0% found this document useful (0 votes)
71 views15 pagesApplications of Laser
M.Sc notes
Uploaded by
knowledgevive42Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF or read online on Scribd
0% found this document useful (0 votes)
71 views15 pagesApplications of Laser
M.Sc notes
Uploaded by
knowledgevive42Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF or read online on Scribd
/ 15
Application of Lasers
44. INTRODUCTION
Lasers are widely used for fundamental studies in physics, chemistry and biology. There
are numerous applications in electronics and medicine. Laser spectroscopy is advantageous
sehere exceedingly high accuracy and resolution are needed.
In this chapter we discuss most significant applications of lasers under different heads.
42. LASERS IN SCIENCE
In this section we discuss the applications of lasers in physics and chemistry involving
basic experiments.
42.1. Raman Spectrosco
If diatomic molecule is regarded as a harmonic oscillator, then the frequency of
oscillations is given by
k
+/+ coi)
2nVm
where & is force constant and m' is the reduced mass. Most vibrational frequencies are
observed in middle infra-red region for which sensitive detectors are not available and
thermal noise creates problem. The Raman effect becomes important because it allows the
transitions in the visible region to be observed. Any material that transmits visible light
implies that it scatters this light. Quantum mechanical scattering means virtual absorption
and re-emission of incident light. If the quantum state of scattering atom or molecule is same
as before after the scattering process, the frequency of scattered light will be the same as that
of the incident light. If scattering centre is left in a different state, the scattered light is
shifted in the frequency according to the equation
nahee [Raman scattering]
where f, is the scattered frequency and f; is incident frequency. AE is the energy difference
between initial and final states. Plus or minus sign depends on whether the scattering centre
is left in the lower or upper state, respectively, after scattering.
___In Raman scattering the intensity of shifted radiation is always small as compared with
incident radiation; when AK is small f, lies close to f;. Raman sources, therefore, should have
gh intensity and narrow spectral width. This condition is easily satisfied with the help of
~ Ruby laser. A ruby laser with output of 0-1 joule is found as useful as conventional red light
(769) 770 Eloments of Spectroscopy
- The CW red He-Ne laser used as Raman source offers the following advantages over
1, The Raman effect is proportional to fourth power of optical frequency used.
2. It is more convenient since it is continuous.
3. Its spectral width is smaller,
4. It gives the facility to use better Raman light scattering geometry.
Laser light is useful for detailed Raman spectroscopy since its direction, polarisation and
frequency are well defined.
Stimulated Raman scattering through Kerr cell (nitrobenzene as active material)
provides radiation with excellent directionality and has about 30% intensity of incident light.
Thus Raman effect can be used as a technique for creating new laser frequencies.
4.2.2. Laser and Ether Drift
Lasers have made possible an experiment to test the presence of ether drift with an accuracy
thousand times better than that in original Michelson Morley experiment. In experimental set
up two He-Ne lasers are mounted at right angles on an extremely rigid support. Two lasers
which oscillate at slightly different frequencies are combined with a beam splitter as shown in
Fig. (4.1) and detected by photomultiplier tube. The slight difference in frequency of laser beams
produces peak at a frequency equal to the difference between the two frequencies. The oscillation
frequency of laser depends on the length of resonant cavity and also on the speed of light in the
cavity. The rotation of apparatus must change the frequency of oscillations of the lasers and
hence the beat frequency, if there was any ether drift.
Laser
Laser ‘Beam splitter
y
@ Photomultiplier
Fig. 4.1. Experimental setup to detect ether drift.
The experiment was repeated by turning the lasers into differnt directions with respect
to the earth motion and no change in beat frequency was observed. The apparatus was
sensitive enough to detect the velocity change as small as 0-03 mm/sec.
4.2.3. Absolute Rotation of Earth
Lasers have been used to measure absolute rotation of earth. The experiment originally’
performed by Michelson and Gale has been reperformed by using lasers. Experimental setup
is shown in the Fig. (4,2). Rotation
O
Mirror
Photodetector{
Fig. 4.2. Experimental setup to detect
absolute rotation of earth : use of ring laser.
Four He-Ne lasers form the sides of a square. Mirrors are placed at the sides of the
square such that light leaving one end of a laser goes around the circuit and return to the
original point. The clockwise rotation of the square makes the path long for light and its
frequency falls whereas anticlockwise rotation increases the frequency. The difference in
frequency can be measured by measuring the beat frequency and then the rate of rotation of
Photo detector
Fig. 4.3. Experimental setup for precision length measurement.
VELOCITY MEASUREMENT
The principle involved in this measurement is Doppler shift. In this experiment, a laser
beam- CW laser is split up by a beam splitter. One component beam is reflected by a fixed
reflector and the other is scattered by the moving object. The two component beams are then
: Qt
combined to interfere. Because of Doppler shift (» = = cos 0
moving object and 6 is the angle between velocity and incident beam, in frequency of
scattered beam beating occurs. The beat frequency is the direct measurement of velocity of
the moving object.
. where vis the velocity of 715
Application of Lasors
4.9. PRODUCTION OF PLASMA
Ahigh electron density(n, = 102° m-*) plasma may be produced by focusing pulsed ruby
laser or neodymium laser on a solid target with peak laser power of the order of 100
magawatt.
Laser methods have been employed to measure electron plasma Klensity sane at
temperature by measuring either the refractive index or the spectral line width. The
formulae employed are
- ()
and o, =| oe 2)
where 1)= refractive index of plasma
gular frequency at which refractive index has been measured
ngular plasma frequency
‘lectron intensity
440. LASER INDUCED FUSION
Ne
A nuclear reaction in which two loosely bound light nuclei produce a tightly bound
nucleus is known as fusion reaction. For fusion reaction to occur the matter is to be heated to
very high temperature (10° K). For two nuclei to fuse together they should have sufficient
kinetic energy to overcome coulomb repulsion. At such a high temperature the matter is in
the plasma state. Thus the problem is two fold :
(i to heat the plasma to a temperature of the order 108 K and
Gi) to confine the plasma so produced for time long enough so that substantial fusion
takes place.
In D-T Fusion reaction
(D+T -J (3-5 MeV) + neutron (14-1 MeV)]
Hfoutput energy exceeds the input energy following criterion should be satisfied
nt 210! em sec
This is known as Lawson's criterion. In magnetic confinement of plasma, the t must be
greater than 0-1 sec. for plasma density of the order of 10" ions/c.c.
In laser induced fusion magnetic field is not required. In such a system when an intense
laser pulse interacts with solid thermonuclear matter, inertial forces are produced which can
Press, confine and heat the matter.
The typical fusion reaction consists of a vacuum chamber with Lithium blanket. At the
centre of this chamber fusion fuel is dropped at regular interval. The fusion fuel isa
deuterium — tritium mixture and in the form of solid sphere of very high density (10° m~*).
‘The surface of this solid sphere as soon as it reaches the centre of the vacuum chamber, is Real Image
AL
3 Fig. 4.6. Recording of a hologram.
Fig, 4.7. Reconstruction of the Image.
T 3 hologram explicitly gives no hint of the image recorded in it but is in a position to
wide enough information for the reconstruction of the object. The requirement is that
photographic emulsion should have high degrce of spatial resolution.
@ Reconstruction of the Image : In reconstruction process, object is removed and
pologram is placed in its original place. A laser beam also known as read out beam is allowed
fpinteract with hologram. Two images are produced as shown in the Fig. (4.7). The virtual
age is formed at the place of object and real image is formed between hologram and the
Myeerver. The real image is inverted in depth i.e, foreground and background are reversed
gnd is not of much use, There is no Tateral inversion in real image. This image is called
‘udoscopic. This image can be photographed directly without the use of lens. Virtual image
ifthe object serves all puspose.
Gas lasers operating in CW mode are often used in holography because of their high
echerence. Moving objects are holographed by using solid state lasers.
Theory : At a point O (not shown) on the plate electric field is due to reference beam and
due to scattered beam from the object. Thus
E=E, +E, .(@)
“here B, is field due to reference beam and Fy is field due to scattered beam. The scattered
‘field is varied appreciably in amplitude and in phase with position whereas E, belongs to &
‘plane wave. Thus
, = £0 exp {i(hrp - 00} a0)
To
B, = Eo, exp {i (P29 - 09} .@)
Ep, and Eo, are the amplitudes of two fields, respectively. rp is the distance between the
ject and the point and Zo is distance between object and plate.
“Intensity at a point P is given by
1=|E, +E?
Bibs 4 2 + Basler exp ikt(ro aq) + Eos Boe oxpib(ca-m)
oe or TN rT
% ‘0
"Eqn. (4) can be conveniently written a
BA cos {e(r = 20) +9) 8
a 4 8 cos wo
I=Ej,+ nee n
————
a Elomonts of Spoctrosco,
778 m ad
where C and $ are suitable constants, : /
Tho presence of cosine term indicates that pattern contains series of maxima and
minima, Interference of plane wave with spherical wave produces a set of
T be tho transmission coofficiont, then
cular fringes, [7
i} al + (6)
zal = ()
where a is some constant,
When hologram is illuminated by a road out wave then the field of transmitted wave
is
given by
-*(1-$1)m, exp {i (k2g - wt)} «- (8)
Substituting the value of J, we have
a Eq, By,
OF exp {i (hry — ot) ~ ; or
On, exp {i 2kzq ~ hry - wt) ++ (10)
0
The first term of eqn.
represents the spherical wave identical to that emanating from the object. This wave surface
sponds to virtual image and third term belongs to real image.
1. The virtual image of hologram is a three dimensional picture of the object.
2. When a portion of a photograph is destroyed, it implies the complete loss of information
related to that portion. In case of a hologram information regarding the point object is
recorded over entire aren of hologram, Each point on the hologram receives light from
all parts of the object and contains information rogarding the entire image,
3. A hologram may contain a number of independent scenes which are recorded by
changing the angle of reference beam which can be done by rotating the plate.
Reconstruction of a particular scene requires a properly oriented read out wave. It has
been estimated that a 100 squaro contimeter contain at least one volume of
Encyclopeadia Britannica,
- Hologram does not possess its negative. It is tho negative itself. The image it produces
is positive, >
sion of 05075 7719
eR COMMUNICATION
sla ining promi
nication because of the following advantages :
Large Information carrying capacity of light waves : ‘The frequency of #1
, al Jies anywhere between 20 ‘and 4000 Hz, Hence in a modulated wave in a band of width
sign! proximately 1000 Hz. is reserved for one speech signal, If frequency of carrier wave
between 100,000 Hz. to 500,000, then about (500,000-100,000)/4006 100 ind ent
ries « ; 6
4 tan be transmitted, If carrier band width is (10! -5 10") Hz
over ot
= yx 10" /4 x 10° = 10! speech signals can be simultancously transmitted. This example
well illustrates enormous information carrying capacity of laser beams. In case of music
e band width is 20 Hz and in case of television the bandwidth is 6 MHz. Comparatively
aller number of television channels can be carried by same carrier frequency band. The
method of sending simultaneously more than one signal along the same channel by assigning
different frequency bands for each channel is known as frequency division multiplexing. It
jmplies that these signals do not overlap in frequency domain but overlap in time domain.
9, Faster rate of Information Transmission ; ‘The rate of information depends upon
the rate on which modulations are impressed on the wave, and these modulations (and
therefore the rate of information transmission) must always be slow compared with the
arrier frequency of the wave. When light waves behave as carrier waves, obviously the rate
if information transmission becomes much larger. This great potential of light waves for
fommunication purposes has become possible only after the development of lasers.
3, Noise-free Information Transmission : A. Light wave communication takes place
through optical fibres and it has the following additional advantages over the conventional
communication.
{@ As there is very low transmission loss (0-2 dbkm at wavelength of 1-55 pm), hence
repeater stations are at a greater distance, this saves substantial cost.
(ii) Optical fibres are basically made of silica or glass and are typically about 100 pm in
diameter. This results in reduction in weight and volume space required.
(ii) Optical fibres are immune to electromagnetic interference and there is no cross talk.
(iv) Optical fibres can be used in explosive as well as high voltage environment due to the
absence of any hazard on account of short circuit etc.
(v) Optical fibres are less susceptible to corrosion and control system failures.
(vi) A very sensible place to errors in modern computers is that their communication lines,
carrying input and output signals, connecting the processors and the memory units
and other hard- ware facilities. These communication systems are complicated strands
of many wires, called busses which are sensitive to noise currents and may be the place
where interference enters to spoil the signal. It is impossible to isolate input from
5 output systems and make the wire immune to variety of pickups.
Optical communication lines are free from this disadvantage. Hence in computer system
_Sbtical fibre lines are used. For better cfficiency, computers are now connected to computer
“‘Retwork. To effect data flow in such networking it is essential that communication between.
omputers be rapid, directional and multichannel. This requirement is satisfied in laser
“fommunication. aa Eloments of Spectroscopy
B. Open space laser communication can be established between satellites-satellites and
nireraft and satellites and station on the ground, Though vagaries of terrestrial atmosphe,,
(rain, fog) ete. hamper the communication, laser communication is preferred because of
higher directivity, higher power concentration of laser beams. In addition these are immune
to distortion by interference, For communication in space and long distance transmission oy,
earth (within tens of kilometors) CO, and Nd-YAG lasers ae used.
4. Communication at the
advent of
time of re-entry of space has become convenient with the
ers. When a spacecraft re-enters the atmosphere enormous amount of heat j
Benerated that forms high temperature plasma into the atmosphere. Plasma acts as 4
conductor for radio waves (y= 10° Hz) as a consequence it gives rise to radio black out
Plasma is transparent for optical frequencies (10! Hz), therefore laser beam can be used to
communicate from space craft to the ground station.
4.13. COMPONENTS OF LIGHT WAVE COMMUNICATION SYSTEM
Optical fiber ; For terrestrial communication o;
Sonasts of central transparent region called the core. The core ie surrounded by a region of
small refractive index called the cladding. If core is of homogeneous refractive index (Step
index type) (Fig. 4.8a). The guidance of light occurs through the phenomenon of total internal
reflection at the core-cladding interface. When refract index of core varies gradually it js
Known as graded index type and guidance of light takes place due to continuous refraction of
Tight rays towards the centre as shown in Fig. (4.8b).
tical fiber is most convenient channel, It
nz(