surface of the eye. It leads to overheating of
the tissue and burning, or ablation of the cornea.
http://www.pro-lite.co.uk/File/laser_safety_biological_effects.php
Electromagnetic radiation is a natural phenomenon found in almost all
areas of daily life, from radio waves to sunlight to x-rays. Laser radiation –
like all light – is also a form of electromagnetic radiation. Electromagnetic
radiation that has a wavelength between 380nm and 780nm is visible to
the human eye and is commonly referred to as light. At wavelengths
longer than 780nm, optical radiation is termed infrared (IR) and is invisible
to the eye. At wavelengths shorter than 380nm, optical radiation is
termed ultraviolet (UV) and is also invisible to the eye. The term "laser
light" refers to a much broader range of the electromagnetic spectrum
that just the visible spectrum, anything between 150nm up to 11000nm
(i.e. from the UV up to the far IR).
The Fundamentals of Laser Operation
The word ‘laser’ is an acronym for Light Amplification by the Stimulated
Emission of Radiation. A laser was first demonstrated in 1960 by Theodore
H Maiman working at the Hughes Corporation, although the term ‘laser’
was first coined by Gordon Gould of Columbia University.
To understand how laser action occurs, one must first consider the atomic
nature of matter. An atom consists of a central nucleus surrounded by a
cloud of electrons. Quantum theory explains that the electrons in atoms
exist in discrete energy states and at thermal equilibrium they are
maintained in the so-called ‘ground state’. The energy state of an atom
can be altered by the emission or absorption of a photon of
electromagnetic radiation (light). The electron cloud in atoms which have
absorbed light is referred to as existing in an ‘excited’ or ‘higher level’
energy state.
In ordinary light sources such as a filament bulb, electrical energy passing
through the tungsten ribbon raises the electrons in the metal atoms into
excited energy states. The electrons return to their ground state
spontaneously and in so doing release packets, or quanta, of optical
energy called photons. Atoms in ordinary light sources radiate photons
independently of each other and no phase relationship exists between
them. In other words, light generated by spontaneous emission is
incoherent. This lack of coherence is one important characteristic that
distinguishes ordinary light sources from lasers.
Einstein was the first scientist to propose that an excited atom can return
to its ground state in either of two processes, which he referred to as
‘spontaneous’ and ‘stimulated’ emission. The conditions for stimulated
emission, and hence laser operation, require that the excited atom is
prompted into emission of a photon by the application of light of the same
frequency (wavelength) as that which the electron decay itself will
produce. Unlike spontaneous emission, the photon emitted as a result of
this electromagnetic stimulation carries the same phase, frequency,
polarisation and direction as that of the stimulating radiation. If one were
to consider the case where all of the atoms have been excited into a
higher energy state (known as a population inversion), the stimulating
radiation would trigger an avalanche of stimulated photons that are all
emitted in phase and in the same direction.
The stimulating radiation in a laser is provided as a result of feedback
within a resonant optical cavity. In practice, a laser cavity is normally
comprised of a 100% reflecting rear mirror and a partially reflecting front
mirror (the partial reflectance is necessary to let some light out of the
laser whilst still providing the feedback required for laser action). When
illuminated (or "pumped") by a radiation source such as a flashlamp or
diode laser, the broadband emission that results from the spontaneous
photon emission travels back and forth and experiences the highest gain
(amplification) at a frequency determined by the configuration of the
resonator and the optical characteristics of the laser medium. That gain
leads to all of the photon emissions occurring on the same, chosen,
transition in the same direction, and all with the same phase
relationship. This amplification of the stimulated emission of coherent,
near-monochromatic, unidirectional photons is what defines laser action.
Laser action can occur in many materials that exist in solid (crystaline or
semiconductor), liquid or gaseous states. An example of a solid state laser
is the Nd:YAG, while semiconductor lasers are commonly referred to as
diode lasers. The dye laser is an example of a liquid laser, while probably
the most common gas laser is the CO2.
Laser Operating Modes
It is often said that almost any material can be made to lase. Lasers differ
not only with respect to the wavelength of the light they produce but also
the optical power and the manner in which the power is emitted. Lasers
which emit a continuous beam of light are termed "cw" (after continous
wave operation), while other lasers produce an output in the form of
pulses of light. For the purpose of classifying their hazards within the laser
safety standards, pulsed lasers are placed into different groups depending
upon the length of their optical pulses. The table below lists the four
groups.
Operating
Mode
Designatio Description
n
Pulse
Length
Continuous
Wave (CW)
Pulsed
D
> 200ms
Giant Pulsed
R
Modelocked
M
I
A laser that produces a continous
output
A laser that produces a single or
sequence of periodically repeated
pulses
A laser that produces very short
pulses (e.g. Q-switched)
A laser that produces ultrashort
pulses (i.e. picosecond or
femtosecond)
> 1µs to
200ms
1ns to 1µs
< 1ns
In the case of pulsed operation with a low pulse repetition frequency (the
number of pulses emitted per second), the critcal parameter from a laser
safety point of view is the peak power of each pulse. If the repetition rate
inceases, the average power becomes the more dominant
parameter. Please note that certain lasers can be operated in more than
one mode.
The Four Laser Hazard Classes
Lasers are categorised into four, general hazard classes based
upon accessible emission limits or AELs. These limits indicate the class of
the laser and are listed in EN 60825-1 and the American National Standard
ANSI Z136.1 for the safe use of lasers. The AEL values for the laser classes
are derived from the medical MPE (Maximum Permissible Exposure)
values. The MPE values specify the danger level for the eye or the skin
with respect to the laser radiation. Since November 2001, the laser
classes are summarised per the table below.
Laser
Class
Description
1
This class of laser does not produce dangerous radiation. No
need for protective equipment.
1M
This class of laser is eye safe when used without optical
instruments but may not be safe if optical instruments are
used. No need for protective equipment if used without optical
instruments.
2
This class of laser is eye safe as a result of normal human
aversion responses, including the blink reflex. No need for
protective equipment.
2M
This class of laser has the same powers as a class 2 beam but
the beam divergence and/or diameter may render it unsafe if
optical instruments are used. No need for protective equipment
if used without optical instruments.
3R
This class of laser emits radiation that exceeds the maximum
permissible exposure (MPE). The radiation is a maximum of 5 x
AEL of class 1 (invisible) or 5 x AEL of class 2 (visible). The risk
is slightly lower than that of class 3B. Dangerous to the
eyes, laser safety glasses are recommended.
3B
For this class of laser, the view into the laser is dangerous.
Diffuse reflections are not considered as dangerous. This is the
old class 3B without 3R. Dangerous to the eyes, laser safety
glasses are obligatory.
4
This class of laser is inherent unsafe. Even scattered radiation
can be dangerous. There is also a danger of fire and a danger
to the skin.Personal safety equipment is necessary (glasses,
screens).
http://www.pro-lite.co.uk/File/laser_safety_laser_basics.php
Laser Safety - Biological Effects
Why is Laser Radiation Dangerous?
The ‘light’ from powerful lasers can be concentrated to power densities
(power per area or Watts/m2) that are high enough to evaporate human
tissue and even metal or ceramics. In the medical field, laser radiation is
used to remove tattoos or to cut human tissue. These are examples of
applications which require high power lasers and as such there is a high
potential risk of accidental exposure of the laser beam to the user.
Because the eyes are much more sensitive to light, they are at increased
risk. In fact, it is possible to cause irreversible ocular injury with just one
glance into a direct or reflected laser beam even at relatively low
power levels. Laser eye protection must be taken extremely seriously.
Laser light can travel over great distances as a nearly parallel, collimated
beam. This means that the beam can travel a considerable distance and
still represent a significant ocular hazard. Compare an ordinary light bulb
with a laser and you will note an important difference. The light bulb emits
light more or less equally in all directions and in accordance with the
inverse squared rule, the irradiance decreases with the square of the
increase in distance travelled. The power received through a 7mm limiting
aperture at a distance of 1m from the lamp would be 100,000 times lower
than for a laser that emits the same optical power. The 7mm aperture is
not just a random area - this is the diameter of the dilated pupil in a dark
adapted eye.
In addition to the quantity of light that can hit the eye, the coherent
radiation produced by a laser can be focussed to extremely small
sizes. While the light bulb creates an image on the retina of approximately
100μm, the laser light is reduced to a spot of just a few micrometers (~
10μm) in diameter. Therefore, the power from the laser that hits the eye
which is already 100,000 higher than for the lamp is further concentrated
into a much smaller focussed spot on the back of the eye. The power
density resulting from this concentration may be sufficient to cause
irreversible damage to the retina. In the worst case, a single laser pulse
can cause a total loss of sight in that eye.
Different Laser Wavelengths Present Different Hazards
The risk of losing your eyesight from an accidental exposure to laser
radiation is due to the special optical properties of the human eye. The
type of ocular hazard depends on the wavelength of laser light you are
exposed to. The eye is only transparent in the wavelength range between
370 and 1400nm.
UV Radiation Below 350nm
Ultra violet light at wavelengths shorter than 350nm either penetrates as
far as the lens or is absorbed on the surface of the eye. A consequence of
exposure to high power laser light at these wavelengths is damage to the
cornea by ablation, or by the creation of a cataract.
Visible Radiation from 380-780nm
Light in the visible wavelength region (380-780nm) penetrates through
the cornea and lens and onto the retina. The eye sees light in this wave
band and has developed natural protective mechanisms against bright
lights. When the light appears too bright, we automatically turn away and
close our eyes. This aversion response is called the blink reflex. This
automatic reaction is effective for radiation up to 1mW power. With higher
power levels, too much energy reaches the retina before the blink reflex
can respond, which can result in irreversible thermal damage and loss of
vision.
IR Radiation Above 780nm
The near infrared wavelengths (780-1400nm) are a type of radiation that
is especially dangerous to the human eye because there is no natural
protection mechanism. The radiation in this band again penetrates to the
retina, but the light is invisible and a dangerous exposure is only noticed
after the damage has been done. Infrared radiation at wavelengths above
1400nm is absorbed at the surface of the eye. It leads to overheating of
the tissue and burning, or ablation of the cornea.
http://www.pro-lite.co.uk/File/laser_safety_biological_effects.php
