Laser Safety Training Manual

Laser Safety Training Manual

Table of Contents

1     Introduction
2     Light
  2.1   Emission and absorption of light
  2.2   Properties of light
    2.2.1 Nature of light
    2.2.2 Reflection of light
    2.2.3 Refraction of light
    2.2.4 Absorption of light
    2.2.5 Scattering of light
    2.2.6 Interference of light
    2.2.7 Diffraction of light
    2.2.8 Dispersion of light
    2.2.9 Polarization of light
    2.2.10 Brewster’s angle
    2.2.11 Beam splitter
  2.3   Non-linear effects
    2.3.1 Raman effect
    2.3.2 Harmonic generation
    2.3.3 Electro-optic effect
3     Lasers
  3.1   Laser levels
  3.2   Laser components
    3.2.1 Lasing medium
    3.2.2 Optical cavity
    3.2.3 Energy pump
  3.3   Laser operation
    3.3.1 Continuous wave lasers
    3.3.2 Pulsed lasers
    3.3.3 Q switched lasers
  3.4   Laser modes
    3.4.1 Axial modes
    3.4.2 Mode-locked lasers
    3.4.3 Transverse modes
4     Laser safety principles
  4.1   General principles of health and safety
  4.2   Controls
  4.3   Principles of laser safety
  4.4   Objective of the ANSI standards
  4.5   Laser classes
    4.5.1 Class 1
    4.5.2 Class 2
    4.5.3 Class 3
    4.5.4 Class 4
5     Laser beam hazards
  5.1   Tissue damage mechanisms
    5.1.1 Photochemical interaction
    5.1.2 Thermal interaction
    5.1.3 Tissue ablation
  5.2   Human eye
    5.2.1 The anatomy of the eye
    5.2.2 The physiology of the eye
    5.2.3 Laser eye injuries
  5.3   Skin
    5.3.1 Structure of the skin
       
       

Introduction

This training manual is intended for the users of open beam class 3B and class 4 lasers. Is primarily aimed at users from a research/university laboratory, but can be successfully used for the laser safety training of users in industry or medical applications. The content covers the requirements of the American Standard for the Safe Use of Lasers for the training of the user personnel routinely working with or potentially exposed to Class 3B or Class 4 laser radiation.

The training manual is based on the laser safety training lectures presented for more than 15 years to students, staff, and faculty at the University of Toronto.

The manual covers basic notions regarding light and optics at the level considered necessary to understand the principles of laser operation and the safety aspects of working with open beam high-class lasers. The fundamentals of laser operation – physical principles, construction, types of lasers, modes, pulse shapes, etc. – are treated in more detail. Starting from the general principles of health and safety, the principles of laser safety are exposed in the same chapter with the laser classification. The hazards to the eye and to the skin are called in the laser safety literature beam hazards. They are explained together with the fundamental notions about the eye and skin structure and function. All other hazards encountered in a laser workplace are covered in the chapter about non-beam hazards.

In the vision of the author, the first defence against hazards is the knowledge of the laser user. Based on this, the laser safety training is essential in controlling the hazards and reducing the risk of working with open beam high-class lasers. Engineering, administrative and procedural controls as well as personal protective equipment are covered in the chapter dedicated to laser hazards control. Fundamentals and examples of laser hazard calculations are presented in the next chapter. Principles of beam alignment and beam measurements are explained in chapter 9.

The best method to ensure compliance with regulations is to have a solid laser safety program. The main components of the program – responsibilities, inventory, laser safety training, laser inspections, program audits, commissioning and decommissioning lasers and laser rooms, etc. – as well as the duties and responsibilities of the Laser Safety Officer are covered in the next chapters.

Principles of laser accident investigations, lessons learned, improvements of the program as a result of incidents and accidents, as well as analysis of some accidents – case studies- are presented in the last chapter.

DISCLAIMER

This training manual uses materials (tables, pictures, diagrams, etc.) available on the world wide web. All materials used are taken from pages anyone can access free, without a password.

2   Light


Fig. 2-1: Optics is Light Work

This picture is taken from the door of one of the laser labs in the Physics building. If we replace the word “OPTICS” with “LASER”, it is easy to understand why we need to know basic notions regarding light emission, absorption, reflection, refraction, etc., to learn about laser safety.

2.1   Emission and absorption of light

In 1905, in the paper explaining the photoelectric effect, Albert Einstein proposed the idea of energy quanta. Luminous energy can be absorbed and emitted only in discrete amounts. This idea combined with the structure of atoms (electrons are located around the nucleus on discrete levels of energy) resulted in explaining one the mystery of physics: how is light produced?


Fig. 2-2: Absorption and emission of photons in an atom
 
Fig. 2-3: Absorption and emission of photons as a result of energy level change of electrons

Excitation of the atoms can be caused by an increase in temperature. Depending on the temperature, many atoms in a light source (like an incandescent bulb) will be in the fundamental state, and others will be in different excited states. For the atoms in the excited state (with electrons in a higher energy state), the de-excitation occurs at a random moment, and radiation (light) is emitted. This process is known as the spontaneous emission of radiation. Since emission happens randomly, from different atoms, the radiation is emitted in all directions, with many energies, at different moments, the emitted radiation is non-coherent. The light emitted in this way has different frequencies (wavelengths), different phases, different polarizations, different directions.

In 1917 Einstein publishes” The Quantum Theory of Radiation” in which he explains the stimulated emission of radiation.


Fig. 2-4: Stimulated emission of radiation

When a photon passes nearby an atom excited in the same energy level as the energy of the photon, de-excitation of the atom occurs. This is called stimulated emission of radiation.

Boltzmann distribution

A system with two energy states (ground state with energy E1 and excited state with energy E2) with the number of atoms/molecules N1 and N2

k =Boltzmann’s constant

T = Temperature

N2 ≤ N1  (equal when T is infinite)

No population inversion can be obtained in a system with two states when thermal equilibrium is reached. At room temperature, N2 is close to zero. At higher temperatures, N2 increases and becomes equal with N1 at infinite temperature. If we try to obtain N2 higher than N1 using different types of excitations (e.g.: by pumping light with the required wavelength into the system) since the probability of absorption is equal with the probability of emission, when the system is saturated N2 = N1 the number of atoms absorbing energy is equal with the number of atoms emitting radiation. No population inversion can be obtained in a system with two levels of energy. To obtain population inversion, the system must have 3 or more states and be brought in non-equilibrium.

In a medium with population inversion, the initial photon is emitted spontaneously. Passing near excited atoms, the photon will stimulate the emission of the next photon, and so on.


Fig. 2-5: Population inversion and stimulated emission

The light emitted by the stimulated emission has the same frequency, the same direction, the same phase, the same polarization as the initial photon. The stimulated emission of radiation in a medium in which the population inversion is obtained is the basic explanation of laser functioning.

2.2   Properties of light

2.2.1   Nature of light

For many centuries, the nature of light was a mystery, rising many controversies. During the XVII century, two main theories were developed. In the first one light was considered a wave and in the second one, the light was considered as being formed of particles of matter emitted in all directions from the source. Both theories were successful in explaining some properties of light.

wave is a disturbance that transfers energy through matter or space. Waves consist of oscillations or vibrations of a physical medium or a field, around relatively fixed locations. There are two main types of waves: mechanical and electromagnetic. Mechanical waves propagate through a physical matter, whose substance is being deformed. Restoring forces then reverse the deformation.

Transverse waves are waves that propagate in a direction perpendicular to the direction of oscillations/vibrations.


Fig. 2-6: Transverse waves

For example, waves propagated in a string when the perturbation happens on a direction perpendicular to the string, is a mechanical transverse wave.

Longitudinal waves propagate in the same direction as the oscillations/vibrations.


Fig. 2-7: Longitudinal wave

Sound waves propagate via air molecules colliding with their neighbours. When the molecules collide, they also bounce away from each other. This keeps the molecules from continuing to travel in the direction of the wave. Sound waves are mechanical longitudinal waves.

In the XIX century, classical physics proved that light is electromagnetic radiation. According to this theory, light is an electromagnetic wave. Electromagnetic waves are generating by moving electrical charges. An electric charge generates an electric field in the surrounding space. If the electric charges move, say it vibrates back and forth, then the motion will be transferred to the electric field lines, which will become wavy. Orsted discovered that a moving electric charge generates a magnetic field. The magnetic field lines also become wavy when the electric charge moves back and forth. The combined electric and magnetic fields waves reinforce one another. This perturbation can be transmitted at distance from the original moving electric charges, as electromagnetic waves. Electromagnetic waves can travel through a vacuum or through a medium.


Fig. 2-8: Electromagnetic wave

The electric and the magnetic fields created by the moving of charged particles, oscillate perpendicular to each other. Electromagnetic wave, and therefore light, is transmitted in a direction perpendicular to the oscillation of the electric field (E) and magnetic field (M). This direction is called a light ray. The speed of light in a vacuum is noted “c” (c = 3*108 m/s). The wavelength of light is noted “λ”, and the frequency is noted “f” (λ = c*f)

The electromagnetic wave theory of light could not explain the energy emission of a black body and the photoelectric effect. The explanation came in the form of a new scientific theory called quantum mechanics. The new theory applies to the microcosmic world and was developed at the beginning of the XX century. According to quantum theory, light is made of quanta of energy.

The quanta of light are called photons. Every photon carries an amount of energy E = h*f, where “h” is called Plank’s constant (a quantum of action equal to 6.626 * 10-34 J*s), and f is the frequency of the electromagnetic radiation measured in Hz.

All fundamental particles are either fermions or bosons. In a quantum system, two or more fermions cannot occupy the same quantum state, while the bosons can. Photons are bosons. All photons from a laser pulse occupy the same quantum state. The fundamental difference between laser light and light from a regular bulb is that laser light is one wave (many photons in the same quantum state), while the regular light is a quantum system with many states mixed together. When properly prepared, laser light can be emitted more parallel than regular light.


Fig. 2-9: Coherent and non-coherent light

2.2.2   Reflection of light

When hitting a plane mirror light is reflected at an angle of reflection equal to the angle of incidence. This type of reflection is called specular reflection. In specular reflection, the laser beam is kept as a beam.

                                
Fig. 2-10: Specular reflection                                                                                 Fig. 2-11: Laser beam – specular and diffuse reflection

When reflecting on a rough surface, the light goes in different directions. This type of reflection is called diffuse. Following a diffuse reflection, the laser beam is replaced by a luminous spot.


Fig. 2-12: Roughness of a surface

If the roughness Ra of a surface is defined as:
     
Specular reflection: λ > Ra
Diffuse reflection:    λ ≤ Ra

Specular reflection happens when λ is greater than Ra, and diffuse reflection when λ is comparable or smaller than Ra.  A He-Ne laser (λ = 633 nm) will create on a wall with the roughness of 3000 nm a diffuse reflection, while a CO2 laser (λ = 10,600 nm) will create a specular reflection.

2.2.3   Refraction of light

The speed of light in a transparent material (noted “v”) is smaller than the speed of light in vacuum. The ratio between the speed of light in vacuum and the speed of light in a certain material is called “indices of refraction” and is noted with “n”. When light passes from one transparent material with indices of refraction n1 to another material with the indices of refraction n2, the direction of the propagation of light changes. This process is called refraction. Refraction is always accompanied by reflection. The amount of light that is reflected depends on the two materials, the polarization of light and on the angle of incidence.


Fig. 2-13: Reflection and refraction

At the separation surface between air and glass, when the incident ray is normal (angle of incidence = 00), 4% of the light will be reflected. When the light passes perpendicular through a window approximately 8% is reflected back. When the angle of incidence is greater than 00, more light will be reflected. This is the reason why all windows in the laser laboratory must be covered.

The reflected ray of a laser beam is called “stray beam”. Most of the laser eye accidents are caused by stray beams.


Fig. 2-14: Stray beams from prisms and windows

The user pays attention to the main beam and forgets about the stray beams. The stray beams get neglected because they are, in general, less intense, however, they can carry enough energy/power to create a serious hazard.

Stray beams from a flat surface are particularly dangerous.


Fig. 2-15: Stray beams from flat and curved surfaces

When refraction happens for light coming from a material with indices of refraction n1 higher to a material with indices of refraction smaller n2, the angle of refraction is greater than the angle of incidence.


Fig. 2-16: Total internal reflection

If the angle of incidence is increased above a critical value, total internal reflection happens (all light is reflected back in the first medium, and no light penetrates in the second medium). The value of the critical angle θc can be found from the equation:

Total internal reflection is used when laser light is transmitted through an optical fiber.

Fig. 2-17: ncore > ncladding


Fig. 2-18: Optical fiber

Fig. 2-19: Optical fiber parts

Fig. 2-20: Optical fiber light acceptance cone

Fig. 2-21: Optical fiber with different jackets

2.2.4   Absorption of light

When radiation passes through a transparent material it is partially absorbed.


Fig. 2-22: Absorption of light

Transmission T is defined as the ratio between the intensity of light that is transmitted and the incident light.


Fig. 2-23: Interaction of light with a typical lens

The incident light on a lens is partially reflected, partially absorbed, and partially transmitted.

2.2.5   Scattering of light

Transmitted or reflected light can be scattered. If the scattered light has the same wavelength, the process is called elastic or Rayleigh scattering. Inelastic scattering is the process through which the scattered light can have a different wavelength (Raman effect – see chapter 2.3.1).

2.2.6   Interference of light

The interference is the phenomenon in which two coherent (with the same wavelength) waves combine to form a resultant wave with the amplitude equal with the combination of the two original amplitudes. The resultant amplitude depends on the phase difference between the original waves.


Fig. 2-24: Light constructive and destructive interference

If the original waves are in phase, the new wave will have an amplitude equal with the sum of the original amplitudes (constructive interference). If the two waves are in opposite phase, and if the waves have the same amplitude, the resultant wave will have zero amplitude (destructive interference).

Since the laser emits coherent light, the interference is used in many laser applications. For example, in dielectric mirrors (also called Bragg mirrors) multiple layers of dielectric layers are used to create mirrors with ultra-high reflectivity (99.999%) for certain wavelengths (by using constructive interference) and transparent for other wavelengths.


Fig. 2-25: Dielectric mirror – The path IA and IB differ exactly by a multiple of integer wavelengths

On the other hand, if the layers are chosen to create destructive interference, the reflection is close to zero for certain wavelengths. These are designed to protect the user against the stray beams (see chapter 2.2.3). By using optics covered with the protective layers against the stray beams for a certain wavelength, when the laser emits a different wavelength the laser user can, in fact, increase the danger of the stray beams.

       n0 <nf < ns
Fig. 2-26: Blocking stray beans – The difference between the optical path is an odd number of half wavelengths.

2.2.7   Diffraction of light

In the wave theory of light when light approaches an obstacle or a slit, light can bend and reach into the region of the geometrical shadow.


Fig. 2-27: Diffraction of light

The intensity of light into the shadow region decreases further from the obstacle.


Fig. 2-28: Light diffraction around small obstacles

Light can go around a small obstacle.


Fig. 2-29: Diffraction effect – wide and narrow gaps

The smaller is the slit, the larger is the diffraction effect.

2.2.8   Dispersion of light

The light coming from the Sun, from an incandescent bulb, or a fire is a mixture of different wavelengths. During the refraction process, the angle at which the light bends depends on the wavelength. This creates the effect called dispersion of light.


Fig. 2-30: Light dispersion in a prism

The same process happens during the diffraction of light.


Fig. 2-31: Light dispersion in a diffraction grating

2.2.9   Polarization of light

As explained in chapter 2.2.1, the transmission of light waves takes place in a direction perpendicular to the oscillation of the vectors E and M.

The polarization is the ability of waves to oscillate in more than one direction. Transverse waves can be vertically or horizontally polarized (oscillations are forced to move in a particular direction). Longitudinal waves are not polarized (the oscillations can only move in the direction of the transmission of the wave).

A polarizer is a device that will force a transverse wave to oscillate only in one direction by blocking all other directions of oscillation.


Fig. 2-32: Vertical and horizontal filters

After the unpolarised transverse wave passes through two perpendicular polarizers the wave will be completely stopped.

This type of polarization is called linearly polarized since the oscillating vector after the wave passes through the polarizer will oscillate in one direction.

When the oscillating vector of the wave that passes through a polarizer rotates only in one sense (counterclockwise or clockwise), the wave is called circularly polarized.

When the oscillating vector rotates counterclockwise the waves are called left hand polarized.


Fig. 2-33: Left-hand polarization

When the oscillating vector rotates clockwise the waves are called right hand polarized.


Fig. 2-34: Right-hand polarization

Light is a transversal wave that can be polarized linear, circular or a mixture of both (elliptical). In quantum mechanics, the polarization of light is a manifestation of the intrinsic angular momentum (the spin) of the photon.

                                     
Fig. 2-35: Linear polarized                           Fig. 2-36: Elliptical polarized

The polarization of light can be obtained by transmission. In this case, the dumped light is absorbed in the polarizer.


Fig. 2-37: Polarization by transmission

Polarization can also be obtained by reflection.


Fig. 2-38: Polarization by reflection

In this case, the light oscillating in one direction penetrates in the second medium and the one oscillating in perpendicular direction returns in the first medium. The light coming back in the first medium is transmitted in a different direction. When this beam is not blocked, can cause laser accidents.

When working with a polarizer the laser user must understand how the polarizer works and where the light goes.

2.2.10   Brewster’s angle

Brewster’s angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection.


Fig. 2-39: Brewster angle

When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized. In this case, the reflected and refracted rays are perpendiculars.


Fig. 2-40: Reflection and refraction at Brewster angle

If the indices of refraction are n1 and n2, θB can be found with the formula:

tan θB = n2/n1           θ= arctan(n2/n1)

Brewster’s angle at the interface between air and glass with refraction index n = 1.5 is around 560.

Two mirrors arranged at the Brewster angle (the first will act as a polarizer and the second as the analyzer) will completely stop unpolarised light.


Fig. 2-41: Two mirrors mounted at a Brewster angle

When the incident ray is polarized and hits the glass at Brewster angle there is no reflection (there is no stray beam). This is used in some lasers to reduce losses in a laser cavity.

            


Fig. 2-42: Brewster windows

2.2.11   Beam splitter

A beam splitter is an optical device, which can split an incident laser beam into two or more beams, which may or may not have the same optical power.

Fig. 2-43: Beam splitter principle of functioning

         


Fig. 2-44: Beam splitter – incident light from both directions

When the direction of the incident beam changes, the direction of the reflected beam, also changes. If a lens is after the beam splitter, the stray beam coming back from the lens will be reflected in the opposite direction of the first reflected beam. The user must block both reflected beams when they are not used.

Cube beam splitter   Window beam splitter
Fig. 2-45: Cube beam splitter         Fig. 2-46: Window beam splitter

One of the uses of the beam splitter is to control the power of the transmitted beam with the help of a polarizer. Since the amount of transmitted and reflected beam is dependent on the polarization of the light, a beam polarizer followed by a beam splitter can replace many filters.

2.3   Non-linear effects

Some non-linear optic effects are important in laser manufacturing, experiments, applications and safety.

2.3.1   Raman effect

When the dimension of the object on which light scattering occurs is much smaller than the wavelength (e.g. when scattering happens on molecules or atoms), most of the scattered light will have the same wavelength as the incident light (Rayleigh scattering). It can occur when light travels through transparent solids and liquids but is most prominently seen in gases. The Rayleigh scattering is inversely proportional tot he wavelength to the power of 4. Therefore, the blue light will be more scattered in the atmosphere than the yellow or red. During the daytime, the sky appears blue due to Rayleigh scattering and the Sun appears yellowish since the direct light is less scattered. Seen from space the sky is black and the Sun is white

When the incoming photons induce intra-molecular vibrations and rotations inside the molecules, the scattered light will have a different wavelength (Raman Effect). When the energy of the scattered photons is reduced by the energy of excited states, there is a Stokes Raman scattering, and when is increased by the energy of the excited states, there is anti-Stokes Raman scattering.  Since the energy of the excited states depends on the molecule, the Raman Effect is successfully used in identifying the molecules (chemical analysis).


Fig. 2-47: Raman effect

2.3.2   Harmonic generation

The harmonic generation is a non-linear process in which two or more photons with the same frequency interacting with a non-linear material, are “combined”, and generate a new photon with energy twice (second harmonic generation SHG) the energy of the incoming photons.


Fig. 2-48: Second-harmonic generator – principle

This is the energy level scheme for a second harmonic generation process.


Fig. 2-49: Non-linear optical medium – both initial frequency and double frequency beams are present

Since this non-linear effect is significant only at high power densities of light, generating the second harmonics was possible only after the invention of lasers (in 1961).


Fig. 2-50: Generating second harmonic

It was proven that the process of generating the second harmonic depends on the material and the square of the power of the laser beam.

P2 = γP12 , where P2 is the power of the double frequency beam, P1 is the power of the pump, and γ is a coefficient that depends on the crystal.

Harmonic generation in the perturbative (weak field) regime is characterized by rapidly decreasing efficiency with increasing harmonic order. This behaviour can be understood by considering an atom absorbing n photons then emitting a single high energy photon. The probability of absorbing n photons decreases as n increases, explaining the rapid decrease in the higher harmonic intensities

The SHG is commercially used to produce a 532 nm beam from a 1064 nm beam of NdYAG lasers. In some non-linear crystals (lithium niobate (LiNbO3), potassium titanyl phosphate (KTP = KTiOPO4), lithium triborate (LBO = LiB3O5), etc.) with a proper phase match between the beam and the crystal, for extremely short pulses (ps or fs), almost 100% of energy can be converted in a beam with double frequency. For lower powers (longer pulses) or phase mismatch, the conversion can be much less, allowing a large amount of laser power with the original wavelength to be present in the laser beam. When the 1064 nm beam is not filtered by the laser manufacturer, laser goggles used to protect the users against the visible 532 nm light, may not be designed to block the 1064 nm.

Starting with a NdYAG beam of 1064 nm, a third harmonic (λ3= λ1/3 = 1064 nm/3 = 355 nm) can be obtained by using a sum-frequency mixing between the 1064 and 532 beams. The fourth harmonic (λ4= λ1/4 = 1064 nm/4 = 266 nm), can be obtained by doubling the frequency of the 532 beam. Because these processes are all non-linear the efficiency of obtaining the third or fourth harmonic can be very low. The first higher harmonic generation (HHG with frequency 5 or higher the original laser beam frequency) was observed in 1977 with a CO2 laser.

2.3.3   Electro-optic effect

A birefringent material has two different indices of refraction, one for each of the two perpendicular components of polarization. When unpolarized light passes through a birefringent material, two different components of polarization travel in two different directions


Fig. 2-51: Birefringent material

Some materials (like quartz or calcite) exhibit birefringence naturally, without the application of an electric voltage. The birefringence is present all the time.

When an electric field is applied on an electro-optic material, the changes in the indices of refraction and polarization properties of the material can occur. The first order non-linear effect (proportional to the electric field applied) is called the Pockels effect, and the second-order (proportional to the square of the electric field intensity), is called the Kerr effect.

The optical Kerr effect is the Kerr effect in which the electric field applied to the material is the electric component of the electromagnetic field of the beam itself. This causes a local variation in the index of refraction, which is proportional to the square of the irradiance of the beam. This refractive index variation is responsible for the nonlinear optical effects and is the basis for Kerr-lens mode-locking (see chapter 3.3.3). The effect only becomes significant with very intense beams such as those from ps or fs lasers.

The change in the birefringence properties of the materials under the influence of the electric field is used in the electro-optical Q-switch to create short laser pulses (see chapter 3.2.3).

2.3.4   Saturable absorption

Saturable absorption is a property of materials where the absorption of light decreases with increasing light intensity. Most materials show some saturable absorption, but often only at very high optical intensities, close to the optical damage of the material. Decreased absorption at the high light intensity, competes with other mechanisms like increase in temperature or formation of colour centers.

For pulsed operation, when the pulse duration, t, is smaller than the relaxation time of the medium, absorption can decrease very fast and increase back after the pulse ends. In some situations, for very short pulses and relaxation times, the material can look unchanged after the laser pulse passes through the material.

Saturable absorption is used in creating very short laser pulses. Also, when the front mirror of the laser cavity is made of saturable absorber with the intention of letting pass the very high intensity, short laser pulses, and return to reflectivity when the intensity of the light is lower.

Saturable absorption can be a challenge for the laser goggles used for ps and fs lasers (see chapter 7.3.5).

Fig. 2-52: Mechanism of increased transmittance at saturable absorption point