Lasers: 4.1 Introduction and Basic Terms
Lasers: 4.1 Introduction and Basic Terms
Lasers: 4.1 Introduction and Basic Terms
4. LASERS
4.1 INTRODUCTION AND BASIC TERMS
Laser beam is used as a high power electromagnetic beam in Engineering and Biological
applications.
LASERs X-rays
1) LASERs are electromagnetic waves 1) X-rays are electromagnetic waves
having wavelength of the order of few having wavelength of few
thousand angstroms. angstroms.
2) LASERs are highly coherent. 2) X-rays are not highly coherent.
3) LASERs are obtained due to phenomenon 3) X-rays are given out when high
called stimulated emission of radiation. speed electrons strike the target of
high atomic number and melting
point
About Light :
Light consists of discrete bundles or chunks (quantum) of energy. Energy of each bundle
is “hν”. - Max Planck
Albert Einstein provided theoretical justification to this and introduced name photon to
this quantum of light energy.
Photon represents minimum energy unit of light.
Each photon carries energy ‘hν’ where ‘ν’ is frequency of light wave.
Light energy cannot have arbitrary values but must be multiple of ‘hν’.
About Matter :
Electrons in an atom cannot have arbitrary amount of energy, but they take only discrete
energies. – Bohr.
Electrons in an atom can have only discrete energy levels which are schematically
represented by horizontal lines drawn to the energy scale –
Nucleus
n=5
n=4
n=3
Energy
n=2
n=1
n=2
Ground State
n=3 n=1
n=4
n=5
Quantum Transition :
Passing of an atom from one energy state to the other state.
Whenever quantum transition occurs between energy states E1 and E2, energy E2 ~ E1 =
hν is absorbed or released as a radiation.
4.5 ABSORPTION
E2 E2 E2
hn = E2- E1
E1 E1 E1
Before Transition During Transition After Transition
Excited atom can stay at the excited level for a limited time known as
(life-time of that state).
After the life-time of the state gets over, the atom is de-excited and come back to the
lower energy level.
During the transition, Excess energy is given in the form of photon of
energy hν = E2 – E1.
This process is called Spontaneous Emission of Radiation.
It is independent of outside circumstances.
It is probabilistic in nature.
Light spreads in all directions around the source.
Light intensity decreases rapidly with distance from the source.
Light is incoherent.
E2 E2 E2
hn = E2- E1
E1 E1 E1
Before Transition During Transition After Transition
If a photon can stimulate an atom to move from a lower energy state E1 to the higher
energy state E2 by means of absorption, then a photon should also be able to stimulate an
atom from the higher energy level E2 to the lower energy level E1 – Albert Einstein.
Consider an atom in the excited energy level E2.
External photon having energy hν = E2 – E1 incident on this system, stimulates this atom
to jump back to lower energy level E1 before its life time gets over.
During this transition, atom emits a photon with same energy as the energy of incident
photon. This is called as stimulated emission.
E2 E2 E2
hn = E2- E1 hn = E2- E1
E1 E1 E1
Before Transition During Transition After Transition
20 21 22 23 2N
5. High Intensity : Intensity of resultant light is proportional to the square of the umber
of atoms emitting that light.
In Normal equilibrium, the lower energy level is more densely populated than the higher
energy level. (N1 >> N2)
Stimulated emission to be effective for light amplification, it should be dominant over the
process of absorption.
This is achieved by adjusting N2 >> N1
Getting more number of atoms in higher energy level than the lower energy level (N2 >>
N1) is called Population Inversion.
4.9 PUMPING
1. Optical Pumping :
Light energy is used for pumping.
Photons are made incident on the active medium.
E.g. Flash discharge tubes, continuously operating lamps, Spark gaps.
2. Electrical Pumping :
Electric current is passed through the active medium.
Electrons collide with atoms and excite them to higher energy states.
Used in gas lasers.
3. Direct Conversion :
Electrical energy is directly converted into light energy.
Electrical current is passed through active medium but atoms are not excited to
higher states.
The current carriers themselves are excited to higher states to achieve
population inversion. E.g. Semiconductor Laser.
Laser Beam
Partially reflecting
Fully reflecting Mirror
Mirror
N3
E3
Spontaneous Emission
Stimulated
hν = Ε2−Ε1 Emission
E1 N1 N2 > N1
E4 N4
Spontaneous Emission
E3 N3 Metastable State
hν = Ε4−Ε1 Stimulated
hν = Ε3−Ε2 Emission
E2 N2 N3 > N2
Spontaneous Emission
E1 N1
Fig. 4.8 Four level system
A conventional light source such as an incandescent lamp or a natural source such as the sum
produces incoherent light since they emit random wavelength light waves with no common
phase relationships. On the other hand, the waves emitted by a laser source will be in phase and
are of the same frequency. Therefore, light generated by a laser is highly coherent. The
coherence length lcoh which is determined by the relation.
lcoh = λ2 /µ∆λ
is typically of the order of a few kilometers in case of lasers; whereas the coherence length of
light radiated by conventional monochromatic sources is of the order of a few millimeters or
centimeters.
4.13.2 Directionality
The conventional sources emit light in all directions. Lasers emit light only in one direction as
photons traveling along the optical axis of the system are selected and augmented with the help
of the optical resonator.
4.13.3 Divergence
Light from conventional sources spreads out in the form of spherical wave fronts and hence it is
highly divergent. This divergence or angular spreads of the laser beam is extremely small. The
little divergence that exists in it arises out of the wave properties of light. When the light issues
out form the front mirror, it undergoes diffraction because the semitransparent mirror acts as a
circular aperture. Accordingly, it spreads our and the angular spread is given by
∆Ө = ( 1.22λ)/d
Where d is the diameter of the front mirror. In case of gas lasers ∆Ө is small as 10-5 to 10-6
radians.
4.13.4 Intensity
The intensity of light from a conventional source decreases rapidly with distance, as it spreads in
the form of spherical waves. One can look at the source without any harm to has eyes. In
contrast, a laser emits light in the form of a narrow beam which propagates in the form of plane
waves. As the energy is concentrated in a very narrow region, its intensity would be
tremendously high. It is estimated that light from a typical 1 -mW laser is 10,000 times brighter
than the light from the sum at the earth’s surface. The intensity of the laser beam stays nearly
constant with distance as the light travels in the form of plane waves.
4.13.5 Monochromaticity
The light from normal monochromatic sources spreads over a wavelength range of the order of
100A to 1000A. The laser light is highly monochromatic. The spread is of the order of a few
angstroms (<10A) only. Such a vast difference arises because conventional sources emit wave
trains of very short duration and light, whereas lasers emit continuous waves of very long
duration.
Nd: YAG Laser is one of the most popular types of solid state laser. It is a four level laser.
Yttrium aluminum garnet, Y3Al5O12, commonly called YAG is an optically isotropic crystal.
Some of the Y3+ ions in the crystal replaced by neodymium ions (Nd3+). The crystal atoms do not
participate in the lasing action but serve as a host lattice in which the active centers namely Nd3+
ions reside.
Nd:YAG rod
Trigger
Pulse
Capacitors
R
Nd:YAG
ROD
Fig. 4.9 Construction of
ELLIPTICAL Nd:YAG Laser
REFLECTOR
Krypton
LAMP
4.14.1 Construction
Fig.4.9 illustrates a typical design of the laser. It consists of an elliptical cylindrical reflector
housing the Nd:YAG rod along one of its focus lines and a krypton arc lamp along the other
focus line. The light leaving one focus of the ellipse will pass through the other focus after
reflection from the silvered surface of the reflector. Thus all the light emitted by the arc lamp
gets focused on the Nd:YAG rod. One fully reflecting mirror and one partially reflecting mirror
are fixed at the two ends which constitute the optical resonator.
4.14.2 Working
The energy levels of the Neodymium ion in YAG crystal are shown if Fig 4.10. The energy level
structure of the free neodymium atom is preserved to a certain extent because of its relatively
low concentration. However, the energy levels are split and the structure is complex. The
pumping of the Nd3+ ions to upper states is done by a krypton arc lamp. The optical pumping
with light of wavelength range 5000 to 8000 A excites the ground state Nd3+ ions to higher
states. The meta-stable state E3 is the upper laser level, while the E2 forms the lower laser level.
The upper laser level E3 will be rapidly populated, as the excited Nd3+ ions quickly make
downward transitions from the upper energy levels. The lower laser level E2 is far above the
ground level and hence it cannot be populated by Nd3+ ions through thermal transitions from the
ground level. Therefore, the population inversion is readily achieved between the E3 level and E2
level. The laser emission occurs in infrared (IR) region at wavelength about 10,600 A (1.06
µm). As the laser is a four level laser emission. Thus Nd: YAG laser can be operated in CW
mode. An efficiency of better than 1% is achieved.
E6
E5 Spontaneous
Emission
E4
Fig. 4.10 Energy Level
E3 Metastable State
10600 A 0
Diagram for Nd 3+ ions (Infrared)
E2
E1
Nd: YAG lasers find many industrial applications such as resistor trimming, machining
operations like welding, hole drilling etc. They are also used in surgery.
Helium-Neon laser was the first successful gas laser. It was build by Ali Javan, W. Bennett and
D. Heriot in 1961.
4.15.1 Construction
Plasma Tube
He + Ne
Gas Discharge
The schematic of a typical He-Ne laser is shown if fig. It consists of a long discharge tube of
length about 50 cm and diameter 1 cm. the tube is filled with a mixture of Helium and Neon
gases in the ration 10:1. Electrodes are provided to produce a discharge in the gas and they are
connected to a high voltage power supply. The tube is hermetically sealed by windows inclined
at Brewster’s angle at its two ends. This arrangement serves the purpose of getting polarized
beam of light. One fully reflecting mirror and one partially reflecting mirror are fixed at the two
ends along the axis of the tube which constitute an optical resonator. The distance between the
mirrors is adjusted such that it equals mλ/2 and supports standing wave pattern.
4.15.2 Working
Helium-neon laser is a four-level laser system. The energy level diagram is shown if fig.4.12.
When the power is switched on, the electric field ionizes some of the atoms in the mixture of
helium and neon gases. Due to the electric field, the electrons and ions will be accelerated
towards the anode and cathodes as shown if fig. Since the electrons have a smaller mass, they
acquire a higher velocity.
The helium atoms are more readily excitable than neon atoms because they are lighter. The
energetic electrons excite helium atoms through collisions to the excited meta-stable levels F2
and F3 which lie at 19.81 eV and 20.61 eV above the ground state respectively. These Helium
atoms can return to the normal state by transferring their energy to Neon atoms through
collisions. Such energy transfer can take place when two colliding atoms have identical energy
levels. Energy E4 and E6 of Neon atoms nearly coincide with energy levels F2 and F3 of Helium
respectively. When Helium atoms in excited energy levels F2 and F3 collide with the Neon atoms
in the ground level, Neon atoms are excited to energy levels E4 and E6 and Helium atoms come
back to the ground state. This is pumping mechanism in He-Ne laser. The neon atoms are much
heavier and could not be pumped efficiently without Helium atoms. The role of Helium atoms is
to excite Neon atoms and cause population inversion. The probability of energy transfer from
Helium atoms to Neon atoms is more as there are 10 Helium atoms per 1 Neon atom in a gas
mixture.
Energy levels E4 and E6 of Neon atoms are meta-stable states. Hence the number of Neon atoms
accumulate in these levels and population inversion exist energy levels E6 and E5, E6 and E3 and
E4 and E3. and lasing action takes place corresponding to the transition due to stimulated
emission of radiation between energy levels E6 and E5, E6 and E3 and E4 and E3.
Energy Transfer
through inelastic
collision
E3 E6
3.39 µm
E5
E2 E4
1.15 µm 6328 A0
E3
Excitation due to Spontaneous emission
collision with E2
electrons
deexcitation due to
collision with walls
E1 E1
Energy Levels of Energy Levels of
Helium Neon
Atoms in energy level E2 loose their energy due to collisions with walls of the discharge tube and
come to the ground state.
He-Ne laser operates in a continuous wave mode and is widely used as a monochromatic source
in interferometer, laser printing, bar code reading etc. They are also used as a reference beam in
surveying, for alignment in pipes etc. He-Ne laser is highly stable. No separate cooling is
needed. But the output power is very low.
A semiconductor diode laser is a specially fabricated pn-junction device that emits coherent light
when it is forward biased. Population inversion is required for producing stimulated emission
and then amplification of light. Semiconductor is not a two level atomic system, but consists of
electrons and holes distributed in the respective energy bands. Therefore, laser action in
semiconductor involves energy bands rather than discrete levels.
4.16.1 Construction
A schematic diagram of a homojuction semiconductor laser is shown if Fig 4.13. The diode is
extremely small in size with sides of the order of 1mm. The junction lies in a horizontal plane
through the center.
Ohmic contact
Active Region
n - region
Ohmic contact
The top and bottom faces are metallized and ohmic contacts are provided to pass current
thorough the diode. The front and rear faces are polished parallel to each other and perpendicular
to the plane of the junction. The polished faces constitute the resonant cavity. The other two
opposite faces are roughened to prevent lasing action in that direction. The active region consists
of a layer of about 1µm thickness.
4.16.2 Working
EC
EC E
EC Fn
Recombination hν=E2-E1
EV
EF EF EV
EC EFp EV
Population
inversion
EV
p-region n-region
p-region n-region
Fig. 4.14 (b) Heavily doped p-n
Fig. 4.14 (a) Energy bands of heavily junction forward biased
doped p-n junction
With very high doping on a n-side the donor levels as well as a portion of the conduction band
are occupied by electrons and the Fermi level lies within the conduction band. Similarly, on the
heavily doped p-side the acceptor levels are unoccupied and holes exist in the valence band and
the Fermi level lies within the valence band. At thermal equilibrium, the Fermi level is uniform
across junction, as shown if Fig.4.14 (a). When a forward bias is applied to the junction, the
energy levels shift and the new distribution as shown fig 4.14(b) will be taken up. Electrons and
holes are injected into the depletion region which results decrease in depletion region width. The
injected electrons and holes appear in high concentrations in this transition region. At low
forward current level, the electron-hole recombination cause spontaneous emission of photons
and the junction acts as an LED. The bandwidth of the emitted light will be larger. As the current
is increased, the intensity of light increases linearly. When the current reaches a threshold value
the carrier concentration in the depletion region will reach very high values. The region contains
a large concentration of electrons within the conduction band and a large concentration of holes
within the valence band, as indicat4ed in the hatched region of fig (b). The upper levels in the
same region are vacant. This is the state of population inversion. The narrow region where the
state of population inversion is achieved is called inversion region or active region. Thus the
forward bias (current) plays the role of pumping agent in semiconductor diode laser. The photons
that propagate in the junction plane induce the conduction electrons to jump into the vacant
states of valence band. The stimulated electron-hole recombination causes emission of coherent
radiation of very narrow bandwidth. At room temperature, GaAs laser emits light at a
wavelength of 9000A in IR region. A GaAsP laser radiates at 6500A in the visible red region.
The pn-junciton lasers are also called injection lasers since the laser action is generated by
minority charge carriers injected across the depletion region of the junction.
The semiconductor diode lasers are simple, compact and highly efficient. They require very little
power and little auxiliary equipment. In contrast to He-Ne gas laser, diode lasers give more
divergent beam having an angular spread of the order 50 to 150. They are less monochromatic
and highly temperature sensitive.
Under thermal equilibrium, the mean population N1 and N2 in the lower energy level (E1) and
upper energy level (E2) respectively must remain constant. This condition requires that the
number of transitions from E2 to E1 must be equal to the number of transitions from E1 to E2.
Thus,
Number of atoms absorbing photons Number of atoms emitting photons
=
per second per unit volume per second per unit volume
The number of absorption transitions occurring in the material at any instance is proportional to
the population of lower energy level and the number of photons per unit volume in the incident
light.
Number of atoms absorbing
photons per second per unit = B12 ρ(v) N 1 --------------------------(1)
volume
where ρ(v) is the energy density of incident light
B12 is constant of proportionality and is known as Einstein’s Coefficient for absorption.
It indicates probability of an induced transition from level 1 to level 2.
Atoms may fall from upper level to lower level due to spontaneous emission and stimulated
emission.
Number of atoms emitting
photons per second per unit = A 21 N 2 + B 21 ρ(v) N 2 -----------(2)
volume
where A21 is Einstein Coefficient for spontaneous emission and
B21 is Einstein Coefficient for stimulated emission.
In equilibrium condition, the number of transitions from E2 to E1 must be equal to the number of
transitions from E1 to E2.
To maintain thermal equilibrium, incident energy density given by (6) and radiated energy
density given by (7) must be equal. This is possible only when
Equation (8) shows that coefficients for both spontaneous emission and stimulated emission are
numerically equal. Thus probability of upward transition is equal to probability of downward
transition.
Equation (9) shows that, ratio of coefficients of spontaneous emission to the coefficients of
stimulated emission is proportional to the 3rd power of frequency of radiation. Spontaneous
emission dominates over the stimulated emission if the energy difference (E2-E1 = hν) between
the two levels is high. So it is difficult to achieve lasing action in higher frequency ranges and
low wavelengths.
4.18 APPLICATIONS
4.18.1 HOLOGRAPHY
In conventional photography a negative is made first and using it a positive print is produced
later. The positive print is only a two dimensional record of light intensity received from a three
dimensional object. It contains information about the square of the amplitude of the light wave
that produced the image but information about the phase of the light wave is not recorded and is
lost. In 1947 Dennis Gabor the English physicist outlined a radically new technique of
photographing objects. He called this technique wavefront construction. According to this
technique both the phase and intensity attributes of the wave are recorded and when viewed the
photograph shows a three dimensional image of the object. This technique is name holography.
Gabor was awarded in 1971 the Nobel prize in physics for this invention.
Fig 4.15 illustrates the principle of holography. A weak but broad beam of laser light is split into
two beams namely a reference beam and object beam. The reference beam is allowed to reach
the photographic plate directly after the reflection from a mirror while the object beam
illuminates the object and is reflected from the object. Part of the light scattered by the object
travels towards the photographic plate and interferes with the reference beam and interference
pattern is called a hologram. Holos means complete in Greek and “gramma” means writing.
Thus a hologram means complete recording. Like any ordinary photographic plate, a hologram is
developed, fixed and stored.
Mirror
Laser Beam
ce
en
e fer am
R Be
Object
Object beam
Photographic plate
(Hologram)
Fig. 4.15 Construction of Hologram
A hologram does not contain a district image of the object. It is only a record of the interference
pattern formed by the superposition of two coherent light beams. The interference pattern on a
hologram consists of a complex pattern of alternate regions of dark and bright fringes. The
hologram is also called a Gabor zone plate in honour of Dennis Gabor who conceived the
principle of holography.
Eye
1st order
0th order
Hologram
1st order
Virtual Image Real Image
Whenever required the object can be viewed by illumination the hologram as shown in fig.4.16.
A laser beam identical to the reference beam is used for the reconstruction of the object. The
reconstruction beam (laser beam) illuminates the hologram at the same angel as the reference
beam. The hologram acts as a diffraction grating and secondary waves from the hologram
interfere constructively in certain direction and interfere destructively in other directions. They
form a real image in front of the hologram and a virtual image behind the hologram at the
original site of the object. An observer sees light waves diverging from the virtual image. An
image of the object appears where the object once stood and that image is identical to what our
eyes would have perceived in all its details. If the observer tilts his head other objects behind the
first one or new details of the object which were not noticed earlier would be observed.
Holography is thus a two stage process. In the first stage a hologram is recorded in the form of
interference pattern. In the second stage the hologram acts as a diffraction grating for the
reconstruction beam and the image of the object is reconstructed form the hologram.
It should be appreciated that an interference pattern cab be obtained only when the reference
beam and the object beam are coherent. The beams can be coherent only when they belong to the
same original wave train. As the two beams travel over large distances it would be difficult to
fulfill this condition with ordinary sources of light. Interference pattern can be obtained when the
optical path difference between two beams, ∆ is very small compared to the length of the wave
train, i.e., coherence length, λ coh. Then only, the two parts of the wave train in the two beams will
be coherent at the point of observation. For conventional monochromatic sources of light, the
coherence length is found to be of the order of a few centimeters and a hologram cannot be
obtained with them. Therefore, holography could not be realized in practice for a long time
though the principle was formulated in 1947. The invention of lasers made holography possible.
Laser beams have coherence length of the order of a few kilometers and, therefore, the
interference of split rays with a large path difference could be achieved. The first practical
hologram was made using a laser beam by two American investigators, E. N. Leith and Y.
Upatneiks in 1964.
OTHER APPLICATIONS :
Lasers are widely used in manufacturing, e.g. for cutting, drilling, welding, cladding, soldering
(brazing), hardening, ablating, surface treatment, marking, engraving, micromachining, pulsed
laser deposition, lithography, alignment, etc. In most cases, relatively high optical intensities are
applied to a small spot, leading to intense heating, possibly evaporation and plasma generation.
Essential aspects are the high spatial coherence of laser light, allowing for strong focusing, and
often also the potential for generating intense pulses.
Laser processing methods have many advantages, compared with mechanical approaches. They
allow the fabrication of very fine structures with high quality, avoiding mechanical stress such as
caused by mechanical drills and blades. A laser beam with high beam quality can be used to drill
very fine and deep holes, e.g. for injection nozzles. A high processing speed is often achieved,
e.g. in the fabrication of filter sieves. Further, the lifetime limitation of mechanical tools is
removed. It can also be advantageous to process materials without touching them.
The requirements on optical power and beam quality depend very much on the application and
the involved materials. For example, laser marking on plastics can be done with fairly low power
levels, whereas cutting, welding or drilling on metals requires much more – often multiple
kilowatts. Soldering applications may require a high power but only a moderate beam quality,
whereas particularly remote welding (i.e., welding with a substantial distance between laser head
and welded parts) depends on a high beam quality.
Laser-aided manufacturing often allows one to produce the essentially same parts with higher
quality and/or lower cost. Also, it is often possible to realize entirely new part designs or the use
of new materials. For example, automobile parts are increasingly made of light materials such as
aluminum, which require tentatively more laser joining operations. Weight reductions are
possible not only by the user of lighter materials, but also e.g. by producing them with shorter
flanges due to higher precision than is feasible with conventional production methods.
− Medical Applications
There is a wide range of medical applications. Often these relate to the outer parts of the human
body, which are easily reached with light; examples are eye surgery and vision correction
(LASIK), dentistry, dermatology (e.g. photodynamic therapy of cancer), and various kinds of
cosmetic treatment such as tattoo removal and hair removal.
Lasers are also used for surgery (e.g. of the prostate), exploiting the possibility to cut tissues
while causing minimal bleeding. Some operations can be done with endoscopic means; an
endoscope may contain an optical fiber for delivering light to the operation scene and another
fiber for imaging, apart from additional channels for mechanical instruments.
Very different types of lasers are required for medical applications, depending on the optical
wavelength, output power, pulse format, etc. In many cases, the laser wavelength is chosen such
that certain substances (e.g. pigments in tattoos or caries in teeth) absorb light more strongly than
surrounding tissue, so that they can be more precisely targeted.
Medical lasers are not always used for therapy. Some of them rather assist the diagnosis, e.g. via
methods of ocular imaging, laser microscopy or spectroscopy (see below).
− Metrology
Lasers are widely used in optical metrology, e.g. for extremely precise position measurements
and optical surface profiling with interferometers, for long-distance range finding and
navigation.
Laser scanners are based on collimated laser beams, which can read e.g. bar codes or other
graphics over some distance. It is also possible to scan three-dimensional objects, e.g. in the
context of crime scene investigation (CSI).
Optical sampling is a technique applied for the characterization of fast electronic microcircuits,
microwave photonics, terahertz science, etc.
Lasers also allow for extremely precise time measurements and are therefore essential
component of optical clocks which are beginning to outperform the currently used cesium atomic
clocks.
Fiber-optic sensors, often probed with laser light, allow for the distributed measurement of
temperature, stress, and other quantities e.g. in oil pipelines and wings of airplanes.
− Data Storage
Optical data storage e.g. in compact disks (CDs), DVDs, Blu-ray Discs and magneto-optical
disks, nearly always relies on a laser source, which has a high spatial coherence and can thus be
used to address very tiny spots in the recording medium, allowing a very high density data
storage. Another case is holography, where the temporal coherence can also be important.
− Communications
Optical fiber communication, extensively used particularly for long-distance optical data
transmission, mostly relies on laser light in optical glass fibers. Free-space optical
communications, e.g. for inter-satellite communications, is based on higher-power lasers,
generating collimated laser beams which propagate over large distances with small beam
divergence.
− Displays
Laser projection displays containing RGB sources can be used for cinemas, home videos, flight
simulators, etc., and are often superior to other displays concerning possible screen dimensions,
resolution and color saturation. However, further reductions in manufacturing costs will be
essential for deep market penetration.
− Spectroscopy
Laser spectroscopy is used in many different forms and in a wide range of applications. For
example, atmospheric physics and pollution monitoring profits from trace gas sensing with
differential absorption LIDAR technology. Solid materials can be analyzed with laser-induced
breakdown spectroscopy. Laser spectroscopy also plays a role in medicine (e.g. cancer
detection), biology, and various types of fundamental research, partly related to metrology (see
above).
− Microscopy
Laser microscopes and setups for optical coherence tomography (OCT) provide images of, e.g.,
biological samples with very high resolution, often in three dimensions. It is also possible to
realize functional imaging.
− Energy Technology
In the future, high-power laser systems might play a role in electricity generation. Laser-induced
nuclear fusion is investigated as an alternative to other types of fusion reactors. High-power
lasers can also be used for isotope separation.
− Military Applications
There are a variety of military laser applications. In relatively few cases, lasers are used as
weapons; the “laser sword” has become popular in movies, but not in practice. Some high-power
lasers are currently developed for potential use as directed energy weapons on the battle field, or
for destroying missiles, projectiles and mines.
In other cases, lasers function as target designators or laser sights (essentially laser
pointers emitting visible or invisible laser beams), or as irritating or blinding (normally not
directly destroying) countermeasures e.g. against heat-seeking anti-aircraft missiles. It is also
possible to blind soldiers temporarily or permanently with laser beams, although the latter is
forbidden by rules of war.
There are also many laser applications which are not specific for military use, e.g. in areas such
as range finding, LIDAR, and optical communications.