optical instrumentation u4

7/30/2019 Optical Instrumentation u4

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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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12

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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13

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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14

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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15

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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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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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.

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