A simple project about production of laser & its different types.

1. 31st May, 2016
LASER
PRESENTED BY,
TRISA DEBNATH
DEPARTMENT OF PHYSICS
NORTH BENGAL UNIVERSITY
UNDER THE SUPERVISION OF DR. S.HALDAR

3. DEFINITION OF LASER

4. THE BOHR ATOM MODEL
Niels Bohr proposed, in 1913, what is now called the Bohr
model of the atom. He suggested that electrons could only
have certain classical motions:
1. Electrons in atoms orbit the nucleus.
2. The electrons can only orbit stably, without radiating, in certain
orbits (called by Bohr the "stationary orbits") at a certain discrete set
of distances from the nucleus. These orbits are associated with
definite energies and are also called energy shells or energy levels. In
these orbits, the electron's acceleration does not result in radiation
and energy loss as required by classical electromagnetics. The Bohr
model of an atom was based upon Planck's quantum theory of
radiation.
3. Electrons can only gain and lose energy by jumping from one
allowed orbit to another, absorbing or emitting electromagnetic
radiation with a frequency ν determined by the energy difference of
the levels according to the Planck relation:

5. THE EMISSION-ABSORPTION PRINCIPLE
• ABSORPTION:
• AN ATOM IN A LOWER LEVEL ABSORBS A PHOTON OF FREQUENCY HΝ AND MOVES TO AN UPPER LEVEL
• SPONTANEOUS EMISSION
• AN ATOM IN AN UPPER LEVEL CAN DECAY SPONTANEOUSLY TO THE LOWER LEVEL AND EMIT A PHOTON OF FREQUENCY HΝ IF THE TRANSITION
BETWEEN E2 AND E1 IS RADIATIVE
• THIS PHOTON HAS A RANDOM DIRECTION AND PHASE
• STIMULATED EMISSION
• AN INCIDENT PHOTON CAUSES AN UPPER LEVEL ATOM TO DECAY, EMITTING A “STIMULATED” PHOTON WHOSE PROPERTIES ARE IDENTICAL TO THOSE
OF THE INCIDENT PHOTON
• THE TERM “STIMULATED” UNDERLINES THE FACT THAT THIS KIND OF RADIATION ONLY OCCURS IF AN INCIDENT PHOTON IS PRESENT
• THE AMPLIFICATION ARISES DUE TO THE SIMILARITIES BETWEEN THE INCIDENT AND EMITTED PHOTONS

6. ABSORPTION
E1
E2

7. SPONTANEOUS EMISSION

8. PRINCIPLE OF STIMULATED EMISSION OF RADIATION
•
• RADIATIVE TRANSITION
•
•

9. STIMULATED EMISSION

10. BACKGROUND PHYSICS
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



IN OTHER WORD, A COHERENT BEAM RESULTED.

11. POPULATION INVERSION
•
• POPULATION INVERSION
•
• PUMPING
•
•
•
•
•
•

12. POPULATION INVERSION
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 a definite time and phase relation to each other,
the light has a high degree of coherence. The two photons that have been produced
can then generate more photons, and the 4 generated can generate 16 etc… etc…
which could result in a cascade of intense monochromatic radiation.

13. POPULATION INVERSION AND PUMPING
•
•
•
•
•
•If there are more atoms in the upper level (N2) than in
the lower level (N1), the system is not at equilibrium
•A situation not at equilibrium must be created by
adding energy via a process known as “pumping” light
in order to raise enough atoms to the upper level
•This is known as population inversion
•Light is amplified when the population inversion is
positive
•Pumping may be electrical, optical or chemical

14. TWO LEVEL LASER NOT POSSIBLE
Optical pumping will at most only achieve equal population of a two-level system. This is because the
probabilities for raising an electron to the upper level and inducing the decay of an electron to the lower level
(simulated emission) are exactly the same! In other words, when both levels are equally populated, the numbers
of electrons "going up" and "down" will be the same, so you cannot achieve population inversion which is
required for lasers.
The solution is to use a third metastable level. The pumping will be between the other two, but electrons in the
upper energy level will quickly decay into the metastable level, leaving the upper level practically unpopulated at
all times. The transition from the metastable level to the ground level has a different frequency: the laser
frequency. The pumping frequency is between upper level and the ground level, so the pumping is off-resonant
to the laser transition and will, thus, not trigger stimulated emission.

15. 3 LEVEL SYSTEM

16. PRINCIPLE OF LASER
•
•
hf
hc
E 


17. OPTICAL FEEDBACK
•
•

18. THRESHOLD CONDITION FOR LASER ACTION
Two conditions must be satisfied for oscillation to occur:
1. The amplifier gain must be greater than the loss in the feedback
system so that net gain is incurred in a round trip through the feedback
loop.
2. The total phase shift in a single round trip must be a multiple of 2𝝅 so
that the fedback input phase matches the phase of the original input.

19. OPTICAL RESONATOR
Only those perpendicular to the mirrors will be reflected
back to the active medium, They travel together with
incoming photons in the same direction, this is the
directionality of the laser.

21. • LASER AMPLIFIER
•
AMPLIFYING OR GAIN MEDIUM
•
•
PUMPING SYSTEM
•
•
1.
2.
3.
OPTICAL RESONATOR (OR CAVITY)
•
•
• LASER OSCILLATOR

22. DIELECTRIC MIRROR
A dielectric mirror, is a type of mirror composed of multiple thin
layers of dielectric material, typically deposited on a substrate of glass or
some other optical material. By careful choice of the type and thickness of the
dielectric layers, one can design an optical coating with specified reflectivity at
different wavelengths of light. Dielectric mirrors are also used to produce
ultra-high reflectivity mirrors: values of 99.999% or better over a narrow range
of wavelengths can be produced using special techniques. Alternatively, they
can be made to reflect a broad spectrum of light, such as the entire visible
range or the spectrum. Dielectric mirrors function based on
the interference of light reflected from the different layers of dielectric stack.

23. CHARACTERISTICS OF LASER
• The second photon has the same energy, i.e.
the same wavelength and color as the first
– laser has a pure color
• It travels in the same direction and exactly in
the same step with the first photon
– laser has temporal coherence
Comparing to the conventional light, a laser is
differentiated by three characteristics. They are:
Directionality,
pure color,
temporal coherence.

24. TYPES OF LASER
1.
2.
3.

25. RUBY LASERS
A ruby laser is a solid-state laser that uses a
synthetic ruby crystal as its gain medium. The first
working laser was a ruby laser made byTheodore H. "Ted"
Maiman at Hughes Research Laboratories on May 16, 1960.
A ruby laser most often consists of a ruby rod that must
be pumped with very high energy, usually from a flashtube, to
achieve a population inversion. The rod is often placed
between two mirrors, forming an optical cavity, which
oscillate the light produced by the ruby's fluorescence,
causing stimulated emission. Ruby is one of the few solid
state lasers that produce light in the visible range of the
spectrum, lasing at 694.3 nanometers, in a deep red color,
with a very narrow linewidth of 0.53 nm.

26. HELIUM-NEON LASER
The HeNe laser operates in a high-voltage (kV), low-current (mA)
glow discharge. Its most familiar output wavelength is 633 nm (red),
but HeNe lasers are also available with output at 543 nm (green), 594
nm (yellow), 612 nm (orange), and 1523 nm (near infrared). Helium is
the major constituent (85 percent) of the gas mixture, but it is the
neon component that is the actual lasing medium. The mechanism
producing population inversion and light amplification in a HeNe
laser plasmaoriginates with inelastic collision of energetic electrons
with ground state helium atoms in the gas mixture. As shown in the
accompanying energy level diagram, these collisions excite helium
atoms from the ground state to higher energy excited states, among
them the 23S1 and 21S0 long-lived metastable states. Because of a
fortuitous near coincidence between the energy levels of the two He
metastable states, and the 3s2 and 2s2 levels of neon, collisions
between these helium metastable atoms and ground state neon
atoms results in a selective and efficient transfer of excitation energy
from the helium to neon.

27. SEMICONDUCTOR DIODE LASERS
The means of generating optical gain in a diode laser, the
recombination of injected holes and electrons (and
consequent emission of photons) in a forward-biased
semiconductor pn junction, represents the direct conversion
of electricity to light. This is a very efficient process, and
practical diode laser devices reach a 50-percent electrical-
to-optical power conversion rate, at least an order of
magnitude larger than most other lasers. Semiconductor
lasers are lasers based on semiconductor gain media,
where optical gain is usually achieved by stimulated
emission at an inter band transition under conditions of a
high carrier density in the conduction band.
• While for other lasers, we need voltage of the order of
kV, in these lasers, a simple pencil battery can be used as
the voltage source. For example, in case of GaAs, the
band-gap energy is 1.434 eV which corresponds to
wavelength of 873 nm. Hence, pencil battery can serve
the purpose of a voltage source in this case.

28. APPLICATIONS OF LASER
In science, lasers are used in many ways, including:-
 A wide variety of interferometric techniques
 Raman spectroscopy
 Laser induced breakdown spectroscopy
 Atmospheric remote sensing
 Investigating nonlinear optics phenomena
 Holographic techniques employing lasers also contribute to a number of measurement techniques.
 Laser based lidar (LIght raDAR) technology has application in geology, seismology, remote sensing
and atmospheric physics.
 Lasers have been used aboard spacecraft such as in the Cassini-Huygens mission.
 In astronomy, lasers have been used to create artificial laser guide stars, used as reference objects for adaptive
optics telescopes.

29. HOLOGRAPHY
Holography is the science and practice of
making holograms. Typically, a hologram is a
photographic recording of a light field, rather than of
an image formed by a lens, and it is used to display a
fully three-dimensional image of the holographed
subject, which is seen without the aid of special
glasses or other intermediate optics. The hologram
itself is not an image and it is usually unintelligible
when viewed under diffuse ambient light. It is an
encoding of the light field as an interference pattern
of seemingly random variations in the opacity,
density, or surface profile of the photographic
medium. When suitably lit, the interference
pattern diffracts the light into a reproduction of the
original light field and the objects that were in it
appear to still be there

30. HOW HOLOGRAPHY WORKS
A beam of coherent light from a laser split into by a
semi-transparent mirror such that one beam can be
scattered by the object to the photographed and
other beam falls directly on the film. The fine speckled
pattern on the film conrtains informations regarding
both the amplitude and the phase. Thus a hologram is
ptroduced.
The nect step is the reconstruction step. The hologram
is illuminated with coherent light called reconstruction
wave, usually of the same wavelrngth as the original
beam. The hologram acts as a diffraction grating
producing two sets of diffracted beam. One set forms
of real image while other forms a 3D virtual image.

31. APPLICATIONS IN MEDICINE
Medical areas that employ lasers include:
 angioplasty
 cancer diagnosis
 cancer treatment
 cosmetic dermatology such as scar revision, skin resurfacing, laser hair removal, tattoo
removal.
 dermatology
 medical imaging
 microscopy
 ophthalmology (includes Lasik and laser photocoagulation)
 optical coherence tomography
 plastic surgery, in laser liposuction

32. THANK
YOU