Engineering Physics
PHY-109
LASER-5
ELECTROMAGNETIC THEORY QUANTUM MECHANICS
LASER WAVES
Dr. Jyoti Rajput
Department of Physics
School of Mechanical Engineering
SOLID STATE PHYSICS
Lovely Professional University
Phagwara, Punjab-144411
FIBER OPTICS
Syllabus
Fundamentals of laser- energy levels in atoms
Radiation matter interaction, Absorption of light
Spontaneous emission of light, stimulated emission of light
Population of energy levels, Einstein A and B coefficients
Metastable state, population inversion, lasing action
Properties of laser, resonant cavity, excitation mechanisms
Nd - YAG, He-Ne Laser, Semiconductor Laser
Applications of laser in engineering, holography.
Advantages and disadvantages of lasers
Ruby Laser Nd-YAG Laser He-Ne Laser Semiconductor Laser
Advantages
1 Easy to construct and Easy to construct and Easy to construct and Easy in operation
operate, hence called operate, operate,
practical laser
2 A Very strong and intense By using Q-swithing, a laser Continuous beam Long life, highly
beam of power 10000 Watt beam pulse frequency and monochromatic, tunable
or 10kWatts is produced. shape can be tailored. May and continuous beam
produce a high energy
beam up to 1 Joule.
3 May produce a laser beam of A short pulse period down Exceptionally Excellent efficiency with
1mm to 25mm in diameter to 10ns may be obtained. monochromatic beam very high operation
with high operation duration (20000Hrs.)
duration (10000hrs)
Disadvantages
1 Its laser beam is only pulse Slower production for It has very low power It has very low power
like and its operation duration thicker materials, Lower about 0.5-5mWatt. about 200mWatt.
is very less (few Hrs.) absorption of radiation of
lighter materials.
Applications of Lasers
Simple photography
• Two dimensional image of the object is formed
• First Negative is obtained by developing the film
• Then Positive is obtained through the printing
• This positive print is a two dimensional record of light intensity received
from the object
• It contains information about the SQUARE OF THE AMPLITUDE of the
light wave that produced the image.
• But the information about the phase of the wave is not recorded and is
LOST.
Holography
It is a technique of photography where the whole information of an
object including its intensity and phase are recorded on a
photographic film. It was invented by Danis Gabor in 1948. He
obtained Nobel prize for the invention in 1971.
Holography= Holos (whole) + Grapho (to write)
It is based on the principle that when object wave or the reflected
wave from any object and the reference wave interfere, then it
records the complete detail of the object in 3 dimension.
These are of many types
1. Transmission hologram
2. Reflection hologram
3. White light hologram
4. Embossed hologram In 1948, Dennis Gabor gave this new
5. Volume hologram technique of photographing the objects
and is known as WAVE FRONT
RECONSTRUCTION
Holography
• Holography is a “LENSLESS PHOTOGRAPHY” in which image is captured as
an interference pattern at the film without a lens.
• A method of obtaining three-dimensional photographic images.
• The records are called holograms.
• It record both “INTENSITY as well as PHASE”
• It is a TWO STEP process
• First step is the Construction of hologram: where the object is
transformed into a photographic record
• Second Step is the Reconstruction: in which the hologram is transformed
into the image
Holography: Working: Recording
Recording process
In order to make hologram two coherent light waves are required.
The first one is called the object wave and the other is called the reference
wave.
The object wave is reflected from the object and carries all the information
about the object.
The reference wave is a plane wave without any information.
These two waves when allowed to fall on a photographic plate interferes
with each other and the interference pattern is recorded in the
photographic film.
For obtaining the stable interference patterns absolutely stable conditions
are required during the exposure of the film.
Each part of the hologram receives
light from every part of the object
and hence even if the hologram is
cut into pieces, each part is capable
of reconstructing the whole image.
Diffraction Zone Plate
A zone plate is a device used to focus light or other
things exhibiting wave character. Unlike lenses or
curved mirrors however, zone plates use diffraction
instead of refraction or reflection. They are sometimes
called Fresnel zone plates. A zone plate consists of a set
of radially symmetric rings, known as Fresnel zones,
which alternate between opaque and transparent.
Light hitting the zone plate will diffract around the
opaque zones. The zones can be spaced so that the
diffracted light constructively interferes at the desired
focus, creating an image there.
Diffraction Zone Plate
A grating mostly has more than one diffraction
order. In a Fresnel zone plate this leads to more
than one focus point. The negative diffraction
orders even result in diverging beams, in other
words: zone plates also behave like a dispersive
lens with virtual focus points. When a zone plate
is used as focusing lens, normally only the first
diffraction order is used and all other orders have
to be blocked by a suitable aperture.
http://www.x-ray-optics.de/index.php/en/types-of-optics/diffracting-optics/fresnel-zone-plates#Fresnel_zone_plates
Holography: Working: Reconstruction
Reconstruction process
The reconstruction is based on the principle of
diffraction. The hologram is illumined by
collimated laser beam.
One set of diffracted beams emerging from the
hologram, appears to diverge from an apparent
object when projected backward. This
constructs the virtual image which is observed
by looking through the hologram.
The other beam produces a real image which
can be recorded on a photographic plate.
By developing the hologram and placing it in its
original position in the reference beam as during
recording, we can reconstruct the image. Both
real and virtual image are formed in the
reconstruction process.
As we look along the reconstructed object wave,
we see a replica of the object.
Holography: Requirements
1. There should not be any path difference between the object wave and the reference wave more than the
coherence length.
2. Spatial coherence is important so that the reference wave and the scattered wave from different regions
can interfere properly.
3. All recording instruments must be in a motionless condition.
