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OPTICAL INSTRUMENTATION
EIC-021
UNIT-4
BY
ANUJ BHARDWAJ
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OPTICAL INSTRUMENTATION
unit-4
Holography
Principle of Holography, On-axis and Off axis Holography ,Application of Holography , Optical data storage
Optical fiber sensors
Active and passive optical fiber sensor, Intensity modulated
displacement type sensors, Multimode active optical fiber sensor
(Microbend sensor ) Single Mode fiber sensor-Phase Modulates
and polarization sensors
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Unit-4 (HOLOGRAPHY)
In conventional photography a negative is made first
and using it a positive print is produced .Positive
print is only a two dimensional record of light
intensity received from a three diamensional record
of high intensity received from a three diamensional
object.It contains information about the square of
the amplitude of the light that produced the image
and the information about the phase of light is
absent.
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Holography
What is not holography
Holodeck from Startrek
What is holography Photography on steroids
Both amplitude and phase is
recorded
Different intensity in different
directions
LA
SER
Photo vs. Holo
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5
History of Holography
Invented in 1948 by Dennis Gabor for use in
electron microscopy, before the invention of
the laser
Leith and Upatnieks (1962) applied laser lightto holography and introduced an important
off-axis technique
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Word Origin
Hologram is from the Greek word holos,
meaning whole and gramma meaning
message.
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How Holograms are Made
Need a laser, lenses, mirror, photographic film,
and an object
The laser light is separated into two beams,
reference beam and object beam
Reference beam enlarged and aimed at a
piece of holographic film
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Making Holograms
Object beam directed at subject to be
recorded and expanded to illuminate subject
Object beam reflects off of object and meets
reference beam at film
Produces interference pattern which is
recorded
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Making Holograms Cont.
Film is developed
Hologram illuminated at same angle as
reference beam during original exposure to
reveal holographic image
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Applications of Holography
Design of containers to
hold nuclear materials
Credit cards carry
monetary value
Supermarket scanners
Optical Computers
Improve design ofaircraft wings andturbine blades
Used in aircraft heads-
up display Art
Archival Recording offragile museum artifacts
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Point object hologram construction:
Intensity distribution on plate
Reference wave
Object wave
Intensity distribution on plate
ROORRROOROyxI
zyxrwhere
oeezyxozyxO
reezyxrzyxR
ikrzyxi
ikzzyxi
****2
222
),,(
),,(
),(
),,(),,(
),,(),,(
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Hologram construction
)cos(2),(
0
)cos(2),,(
22
22
krororyxI
planefilmz
ororzyxI
Maxima for kr=2m or r=m
i.e. if the OPL difference OZ OP is an integral number of wavelengths, thereference beam arrives at P in step with the scattered (i.e. object) beam.
Gabor zone plate
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Hologram
When developed the photographic plate will have a
transmittance which depends on the intensity
distribution in the recorded plate
tb backgrond transmittance due to |R|2 term
B parameter which is a function of the recording andeveloping process
)(**2
ORROOBtt b
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Hologram reconstruction
When illuminated by a coherent wave, A(x,y), known as the
reconstruction wave, the optical field emerging from the
transparency is,
i.e. a superposition of 4 waves
If A(x,y)=R(x,y), i.e. reconstruction and reference waves are
identical,
ORBOBRRBOOttyxR
ABORRABOABOOAttyxA
bp
bp
2*2*
***
)(),(
),(
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Hologram reconstruction
Three terms in the reconstructed wave
ORBOBRRBOOttyxR bp2*2*
)(),(
Direct wave
identical to reference
wave except for an
overall change in
amplitude
Object wave
identical to object
wave except for a
change in intensity
Conjugate wave
complex
conjugate ofobject wave
displaced by a
phase angle 2
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Transmission hologram: reference and object waves traverse
the film from the same side
Reflection hologram: reference and object waves traverse the
emulsion from opposite sides
Hologram Reflection vs. Transmission
View in Transmission View in reflection
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Hologram: Some Applications
Microscopy M = r/s Increase magnification by viewing hologram with longer wavelength
Produce hologram with x-ray laser, when viewed with visible light M ~106
3-d images of microscopic objects DNA, viruses
Interferometry Small changes in OPL can be measured by viewing the direct image ofthe object and the holographic image (interference pattern producefinges l)
E.g. stress points, wings of fruit fly in motion, compression wavesaround a speeding bullet, convection currents around a hot filament
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Holography A Tase Of
Principle Fundamental technology Diffraction grating bends light
Can be superposed
Effect (bending) persists superposition
Hologram super complex diffraction
grating
Effect of diffraction grating on a direction of light
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Holography principle
Recording
Encoding phase and amplitude as interference fringe pattern
Two beams interfering
Reference beam known properties
Scene beam recorded light field
Complex diffraction grating is created hologram
Reconstructing
Hologram illuminated with reference beam
Diffraction occurs
Resulting light field contains original scene beam
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Holography Principles in
Pictures Recording
Reconstruction
Photographic
plate
Object
MirrorLaserbeam
Hologram
Image
Mirror
Laser
beam
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OFF AXIS HOLOGRAPHY
Laser beam splitted
into two beam:
Reference beam Object beam
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Off-axis Hologram
Recording
Non-zero angle between reference wave
and object wave
3D opaque objects
Higher spatial frequency
Reconstruction Orders diffracted into different directions
Clean original optical field
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IN LINE HOLOGRAM
Here the light
diffracted by the
figure interferes with
the undiffracted light
.The recordof this
interference pattern
forms the hologram.
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In-line Hologram
Recording
Reference, object, hologram aligned in line
Mostly transparent and planar objects
Lower spatial frequency
Reconstruction
Images disturbed by blurred counterparts
and zero order Special setup: blurred image became
background
Hologram
Object
Reference
planar wave
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In-line Hologram
The radiations associated
with the virtualand real
images,propagate in the
same diraction.Hence when
the real image is viewed the
virtual image is
superimposed but it is well
out of focus.
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Lens & Fourier Hologram
Lens
Different optical material: slowdown/diffraction of waves
Use of thin lens: assumption on lack of diffraction
Back focal plane = {front focal plane}
Fourier Hologram
Recording through lens
{planar image} + {point source}
Reconstruction through lens Both virtual & real image in focus
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Other holograms
Holographic Stereograms Recording of multiple views through slit
Reconstruction: only single focus depth
Rainbow Hologram
2 Stages of recording Record regular hologram
Record rainbow hologram through slit
Visible on white light: multiple color images
Color Hologram
Common hologram: rainbow due to diffraction
3 holograms + 3 wavelengths: larger gamut
Achromatic holograms: holographic stereograms
Overlapping/coplanar colors
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Physical Representations
Thin Amplitude Hologram Zero and first order only
First order: 6 % of energy
Thin Phase Hologram Multiple orders
First order: 33 % of energy
Volume Hologram Multiple layers of fringes
Reflective transmission Sensitive only to selected wavelength
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Applications of holography
Three diamensional photography
a) image photography b) pulse laser photography of
moving object
Image recognition
Volume hologram
Interferometery
Computer generated holograms
Lensless optics
Three diamensional microscopy
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Optical Fiber Sensor
Optical fiber sensor: A sensor that measures a physical
quantity based on its modulation on the intensity,
spectrum, phase, or polarization of light traveling
through an optical fiber.
Compact size
Multi-functional
Remote accessible
Multiplexing
Resistant to harsh environment
Immunity to electro-magnetic interference
Advantages of optical fiber sensors
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Optical Fiber Sensor Types
Intrinsic: the effect of the measurand on the
light being transmitted take place in the fiber
Extrinsic: the fiber carries the light from thesource and to the detector, but the modulation
occurs outside the fiber
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Optical Fiber Sensor Types
Point sensor: detectmeasurand variation
only in the vicinity of
the sensor
Multiplexed sensor:
Multiple localized sensorsare placed at intervals along
the fiber length.
Distributed sensor:
Sensing is distributed
along the length of the
fiber
Opto-
electronics
Output, M(t, Zi)
Opto-
electronics
Output, M(t,z)
Opto-electronics
Sensing
element
Output, M(t)
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Optical Fiber Sensor Types
Intensity-based: measure physic measurandbased on the intensity of the light detected
through the fiber, e.g. fiber break, OTDR
Interferometric (phase modulation):
Fabry-Perot Interferometry
Grating based (wavelength modulation)
Fiber Bragg Grating (FBG)
Long Period Fiber Grating (LPFG)
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Intensity-based Optical Fiber Sensor
Advantages:
Simple signal processing
Inexpensive measurement instrument
Disadvantages:
Susceptible to power fluctuation of the light source
Susceptible to fiber bending losses
Variation in modal power distribution in Multi-mode fiber
(MMF)
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Intensity-based Optical Fiber Sensor
Reference: Split-spectrum intensity-based opticalfiber sensors for measurement of
microdisplacement, strain, and pressure, by Anbo
Wang et al.
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Optical Fiber Components
Fiber connector
Broadband light source (BBS)
Fiber coupler/circulator
Mode scrambler
Index matching fluid
Wavelength division multiplexer
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Fiber Connector
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Fiber Connector Type
FC/PC: polished curved
FC/UPC: ultra-PC
FC/APC: angle PC
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Broadband Light Source
Definition: a light source that emit lights
over a large wavelength range
Examples:
ASE source EELED
SLED
LED spectrum ASE spectrum
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Fiber Coupler
Definition: an optical device that combines or splits power from optical fibers
1X2 coupler
(95/5, 90/10, 80/20, 50/50)
2X2 coupler
1X2 coupler
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Circulator
Definition: a passive three-port device that couple light
from Port 1 to 2 and Port 2 to 3 and have high isolation
in other directions.
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Mode scrambler
Mode Scrambler: an optical device that mixes optical power in fiber toachieve equal power distribution in all modes.
Mode stripper: an optical device that removes light in the cladding of an
optical fiber.
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Other Mode Scrambler
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Index matching fluid
Definition: A liquid with refractive index similar to glass
that is used to match the materials at the ends of two
fibers to reduce loss and back reflection.
Applications:
Reduce back reflection
increase coupling between two fibers
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Wavelength division multiplexer
Definition: a device that combines and split
lights with different wavelengths
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Intensity-based Distance Sensor
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OPTICAL DATA STORAGE
The laser beam
passes through
splitter to form
two beams
Reference beam
Object beam
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Optical sensor system
Optical sensor system:Opticalfiber sensors may be implemented as
intrinsic or extrinsic devices. The
former type is arranged such that the
physical parameter to be sensed acts
on the fiber itself to cause a change
in the transmissioncharacteristics.The latter type uses
the fiber as a light guide to and from
the sensor, which is configured to
allow the measured to a change the
coupling characteristics between the
feed and return fiber .
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Phase and polarisation fiber sensor
These devices cause
interference of coherent
monochromatic light
propagating in a strained or
temperature varying fiber
with light directly from the
laser source guided by areference fiber isolated
from the external
influence.
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Active optical sensor
In this device a multimode optical fiber is
inserted across a pipe such that the liquid flows
past the transversely stretched fiber.The
turbulence resulting from the fibers presence
causes it to oscillate at a frequency roughly
proportional to the flow rate.This results in a
corresponding oscillation in the mode powerdistribution within the fiber giving a similarly
modulated intensity profile at the optical
receiver.
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MICROBEND SENSOR
A popular technique for therealization of an intrinsic multimodefiber sensor involves microbending ofthe fiber in the modulationregion.Deformation of the fiber on a
small scale causes light to be coupledfrom the guided optical modespropagating in the fiber core into thecladding region where they are lostthrough radiation into the
surrounding region.
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The test data corresponding to this
configuration is given in this figure.It
must be noted ,however that a
requirement of this sensor type is the
removal of the cladding modes both
immediately prior to , and after the
modulation zone.
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Passive fiber sensor
This shows the operation of a simple
optical fluid switch . When the fluid , which
has a refractive index greater than the glass
forming optical dipstick,reaches the
chamfered end ,total internal reflection
ceases and the light is transmitted into the
fluid.Hence an indication of the fluid levelis obtained at the optical detector.
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Intensity modulated displacement type sensor
In this light reflected from the target
is collected by a return fiber and is a
function of the distance between
the fiber ends and the target.Hence
the position or displacement of the
target may be registered at the
optical detector.