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University Question Key- April 2010

Part-A

1. Characteristics of optical sources

It must emit the required wavelengths

It should be reliable,durable and inexpensive

It should have compact size and high efficiency

It must require very small time for its operation

2. Types of optical splicers

Fusion splicing- by flame or by an electric arc

Mechanical splicing

3. The potential applications of fiber optic sensors

Measurement of physical properties such as strain, displacement, temperature, pressure,

velocity, and acceleration in structures of any shape or size.

Monitoring the physical health of structures in real time.

4. Principle of Electro optic modulator

Many elements like potassium tantalite niobate , nitrobenzene,nitrotoaluene

exhibit electro optic effect in presence of applied electric field

Kerr effect- change in the refractive index of the medium which is proportion to

the square of the applied electric field or voltage

5. a. population inversion

for light amplification, the rate of emission of photons should exceed the rate of

absorption of photons

population inversion is the condition for light amplification by stimulated

emission

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b. optical pumping

In a laser we strive to create a "population inversion" where most or all of the particles

are in the excited state.

This is achieved by adding energy to the laser medium (usually from an electrical

discharge or an optical source such as another laser or a flash lamp); this process is called

pumping

6. SLM and MLM lasers

7. features of laser trimming

is the controlled alteration of the attributes of a capacitor or a resistor by a laser

action

8. Basic principle of velocity measurement

Based on the Doppler effecr, the velocity of the particle in the fluid flow can be

measured from the Doppler frequency shift produced in the laser beam

9. components of holography

Holographic optical elements (HOE) can perform the functions of mirrors, lenses,

gratings, or combinations of them, and they are used in myriad technical devices

10. Requirements for laser instruments for medical applications

The highly collimated beam of a lasercan be further focused to a microscopic dot of extremely

high energy density.

This makes it useful as a cutting and cauterizing instrument.

Lasers are used for photocoagulation of the retina to halt retinal hemorrhaging and for the tacking

of retinal tears.

Higher power lasers are used after cataract surgery if the supportive membrane surrounding the

implanted lens becomes milky.

http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/qualig.html#c5http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/qualig.html#c5http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/qualig.html#c5http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/qualig.html#c5

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PART- B

11.a.i) Different types of optical fibers

Types of optical fiber based on different classification -> 2 marks

Each fiber with neat sketch -> 3 marks

Applications ->2 marks

11.a.ii) Skew ray scattering loss

Define scattering loss ->2 marks

Types of scattering loss -> 1 mark

Define skew Ray scattering loss -> 2 marks

Explanation -> 2 marks

Define dispersion -> 2 marks

11.b.i) Compare optical sources with optical detectors

Optical source Optical detector

1. to convert electrical signal to light

signal

1. to convert light signal to electrical signal

2. Source- LED, LASER diode 2. Detector - Avalanche photo diode, p-i-n

photo diode

3. It must emit required wavelengths 3. it have high quantum efficiency

4. requires low current 4. requires low dark current

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11.b.ii) LED

LED ->2 marks

Symbol -> 1 mark

Principle -> 2 marks

Operation and charcteristics -> 5 marks

12. a.i) Acousto optic modulator

Define acousto optic modulator ->2 marks

Neat sketch -> 2 marks

Special feature and working ->4 marks

12. a.ii) Interferometric method for the measurement of length -> 4 marks

Define interferometric method

Neat sketch

Working

Fiber sensor for measurement of pressure and temperature -> 4 marks

Neat sketch

Working

Characteristics graph

12. B.Fiber optic sensors

- for the measurement of liquid level, length and strain

Neat diagram -> 6 marks

Working for each quantity ->10 marks

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13.a.i) Fabry perot cavity

Fiber-based Fabry-Perot interferometer (FFPI) is a typical multiple-beam interferometer. It consists of a single-

mode fiber with cleaved end faces and a sensing element, surface.

o The space separating of the reflecting surface is called the cavity length.

o The reflected light in the FFPI is wavelengthmodulated in exact accordance with the cavity length. It can

be used in various sensitive applications such as measuring velocity, displacement, strain, temperature

and stiffness measurements.

Fabry-Perot interferometer

For dynamic displacement measurements. A vibrating target was excited by a mechanical vibrator via a function

generator in various excitation frequencies.

An interference signal was generated from the modulation between the two sets of reference and sensing

signals at the output arm of the fiber interferometer detected by a detector.

A fringe-counting technique was used for counting the number of interference fringes. This number was thenconverted to displacement information.

One fringe period is equivalent to a displacement of a half of the wavelength (/2).

A commercial displacement sensor was employed as a reference displacement sensor for comparison with the

measured information from the fiber interferometer.

PRINCIPLE OF OPERATION

In general, the classical configuration of the fiber-based Fabry-Perot interferometer for dynamic

displacement measurement is illustrated in Fig.

The optical FFPI can be approximated as a two-beam interferometer due to the inherent property of the fiber

Interferometer.

When the laser diode light arrives at the fiber end-face, a portion is reflected off the fiber/air interface (R1) and

the remaining light propagates through the air gap (L) with a second reflection occurring at the air/fiber interface

(R2).

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In an interferometric sense, R1 is the reference reflection called the reference signal (I1) and R2 is the sensing

reflection or sensing signal (I2).

These reflective signals interfere constructively or destructively based on the optical path length difference

between the reference and sensing signals which is called the interference signal.

Therefore, small movements of the vibrating target cause a change in the gap length, which

changes the phase difference between the sensing and reference signals producing fringes.

13. a ii) Gas laser

Heliumneon laser

A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. HeNe lasers have many industrial and

scientific uses, and are often used inlaboratorydemonstrations ofoptics. The best known form operates at a

wavelengthof 632.8 nm, in theredportion of the visible spectrum.[1]

The gain medium of the laser, as suggested by its name, is a mixture ofhelium and neon gases, in a 5:1 to 20:1 ratio,

contained at low pressure He at 1torr and Ne at 0.1torr (an average 50 Pa per cm of cavity length) in a glass

envelope. The energy or pump source of the laser is provided by an high voltage electrical discharge through an

anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical forCW operation.[The optical

cavity of the laser typically consists of a plane, high-reflecting mirrorat one end of the laser tube, and a concave

output couplermirror of approximately 1% transmission at the other end.

HeNe lasers are normally small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging

from 1 mW to 100 mW. The precise operating wavelength lies within about 0.002 nm of this value, and fluctuates

within this range due to thermal expansion of the cavity. Frequency stabilized versions enable the wavelength to be

maintained within about 2 parts in 1012[4]for months and years of continuous operation.

The laser process in a HeNe laser starts with collision ofelectrons from the electrical discharge with the helium atoms

in the gas. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states.

Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon

http://en.wikipedia.org/wiki/Gas_laserhttp://en.wikipedia.org/wiki/Gas_laserhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Laboratoryhttp://en.wikipedia.org/wiki/Laboratoryhttp://en.wikipedia.org/wiki/Laboratoryhttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-0http://en.wikipedia.org/wiki/Laser_constructionhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Continuous_wavehttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Optical_cavityhttp://en.wikipedia.org/wiki/Optical_cavityhttp://en.wikipedia.org/wiki/Mirrorhttp://en.wikipedia.org/wiki/Output_couplerhttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Frequency_drifthttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Niebauer-3http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Niebauer-3http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Niebauer-3http://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Ground_statehttp://en.wikipedia.org/wiki/Metastablehttp://en.wikipedia.org/wiki/Gas_laserhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Laboratoryhttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-0http://en.wikipedia.org/wiki/Laser_constructionhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Neonhttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Continuous_wavehttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Optical_cavityhttp://en.wikipedia.org/wiki/Optical_cavityhttp://en.wikipedia.org/wiki/Mirrorhttp://en.wikipedia.org/wiki/Output_couplerhttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Frequency_drifthttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Niebauer-3http://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Ground_statehttp://en.wikipedia.org/wiki/Metastable

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atoms, exciting neon electrons into the 3s2 level[3]. This is due to a coincidence ofenergy levels between the helium

and neon atoms. This process is given by the reaction equation:

He(21S)* + Ne + E He(11S) + Ne3s2*

where (*) represents an excited state, and E is the small energy difference between the energy states of the two

atoms, of the order of 0.05 eV or 387 cm-1, which is supplied by kinetic energy.[3]. The number of neon atoms entering

the excited states builds up as further collisions between helium and neon atoms occur, causing a population

inversion. Spontaneous andstimulated emissionbetween the 3s2 and 2p4 states results in emission of 632.82 nm

wavelength light, the typical operating wavelength of a HeNe laser. After this, fast radiative decay occurs from the 2p

to the 1s ground state. Because the neon upper level saturates with higher current and the lower level varies linearly

with current, the HeNe laser is restricted to low power operation to maintain population inversion. Prior to the

invention of cheap, abundant diode lasers, HeNe lasers were used inbarcode scanners

13. b.i) Resonator configuration

Optical resonators

A simple laser resonator diagram

Features and importance of resonator design

Principle of semiconductor laser

The laser consists of a semiconductor p-n diode cleaved into a small chip, as shown to the left. Electrons are

injected into the n-region, and holes into the p-region. The electrons and holes recombine in the active region at

the junction and photons are emitted.

The laser cavity is formed by using the cleaved facets of the chips. The refractive index of a typical

semiconductor is in the range 34, which gives about 30% reflectivity at each facet. This is enough to supportlasing, even in crystals as short as ~1 mm, because the gain in the semiconductor crystal is very high.

A highly reflective coating is often placed on the rear facet to prevent unwanted losses through this facet and

hence reduce the threshold. The energy of the photons emitted is equal to the bandgap of the semiconductor.

The drive voltage must be at least equal to Eg/e, where Eg is the band gap, and e is the electron charge.

The semiconductor must have a direct band gap to be an efficient light emitter. Silicon has an indirect band gap,

and is therefore no good for laser diode applications. The laser diode industry is based mainly on the compound

semiconductor GaAs, which has a direct band gap at 1.4 eV (890 nm).

13. b.ii) Liquid laser ->8 marks

Dye laser

Construction

Diagram

Working

Advantages

14.a. Measurement of distance and length

http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Electronvolthttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Stimulated_emissionhttp://en.wikipedia.org/wiki/Stimulated_emissionhttp://en.wikipedia.org/wiki/Stimulated_emissionhttp://en.wikipedia.org/wiki/Barcodehttp://en.wikipedia.org/wiki/Barcodehttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Electronvolthttp://en.wikipedia.org/wiki/Helium-neon_laser#cite_note-Verdeyen-2http://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Population_inversionhttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Stimulated_emissionhttp://en.wikipedia.org/wiki/Barcode

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Laser for measuremet of distance operation with neat diagram

Disadvantages

Overcoming technique

Laser for measuremet of length operation with neat diagram

14.b. i)Laser melting and welding

Welding and Brazing

Against many predictions laser beam welding has found its place in industrial production with a

rapidly growing market.

Some years ago first laser welded aluminium parts in the automotive industry were introduced,

meanwhile aluminium car bodies with more than 30 m of laser welded seams are in production.

More and more branches discover new chances in product development by laser welding. With a

shortening of production chains and new designs for example in ship building industry by

combined production steps for cutting and welding of panels large savings are forecasted

In aircraft industry a partially replacement of the rivet technology enables new constructive

benefits which lead to low weight constructions and higher up-times

laser surface alloying (LSA) involves melting of a deposited layer along with a part of the

underlying substrate to form an alloyed zone for improvement of wear, corrosion and oxidation

resistance.

It includes three major parts: a laser source with a beam focusing and delivery system, a lasing

chamber with controlled atmosphere and a microprocessor controlled sweeping stage where the

specimen is mounted for lasing.

The process includes melting, intermixing and rapid solidification of a thin surface layer with

pre/codeposited alloying elements

14.b.ii) Industrial applications of laser

Any two industrial applications of laser with neat sketch

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15. a . i) Principle of holography

Holography is a much broader field than most people have perceived.

Recording and displaying truly three-dimensional images are only small parts of it.

Holographic optical elements (HOE) can perform the functions of mirrors, lenses, gratings, or

combinations of them, and they are used in myriad technical devices

Basic hologram setup

Hologram formation

15.a.ii) Holographic interferometry

Microscopic changes on an object can be quantitatively measured by making two exposures on a

changing object.

The two images interfere with each other and fringes can be seen on the object that reveal the

vector displacement.

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In real-time holographic interferometry, the virtual image of the object is compared directly with

the real object. Even invisible objects, such as heat or shock waves, can be rendered visible.

There are countless engineering applications in this field of holometry

Holography for non-destructive testing

Optical Holographic techniques can be used for nondestructive testing of materials (HNDT).

Nonoptical Holography techniques include Acoustical, Microwave, X-Ray and Electron beam

Holography.

HNDT essentially measures deformations on the surface of the object. However, there is

sufficient sensitivity to detect sub- surface and internal defects in metallic and composite

specimens.

In HNDT techniques, the test sample is interferometrically compared with the sample after it has

been stressed (loaded). A flaw can be detected if by stressing the object it creates an anomalous

deformation of the surface around the flaw.

Optical holography is an imaging method, which records the amplitude and phase of light

reflected from an object as an interferometric pattern on film. It thus allows reconstruction of the

full 3-D image of the object.

In HNDT, the test sample is interferometrically compared in two different stressed states.

Stressing can be mechanical, thermal, vibration etc. The resulting interference pattern contours

the deformation undergone by the specimen in between the two recordings. Surface as well as

subsurface defects show distortions in the otherwise uniform pattern.

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In addition, the characteristics of the component, such as vibration modes, mechanical properties,

residual stress etc. can be identified through holographic inspection.

Applications in fluid mechanics and gas dynamics also abound.

HNDT is widely applied in aerospace to find impact damage, corrosion, delamination,

debonds, and cracks in high performance composite aircraft parts as well as turbine

blades, solid propellant rocket motor casings, tyres and air foils.

But Holography is also finding new applications in commercial and defense related

industries to investigate and test object ranging from microscopic computer chips and

circuits to cultural articles, paintings and restoration.

15.b.i) Plastic Surgery

Laser instruments used For the treatment of angiomas and other vascular lesions

For the treatment of port wine stains and for the coagulation of pigmented lesions

15.b.ii)Gynecology

-refers to the surgical speciality dealing with the health of female reproductive system

Main conditions dealt with the gynaecologist

Lasers used

Oncology