Laser

What is a Laser?


Laser is the acronym of Light Amplification by Stimulated Emission of
Radiation. The laser world is really rich and interesting. Laser is light of
special properties, light is electromagnetic (EM) wave in visible range.The
first laser was invented by Maiman in May, 1960. It was a solid ruby laser.
Many kinds of laser were invented soon after the solid ruby laser-Uranium
Laser, Helium-Neon Laser, semiconductor laser, Nd:YAG laser, CO2 laser
etc.

As a light source, a laser can have various properties, depending on the
purpose for which it is designed and calibrated. A typical laser emits light in
a narrow, low-divergence beam and with a well-defined wavelength
(corresponding to a particular color if the laser is operating in the visible
spectrum). This is in contrast to a light source such as the incandescent light
bulb, which emits into a large solid angle and over a wide spectrum of
wavelength. These properties can be summarized in the term coherence.
A laser consists of a gain medium inside an optical cavity (resonator), with a
means to supply energy to the gain medium. The gain medium is a material
(gas, liquid, solid or free electrons) with appropriate optical properties. In its
simplest form, a cavity consists of two mirrors arranged such that light
bounces back and forth, each time passing through the gain medium.
Typically, one of the two mirrors, the output coupler, is partially transparent.
The output laser beam is emitted through this mirror.

Light of a specific wavelength that passes through the gain medium is
amplified (increases in power); the surrounding mirrors ensure that most of
the light makes many passes through the gain medium. Part of the light that
is between the mirrors (i.e., is in the cavity) passes through the partially
transparent mirror and appears as a beam of light. The process of supplying
the energy required for the amplification is called pumping and the energy istypically supplied as an electrical current or as light at a different
wavelength. In the latter case, the light source can be a flash lamp or another
laser. Most practical lasers contain additional elements that affect properties
such as the wavelength of the emitted light and the shape of the beam.


Basic Principles of Lasers


To explain how laser light is generated, we need first to investigate the
energy transition phenomena in atoms or molecules. These phenomena
include: spontaneous emission, stimulated emission/absorption and
nonradiative decay.

Absorption and Emission


s with matter as an example, absorption of a photon will occur only when the
quantum energy of the photon precisely matches the energy gap between the
initial and final states. In the interaction of radiation with matter, if there is
no pair of energy states such that the photon energy can elevate the system
from the lower to the upper state, then the matter will be transparent to that
radiation.Energy levels associated with molecules, atoms and nuclei are in general
discrete, quantized energy levels and transitions between those levels
typically involve the absorption or emission of photons. Electron energy
levels have been used as the example here, but quantized energy levels for
molecular vibration and rotation also exist. Transitions between vibrational
quantum states typically occur in the infrared and transitions between
rotational quantum states are typically in the microwave region of the
electromagnetic spectrum.

Stimulated Emission

If an electron is already in an excited state (an upper energy level, in contrast
to its lowest possible level or "ground state"), then an incoming photon for
which the quantum energy is equal to the energy difference between its
present level and a lower level can "stimulate" a transition to that lower
level, producing a second photon of the same
When a sizable population of electrons resides in upper levels, this condition
is called a "population inversion", and it sets the stage for stimulated
emission of multiple photons. This is the precondition for the light
amplification which occurs in a laser, and since the emitted photons have adefinite time and phase relation to each other, the light has a high degree of
coherence.
Like absorption and emission, stimulated emission requires that the photon
energy given by the Planck relationship be equal to the energy separation of
the participating pair of quantum energy states.

Population Inversion

· The achievement of a significant population inversion in atomic or
molecular energy states is a precondition for laser action. Electrons
will normally reside in the lowest available energy state. They can be
elevated to excited states by absorption, but no significant collection
of electrons can be accumulated by absorption alone since bothspontaneous emission and stimulated emission will bring them back down.
· A population inversion cannot be achieved with just two levels
because the probabability for absorption and for spontaneous emission
is exactly the same, as shown by Einstein and expressed in the
Einstein A and B coefficients. The lifetime of a typical excited state is
about 10^8 seconds so in practical terms, the electrons drop back down
by photon emission about as fast as you can pump them up to the
upper level. The case of the helium-neon laser illustrates one of the
ways of achieving the necessary population inversion.

Laser pumping


Laser pumping is the act of energy transfer from an external source into the
gain medium of a laser. The energy is absorbed in the medium, producing
excited states in its atoms. When the number of particles in one excited state
exceeds the number of particles in the ground state or a less-excited state,
population inversion is achieved. In this condition, the mechanism of
stimulated emission can take place and the medium can act as a laser or an
optical amplifier. The pump power must be higher than the lasing threshold
of the laser.
There are a number of techniques for pumping. The pump energy is usually
provided in the form of light or electric current, but more exotic sources
have been used, such as chemical or nuclear reactions. In optical pumping a
light source such as a flash discharge tube is used. This method is adopted in
solid state lasers.
Electric glow discharge method is common in gas lasers. For example, in the
helium-neon laser the electrons from the discharge collide with the helium
atoms, exciting them. The excited helium atoms then collide with neon
atoms, transferring energy. This allows an inverse population of neon atoms
to build up. In semiconductor lasers, a direct conversion of electrical energy
into light energy takes place.
Before we study individual lasers, let’s first examine the properties of laser
beams.Properties of Laser Beams

Monochromaticity:


This property is due to the following two factors. First, only an EM wave of
frequency = (E2-E1)/h can be amplified, has a certain range which is
called linewidth, this linewidth is decided by homogeneous broadening
factors and inhomogeneous broadening factors, the result linewidth is very
small compared with normal lights. Second, the laser cavity forms a resonant
system, oscillation can occur only at the resonance frequencies of this cavity.
This leads to the further narrowing of the laser linewidth, the narrowing can
be as large as 10 orders of magnitude! So laser light is usually very pure in
wavelength, we say it has the property of monochromaticity.

Coherence:


For any EM wave, there are two kinds of coherence, namely spatial and
temporal coherence.
Let’s consider two points that, at time t=0, lie on the same wave front of
some given EM wave, the phase difference of EM wave at the two points at
time t=0 is k0. If for any time t>0 the phase difference of EM wave at the
two points remains k0, we say the EM wave has perfect coherence between
the two points. If this is true for any two points of the wave front, we say the
wave has perfect spatial coherence. In practical the spatial coherence occurs
only in a limited area, we say it is partial spatial coherence.
Now consider a fixed point on the EM wave front. If at any time the phase
difference between time t and time t + dt remains the same, where "dt" is the
time delay period, we say that the EM wave has temporal coherence over a
time dt. If dt can be any value, we say the EM wave has perfect temporal
coherence. If this happens only in a range 0<dt<t0, we say it has partial
temporal coherence, with a coherence time equal to t0.
We emphasize here that spatial and temporal coherence are independent. A
partial temporal coherent wave can be perfect spatial coherent. Laser light is
highly coherent, and this property has been widely used in measurement,
holography, etc.


Divergence and Directionality:


Laser beam is highly directional, which implies laser light is of very small
divergence. This is a direct consequence of the fact that laser beam comes
from the resonant cavity, and only waves propagating along the optical axis
can be sustained in the cavity. The directionality is described by the light
beam divergence angle. Please try the figure below to see the relationship
between divergence and optical systems.
For perfect spatial coherent light, a beam of aperture diameter D will have
unavoidable divergence because of diffraction. From diffraction theory, the
divergence angle is:
q= b l /D
Where and D are the wavelength and the diameter of the beam
respectively, is a coefficient whose value is around unity and depends on
the type of light amplitude distribution and the definition of beam diameter.
is called diffraction limited divergence.
If the beam is partial spatial coherent, its divergence is bigger than the
diffraction limited divergence. In this case the divergence becomes:q = b l /(Sc)
1/2 where Sc is the coherence area.

Intensity:
The intensity of light from a conventional source decreases rapidaly with
distance, as it spreads in form of spherical waves. One can look at the source
without any harm to his 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 very high.It is
estimated that light from a typical 1-mW laseris 10,000 times brighter than
the light from the sun at the earth’s surface. The intensity of laser beam stays
nearly constant with distance as the light travels in the form of plane waves.

Helium-Neon Laser


· Most common and inexpensive gas laser,
· Helium-neon laser is usually constructed to operate in the red at 632.8
nm.
· It can also be constructed to produce laser action in the green at 543.5
nm and in the infrared at 1523 nm.· One of the excited levels of helium at 20.61 eV is very close to a level
in neon at 20.66 eV, so close in fact that upon collision of a helium
and a neon atom, the energy can be transferred from the helium to the
neon atom.
· Helium-neon lasers are common in the introductory physics
laboratories, but they can still be dangerous!
· An unfocused 1-mW He-Ne laser has brightness equal to sunshine on
a clear day (0.1 watt/cm 2 ) and is just as dangerous to stare at directly.
· The helium gas in the laser tube provides the pumping medium to
attain the necessary population inversion for laser action.
Semiconductor Laser
Laser action (with the resultant monochromatic and coherent light output)
can be achieved in a p-n junction formed by two doped gallium arsenide
layers. The two ends of the structure need to be optically flat and parallel
with one end mirrored and one partially reflective. The length of the junction
must be precisely related to the wavelength of the light to be emitted. The
junction is forward biased and the recombination process produces light as
in the LED (incoherent). Above a certain current threshold the photons
moving parallel to the junction can stimulate emission and initiate laser
action.
Application of Laser


There lasers find applications in many fields e.g. medical, welding, cutting,
holography etc.They bought amazing changes in many areas and caused
spectular developments in the field of communication. Laser made optical
communication possible. A few applications of lasers are given below.

· Medical Uses of Lasers


The highly collimated beam of a laser can be further focused to a
microscopic dot of extremely high energy density. This makes it useful as a
cutting and cauterizing instrument. Lasers are used for photocoagulation of
the retina to halt retinal hemorrhaging and for the tacking of retinal tears.
Higher power lasers are used after cataract surgery if the supportive
membrane surrounding the implanted lens becomes milky. Photodisruption
of the membrane often can cause it to draw back like a shade, almost
instantly restoring vision. A focused laser can act as an extremely sharp
scalpel for delicate surgery, cauterizing as it cuts. ("Cauterizing" refers to
long-standing medical practices of using a hot instrument or a high
frequency electrical probe to singe the tissue around an incision, sealing off
tiny blood vessels to stop bleeding.) The cauterizing action is particularly
important for surgical procedures in blood-rich tissue such as the liver.
Lasers have been used to make incisions half a micron wide, compared to
about 80 microns for the diameter of a human hair.

· Welding and Cutting


The highly collimated beam of a laser can be further focused to a
microscopic dot of extremely high energy density for welding and cutting.
The automobile industry makes extensive use of carbon dioxide lasers with
powers up to several kilowatts for computer controlled welding on auto
assembly lines.

An interesting application of CO2 lasers to the welding of stainless steel
handles on copper cooking pots. A nearly impossible task for conventional
welding because of the great difference in thermal conductivities between
stainless steel and copper, it is done so quickly by the laser that the thermal
conductivities are irrelevant.· Lasers in Communication
Fiber optic cables are a major mode of communication partly because
multiple signals can be sent with high quality and low loss by light
propagating along the fibers. The light signals can be modulated with the
information to be sent by either light emitting diodes or lasers. The lasers
have significant advantages because they are more nearly monochromatic
and this allows the pulse shape to be maintained better over long distances.
If a better pulse shape can be maintained, then the communication can be
sent at higher rates without overlap of the pulses.

 Laser Printers


The laser printer has in a few years become the dominant mode of printing
in offices. It employs a semiconductor laser and the xerography principle.
The laser is focused and scanned across a photoactive selenium coated drum
where it produces a charge pattern which mirrors the material to be printed.
This drum then holds the particles of the toner to transfer to paper which is
rolled over the drum in the presence of heat. The typical laser for this
application is the aluminum-gallium-arsenide (AlGaAs) laser at 760 nm
wavelength, just into the infrared.