LASER CHARACTERISTICS MONOCROMATICITY DIRECTIONALITY.pdf

INCLUES DIFFERENT CHERECTERISTICS OF LASER LIGHT
LIKE DIRECTIONALITY MONOCROMATICITY
COHERANCE ETC

1. LASER
CHARACTERISTICS
ARYANANDA P K
99AXIPH008

2. ■Directionality
■High intensity
■High degree of coherence
■Extraordinary monochromaticity

3. DIRECTIONALITY
■In the case of laser The active material is in a cylindrical resonant
cavity
■The beam is highly focused and travels in one direction with minimal
divergence
■The directionality of a laser beam expressed in terms of Beam
divergence

4. DIVERGENCE
■The distance from the laser over which the light rays remains parallel is known as
RAYLEIGH RANGE.

5. ■There are two parameters which cause beam divergence
i . Size of the beam waist
ii . Diffraction
■The divergence angle is measured from the centre of the beam to the edge of
the beam
■Edge is defined as the location in the beam where the intensity decreases to
1/e of that at the centre
■Twice the angle of divergence is known as the full angle beam divergence.
This angle tells how much the beam will spread as it travels through space.

6. z
w
2??????
0
??????
Relation between beam waist and
divergence of laser beam
2??????=
4??????
????????????0
Where ,
•??????
0=2??????
0 is the diameter of
the beam waist
•Divergence is inversely
proportional to ??????
0
•Divergence is large for a
beam of small waist

7. •The beam divergence due to diffraction is determined
from Reyleigh criterion
??????=1.22
??????
??????
Where D is the diameter of laser aperture
• in case of gas lasers , the diffraction divergence is about
twice as large as the beam waist divergence.
•Typical value of divergence for a He-Ne laser is 10
−3

8. INTENSITY
■ The intensity of laser beam approximately given by
??????=
10
??????
2
??????
p is the power radiated by the laser.
■In case of He-Ne laser ??????=6328 X 10
−10
m
??????=
100�10
−3
�
6328 � 10
−102
??????
2
= 2.5X10^11 w/m^2

9. COHERENCE
■Light waves are said to be coherent if they are in phase with each other
■The light from a laser is a resultant of a large number of identical which are in phase
and therefore exhibits a high degree of coherence.

10. The light that emerges from a conventional light source is a
jumble of short waves which combine with each other in a
random manner .The resultant light is incoherent and the
wavefront varies from point to point and changes from instant to
instant. On the other hand , the light from a laser is a resultant
of a large number of identical photons which are in phase and is
therefore exhibit a high degree of coherence.

11. ➢Accordingly there are two classes of coherence
1. temporal coherence/longitudinal coherence
Temporal coherence refers to the constancy and predictability of phase as a
function of time when the waves travel along the same path at slightly
different times.
2. spatial coherence/transverse coherence
Spatial coherence refers to the phase relationship between
waves travelling side by side at the same time but at some distance from one
another.

12. TEMPORAL COHERENCE
■ Let us consider a single wave propagating along x-direction. Let us note the
electric field at one point in space at two different times ??????
1 and ??????
2. phase difference
between the field ??????
1 at ??????
1, and the field E₂ at ??????
2 be ??????
1.
■electric fields at later times ??????
3 and ??????
4, and, where (??????
4−??????
3) = (??????
2−??????
1). Let the
phase difference be now ??????
2.

13. •If ??????
�=??????
�, and it is true for any time interval of same
duration, then the wave is said to be temporally
coherent. If the phase difference and changes from
interval to interval and in an irregular fashion, then the
wave is said to the incoherent.
•Temporal coherence is characterized by two
parameters ,coherence length l
coh and coherence time
t
coh
•Both the coherence length and coherence time
measure how long light wave remains in phase as they
travel in space.

14. •The coherence length depends on the central wave length λ And the band width ∆λ Of the
wave length is given by
.

15. SPATIAL COHERENCE
■Let us consider two identical waves travelling along the same direction but are at a
distance from each other.

16. •If the phase difference between the two
electric vectors remains zero for all
times, then the two waves are said to
have spatial coherence
•It implies that spatial coherence
measure the area over which light is
coherent

17. MONOCHROMATICITY
■If light coming from a source has only one frequency of oscillation , the light is
said to be monochromatic and the source a monochromatic source.
■Light coming out of any source consists of a band of frequencies closely spaced
around a central frequency ??????
0 . The band of frequencies ∆??????
0 is called the line
width or band width.
■The light from conventional sources has large linewidths of the order of ��
��
Hz
or more. On the other hand, light from lasers is more monochromatic, having
line-widths of the order of 100 Hz.

18. POLARIZATION
■The conventional light source produces unpolarized light . External devices such as
polarizers and crystal plates are used to convert unpolarized light into light of
desired polarization .
■In case of lasers most of them also emit unpolarized light.
■Laser output can be made linearly polarized light by adding suitable device.
Normally, Brewsters window is used to obtain linearly polarized light from a laser

19. SPECKLES
■If a surface , such as of a wall
or of a metal , is illuminated by
a laser beam it appears to be
granular . The granules are
randomly distributed over the
surface and appear to be
shifting. Such grainy patterns
are called laser speckles.
■The shifting speckles is an
interference pattern created by
light differences in the paths
that light travel.

20. THANK YOU