STEP-INDEX MULTI MODE FIBER

STEP-INDEX MULTI MODE FIBER has a large core, up to 100 microns in diameter. Some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits bandwidth that is, the amount of information that can be sent. So this type of fiber is best suited for transmission over short distances.

Posted by: Wasim Javed

OPTICAL FIBER

Fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper     lines. At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are funneled into the fiber-optic medium where they travel down the cable. The light is most often 850nm for shorter distances and 1,300nm for longer distances on Multi-mode fiber and 1300nm for single-mode fiber and 1,500nm is used for longer distances.

Light pulses move easily down the fiber-optic line because of a principle as total internal reflection. "This principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses. The core must a very clear and pure material for the light or in most cases near infrared light (850nm, 1300nm and 1500nm). The core can be Plastic (used for very short distances) but most are made from glass. Glass optical fibers are almost always made from pure silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for longer-wavelength infrared applications. 

Single Mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission.  Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.

Assignment on M.Phil (All Questions)

Questions of Assignment

Q.1 Write three monomers for each which can polymerize through:

(a) Addition polymerization

(b) Condensation polymerization

(c) Emulsion polymerization

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Q.2 Describe at least three methods or techniques for degradation of

polymers ?

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Q.3 Generate data points , calculate the number average (Mn) , weight

average (Mw) molecular weights and degree of dispersity(P).

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Q.4 Describe at least three methods for the preparation of colloidal

particles?

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Q.5 Generate data points and calculate potential of three colloidal

particles?

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Q.1 Write three monomers for each which can polymerize through:

(a) Condensation polymerization
(b) Addition polymerization
(c) Emulsion polymerization

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a) Condensation polymerization

The condensation polymers are formed by repeated condensation reaction

between two different bi-functional or tri-functional monomeric units. In these

polymerisation reactions, the elimination of small molecules such as water,

alcohol, hydrogen chloride, etc. take place.

Example.1

Bakelite is a condensation polymer of phenol and formaldehyde.

Example.2

A carboxylic acid monomer and an alcohol monomer can join in an ester

linkage.

Example.3

A carboxylic acid monomer and an amine monomer can join in an amide linkage.

The polymer made from these two six-carbon monomers is known as nylon-6,6.

b) Addition polymerization

Addition polymerization is a chemical reaction in which simple molecules

(monomers) are added to each other to form long-chain molecules (polymers)

without by-products.

Example.1

PVC (polyvinyl chloride) is formed from vinyl chloride (H₂C=CHCl).

Example.2

Polypropylene made from the monomer propylene.

Example.3

Polystyrene is an aromatic polymer made from the monomer styrene.

c) Emulsion polymerization

A polymerization reaction that occurs in one phase of an emulsion. Emulsion polymerization involves the formation of synthetic latexes and resins by the

polymerization of amonomer-in-water emulsion.

Example.1

Polytetrafluoroethylene ( PTFE) is synthesized by the emulsion polymerization

of tetrafluoroethylene monomer.

Example.2

Styrene-Butadiene-Rubber (SBR) is a synthetic rubber copolymer consisting of

styrene and butadiene.

Example.3

Acrylonitrile is used principally as a monomer to prepare the polyacrylonitrile.

Q.2 Describe at least three methods or techniques for degradation of polymers ?

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Polymer degradation

Polymer degradation is a change in the properties tensile strength, color, shape,

or molecular weight of a polymer or polymer based product under the influence

of one or more environmental factors, such as heat, light, chemicals and, in

some cases, galvanic action.

Following are the method or techniques for degradation of polymers:

1. Thermal degradation

2. UV degradation

3. Chemical degradation

1. Thermal degradation of polymers

Thermal degradation of polymers is molecular deterioration as a result of

overheating. At high temperatures the components of the long chain backbone

of the polymer can begin to separate (molecular scission) and react with one

another to change the properties of the polymer. Thermal degradation can

present an upper limit to the service temperature of plastics as much as the

possibility of mechanical property loss. Indeed unless correctly prevented,

significant thermal degradation can occur at temperatures much lower than

those at which mechanical failure is likely to occur. The chemical reactions

involved in thermal degradation lead to physical and optical property changes

relative to the initially specified properties. Thermal degradation generally

involves changes to the molecular weight (and molecular weight distribution) of

the polymer and typical property changes include reduced ductility and

embrittlement, chalking, color changes, cracking, general reduction in most

other desirable physical properties.

Ways of Polymer Thermal Degradation

Depolymerisation

Under thermal effect, the end of polymer chain departs, and forms low free

radical which has low activity. Then according to the chain reaction mechanism,

the polymer loses the monomer one by one. However, the molecular chain

doesn’t change a lot in a short time.This process is common for

polymethymethacrylate (perspex).

CH2-C(CH3)COOCH3-CH2-C*(CH3)COOCH3→CH2-C*(CH3)COOCH3+ H2=C(CH3)COOCH3

Random Chain Scission

The backbone will be break down randomly, could be occurred at any position

of the backbone. The molecular weight decreases rapidly, and cannot get

monomer in this reaction, this is because it forms new free radical which has

high activity can occurs intermolecular chain transfer and disproportion

termination with the CH2’group.

CH2-CH2-CH2-CH2-CH2-CH2-CH2’→ CH2-CH2-CH=CH2 + CH3-CH2-CH2’ or CH2’+CH2=CH-CH2-CH2-CH2-CH3

Side-Group Elimination

Groups that are attached to the side of the backbone are held by bonds which are

weaker than the bonds connecting the chain. When the polymer was being

heated, the side groups are stripped off from the chain before it is broken into

smaller pieces. For example the PVC eliminates HCL, under 100-120oC.

CH2(Cl)CHCH2CH(Cl)→CH=CH-CH=CH+2HCl

2. UV degradation of polymers

Many natural and synthetic polymers are attacked by ultra-violet radiation and

products made using these materials may crack or disintegrate. The problem is

known as UV degradation, and is a common problem in products exposed to

sunlight. Continuous exposure is a more serious problem than intermittent

exposure, since attack is dependent on the extent and degree of exposure.

Sensitive polymers include thermoplastics, such as polypropylene, polyethylene,

and poly(methyl methacrylate) as well as speciality fibers like

aramids. UV absorption leads to chain degradation and loss of strength at

sensitive points in the chain structure. They include tertiary carbon atoms,

which in polypropylene occur in every repeat unit.

3. Chemically assisted degradation of polymers

Chemically assisted degradation of polymers is a type of polymer degradation

that involves a change of the polymer properties due to achemical reaction with

the polymer’s surroundings. There are many different types of possible

chemical reactions causing degradation however most of these reactions result

in the breaking of double bonds within the polymer structure.

Example of chemically assisted degradation

Degradation of rubber by ozone

One common example of chemically assisted degradation is the degradation of

rubber by ozone particles. Ozone is a naturally occurring atmospheric molecule

that is produced by electric discharge or through a reaction of Oxygen with solar

radiation. Ozone is also produced with atmospheric pollutants reacted with

ultraviolet radiation. For a reaction to occur, ozone concentrations only have to

be as low as 3-5 parts per hundred million (pphm) and when these

concentrations are reached, a reaction occurs with a thin surface layer (5x10-7 metres)

of the material. The ozone molecules react with the rubber which in

most cases is unsaturated (contains double bonds), however a reaction will still

occur in saturated polymers (those containing only single bonds).

When reaction occurs, scission of the polymer chain (breaking of double covalent bonds)

takes place forming decomposition products:

Chain scission increases with the presence of active Hydrogen molecules (for

example, in water) as well as acids and alcohols. Along with this type of

reaction, cross linking and side branch formations also occur by an activation of

the double bond and these make the rubber material more brittle. Due to the

increase in brittleness due to the chemical reactions, cracks form in areas of

high stress. As propagation of these cracks increases, new surfaces are opened

for degradation to occur.

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Q.3 From the following data points , calculate the number

average (Mn) , weight average (Mw) molecular weights and

degree of dispersity(P).

Number of Molecules	Mass of each molecule
7	1
5	3
4	6
3	9

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Solution: Formula for the calculation of number average molecular weight:

Mn = ΣNiMi

Σni

Step1: Multiply the weight of the molecule by the number of molecules of that weight

NiMi would be: 7(1) + 5(3)+ 4(6)+3(9) = 73

Σni would be: 7+5+4+3 = 19

Therefore, Number Average Molecular Weight Mn = 73 / 19 = 3.842

Step 2: Formula for the calculation of weight average molecular weight

Mw = ΣNiMi ²/ΣNiMi = Σwi Mi /Σwi

The calculation of the weight average molecular weight requires the weight fraction, Wi, of each type of molecule. Weight fraction of individual molecules is the mass of each molecule (NiMi) divided by the total weight (ΣNiMi).

Ni	Mi	NiMi	Wi = NiMi/ΣNiMi	MiWi
7	1	7	0.095	0.095
5	3	15	0.205	0.615
4	6	24	0.328	1.968
3	9	27	0.369	3.321

ΣNiMi = 73

Weight Average Molecular Weight is ΣWiMi = 5.999

The ratio of Mw/Mn is a measure of polydispersity.

P = Mw/Mn

5.999/3.842 = 1.561

Q.4 Describe at least three methods for the preparation of colloidal particles?

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Preparation of Colloidal Solutions

(a) Preparation of Lyophilic Sols:

Hydrophilic colloids such as starch, gum, gelatin, etc. form colloidal solution

when warmed or left in contact with water for a long time. The particles are

already of colloidal size and they are readily dispersed in water forming

colloidal solution. Sols of water insoluble high molecular weight compounds

may similarly be obtained by their dispersal in suitable liquids.

(b) Preparation of Lyophobic Sols:

Lyophobic sols cannot be prepared by direct mixing of dispersed phase and

the dispersion medium. These are prepared by using special techniques.

The methods employed for the preparation of lyophobic colloids fall into

two categories:

(i) Dispersion methods in which larger macro-sized particles are broken down

to colloidal size.

(ii) Condensation methods in which particles of colloidal size are produced by

aggregation of single ions or molecules.

(i) Dispersion Methods

1. Bredig’s Electric Arc Method:

This method is employed to prepare sols of metals such as copper, silver, gold

or platinum. The two electrodes used in this method are made of the metal

whose sol is to be prepared. The electrodes are immersed in dispersion medium

such as water. The dispersion medium is cooled by immersing the container

in an ice bath and a trace of alkali is added. An arc is struck between the metal

electrodes held close together. The tremendous heat generated by the spark

across the electrodes vapourises some of the metal and the vapour condenses

immediately in the dispersion medium to give colloidal solution. The trace of

added alkali helps to stabilise the sol.

2. Mechanical Dispersion:

It is done with the help of a colloid mill. The mill consists of two steel discs

with a small gap between them. The discs rotate in opposite direction at high

speed. The substance whose sol is to be prepared is first ground as finely as

possible and then shaken with the dispersion medium to get a suspension. The

suspension is added to the colloid mill.The speed of the rotating discs is

adjusted so that the particles of the suspension are broken to produce the

particles of colloidal size.

3. Ultrasonic Dispersion:

Sound waves having frequency more than that of the audible sound are called

ultrasonic waves. These waves can produce particles of colloidal size from

coarse suspension. This method is used to prepare a sol of mercury in water.

The ultrasonic waves produced from a quartz generator propagate through the

oil and strike the beaker containing mercury under water. The ultrasonic waves

transfer their energy to the atoms of mercury. Mercury gets vapourised and

the vapours disperse in water producing colloidal solution. This is the latest method

for the preparation of metal sols from their coarse suspension.

(ii) Condensation or Aggregation Methods

1. Lowering of solubility by exchange of solvent:

In this method, a substance is dissolved in a solvent and then the solution is

added to another solvent in which it is less soluble. For example if an alcoholic

solution (true solution) of sulphur is added in excess of water, a colloidal

solution of sulphur results. Sulphur is insoluble in water.

2. Passing vapours of an element into a liquid:

When the vapours of an element are passed through a liquid, condensation takes

place to give a colloidal solution. For example, colloidal solution of mercury

can be obtained by passing the vapours of mercury into cold water containing

suitable stabilising agents such as ammonium salts or citrates.

3. Excessive cooling:

The method can be used to get colloidal solution of ice in an organic solvent

like chloroform or ether. A solution of water in the required solvent is frozen.

The molecules of water, which can no longer be held in the solution, get

together to form particles of colloidal size.

Q.5 Generate data points and calculate potential of three colloidal particles?

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By directly measuring the electrophoretic mobility of a particle, the zeta

potential may then be determined using the Henry Equation:

UE = 2εzf(Ka)/3η

where UE is the electrophoretic mobility, ε is the dielectric constant, z is the

zeta potential, f(Ka) is Henry’s function, and η is the viscosity. Henry’s function

generally has value of either 1.5 or 1.0. For measuring zeta potential in aqueous

solutions of moderate electrolyte concentration, a value of 1.5 is used and this is

referred to as the Smoluchowski approximation.

1. Calculate the Zeta Potential of colloidal particle when the Particle size =235nm,Dielectric constant of water =78.5, Electric field = 8.13 V/cm,Viscosity of water = 0.890 cP and Mobility= -4.45 μ/s ?

UE = 2εzf(Ka)/3η

-4.45 = 2(78.5)z(1.5)/3(0.890)

-4.45 = z(235.5)/2.67

z(235.5) = (4.45)(2.67)

z(235.5) = 11.881

z = 11.881/235.5

z = +0.0504 mv

2. Calculate the Zeta Potential of colloidal particle when the Particle size = 150nm, Dielectric constant of water = 78.5, Electric field = 27.04 V/cm, Viscosity of water = 0.890 cP and Mobility = -1.07 μ/s?

UE = 2εzf(Ka)/3η

-1.07 = 2(78.5)z(1.5)/3(0.890)

-1.07 = z(235.5)/2.67

z(235.5) = (1.07)(2.67)

z(235.5) = 2.856

z = 2.856/235.5

z = +0.0121 mv

3. . Calculate the Zeta Potential of colloidal particle when the Particle size = 200nm, Dielectric constant of water = 78.5, Electric field = 25.78 V/cm, Viscosity of water = 0.890 cP and Mobility = -2.07 μ/s?

UE = 2εzf(Ka)/3η

-2.07 = 2(78.5)z(1.5)/3(0.890)

-2.07 = z(235.5)/2.67

z(235.5) = (2.07)(2.67)

z(235.5) = 5.526

z = 5.526/235.5

z = +0.0234 mv

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References:

(i) D. J. Shaw, Introduction to Colloid and Surface Chemistry, Butterworth (1980)

(ii) J. W. McBain, Colloid Science, D.C. Heath and Company, Boston (1950)

(iii) F.W. Billmeyer, Text Book of Polymer Science, Interscience Publishers Inc, New York (1962)

(iv) R.J. Hunter, ‘Zeta Potential in Colloids Science’, Academic Press, NY, 1981

(v) Principles of polymerization by George Odian New York fourth Edition

(vi) PhD Thesis ‘Mobility measurements By phase analysis’, Walther W.Tscharnuter.

(vii) Physical Chemistry ‘Surface Chemistry and Colloids’, By Dr A K Ghosh

(viii) Everett, D.H. (1994) Basic Principles Of Colloid Science, The Royal Society of Chemistry, UK.

(ix) Ross, S. and Morrison, I.D. (1988) Colloidal Systems and Interfaces,

John Wiley and Sons, USA.

(x) A.W. Admason and A.P. Gast, Physical Chemistry of Surfaces, Wiley, New York (1997).

Site Links:

i) en.wikipedia.org/wiki/Condensation_polymer

ii) dictionary.reference.com/browse/condensation+polymerization

iii) en.wikipedia.org/wiki/Addition_polymer

iv) en.wikipedia.org/wiki/Polymer_degradation

v) en.wikipedia.org/wiki/Colloid

vi) en.wikipedia.org/wiki/Colloidal_particle