Dr. Ravindra H J & Dr. P.S. Aithal

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15 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

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Dr. Ravindra H J & Dr. P.S.
Aithal



Introduction to Superconductivity, temperature dependence of resistivity



Effect of magnetic field, Meissner effect, Type I and Type II superconductors,
Temperature dependence of critical field



BCS theory and High temperature superconductors



Applications of superconductors
-

superconducting magnet and Maglev
vehicles



Fundamentals of Optical Fiber, construction, principle, mechanism of light
propagation, acceptance angle, and numerical aperture of optical fibers



Types of optical fibers and attenuation in fibers



Applications of optical fibers


Discovered by


Heike
Kammerlingh

Onnes


1908


Liquefying of gases





䕬散瑲b捡c

牥獩獴慮捥



獯浥



瑨t

浥瑡汳

⡬敡d

慮a

浥m捵特c

扥co浥m

穥牯



汯l

瑥浰敲t瑵牥

(

near

to

absolute

zero)”


discovered

in

the

year



ㄹㄱ


Nobel

pri穥



J

ㄹㄳ


As the temperature increases more number of
electrons are liberated from atoms


If the temperature is decreased then all the
electrons will return to atoms.


Total resistivity of the given metal
=
residual
resistivity

+
resistivity due to lattice vibration




Bardeen


Cooper


Sheiffer



Cooper

pair



bound

pair

of

electrons

formed

by

the

interaction

between

the

electrons

with

opposite

spin

and

momentum

in

the

phonon

filed
.



Supercondu
ctor

Symbol

T
c

(k)

Gallium

Aluminium

Indium

Lanthanum

Neobium

Mercury

Ga

Al

In

La

Nb

Hg

1.15

1.20

3.40

6.00

9.50

4.2


A diamagnetic
property exhibited by
superconductors.


`
End result is the
exclusion of
magnetic field from
the interior of a
superconductor.



Experimentally it is found that





2
c
o
c
T
T
1
H
)
T
(
H


Critica
l field

Lecture 4


The strength of minimum field required to
switch a material from superconducting state
to normal state

Type I (or Soft) Superconductors

H

Normal state
cores

Superconducting
region



copper oxide

based superconductors


Perovskite

crystal structure


1986 publication by Georg Bednorz and K. Alex
Müller
-

superconductivity at 30 K in an oxide of
barium, lanthanum, and copper


superconductivity at about 92 K in an oxide of
yttrium, barium, and copper (YBa
2
Cu
3
O
7
).


superconductivity at 105 K in an oxide of
bismuth, strontium, calcium, and copper.


superconductivity at temperatures as high as
150 K in an oxide containing mercury.


JOSEPHSON EFFECT,
the flow of electric current, in
the form of electron pairs (called Cooper pairs),
between two superconducting materials that are
separated by an extremely thin insulator.



A steady flow of current through the insulator can be
induced by a steady magnetic field.



The current flow is termed Josephson current, and
the penetration ("tunneling") of the insulator by the
Cooper pairs is known as the
Josephson effect
.



Named after the British physicist Brian D. Josephson,
who predicted its existence in 1962.


Detection of very
weak magnetic fields


10
-
14

Tesla


Earth magnetic field


~ 0.5 x10
-
4

Tesla

Squids



formed by
incorporating two
Josephson’s Junction
in the loop o映
superconducting
materials


Superconducting Magnets


Magnetic levitation and permanent magnets


Wires and cables for motors and power
distribution systems


Microwave filters for cellular telephone
networks


Superconducting quantum interference device
(SQUID) as sensors of heart beats and brain
signals, and


Single flux quantum logic in quantum
computing system



Fibers of glass


Usually 120 micrometers in diameter


Used to carry signals in the form of light over
distances up to
50
-
100
km.


No repeaters needed
.




Thin strand of metal is called
wire


Thin strand of dielectric material is called
fiber
.


1880


Alexander Graham Bell


1930


Patents on tubing


1950


Patent for two
-
layer glass wave
-
guide


1960


Laser first used as light source


1965


High loss of light discovered


1970s


Refining of manufacturing process


1980s


OF technology becomes backbone
of long distance telephone networks in NA.



Thinner


Less Expensive


Higher Carrying Capacity


Less Signal Degradation& Digital Signals


Light Signals


Non
-
Flammable


Light Weight


Telecommunications


Local Area Networks


Cable TV


CCTV


Optical Fiber Sensors


Core


Glass or plastic with a higher index
of refraction than the cladding


Carries the signal


Cladding


Glass or plastic with a lower index
of refraction than the core


Buffer


Protects the fiber from damage
and moisture


Jacket


Holds one or more fibers in a cable

Conditions for total
internal reflection
:

1.
The ray of light must
travel from denser
medium towards rarer
medium.

2.
The angle of incidence
in the denser medium
must be greater than the
critical angle for the pair
of the media in contact.

Snell’s law:

n
1

sin

1
=n
2

sin

2


c = sin
-
1
(n
2
/n
1
)


Based on refractive index profile:


Step index fiber


Graded index fiber

Based on modes of light propagation:

Single mode fiber

multimode fiber

Based on materials:

Glass/glass fibers

Plastic/plastic fibers

Polymer clad silica fiber(
pcs
)


Three classes of fibers



Single mode step index fiber (SMF)


Multimode step index fiber (MMF)


Graded index multimode fiber (GRIN)




If the medium surrounding the
fiber is air then n=1



2
2
2
1
2
2
sin
n
n
n
i





i

is

called wave guide acceptance angle or acceptance cone half angle

`
Sin

i

is called numerical aperture (light gathering capacity of the optical
fibers

-

The ratio of the refractive index difference
between the core and the cladding to the
refractive index of core.

1
2
1
)
(
n
n
n




r <

i

Sin

r < sin

i

Sin

r < NA





But ,






Since n1=n2

1
2
1
)
(
n
n
n











1
2
1
2
1
2
1
2
2
2
1
)
(
.
)
)(
(
.
.
n
n
n
A
N
n
n
n
n
A
N
n
n
A
N




2
.
2
.
1
2
1
n
A
N
n
A
N

The number of modes supported for propagation in the
fiber is determined by a parameter called
V
-

number


If the medium is air then





d


core diameter


-

wavelength of the light propagating in the fiber

For the medium other than air V
-

number is



For V>>1, the number of modes supported by fiber is
given by V
2
/2

)
.
(
2
2
2
1
A
N
d
V
or
n
n
d
V







0
2
2
2
1
n
n
n
d
V





-

the loss of power suffered by the optical
signal as it propagates through the fiber

Attenuation in optical fibers takes through
three mechanisms


Absorption losses


Scattering losses


Radiation losses



a)
absorption by impurities
( transition metal
ions

iron, chromium, copper and cobalt &
OH ions


b)
Intrinsic absorption
(self absorption)


-

absorption due to fiber ( assuming no
impurities, no inhomoginities)


a)

Rayleigh scattering
occurs whenever a light
wave travels through a medium having
scattering objects whose dimensions are
smaller than a wavelength

Scattering of photons due to sharp changes in the
refractive index

Change of refractive index is due to
inhomoginities ( occurs during the solidification
of fibers)

b)

Others

Defects


gas bubbles, unreacted starting materials

And crystallized regions in glass.


a) macroscopic bends




b) microscopic bends



Lambert’s law


The rate of decrease of intensity of light with distance travelled
in a homogeneous medium is proportional to the initial intensity






x
Where


is a constant called


attenuation coefficient
or
attenuation

or
fiber loss




x
Integrating above equation we get


P
dL
dP
or
P
dL
dP





dL
P
dP







dL
P
dP





L
P
P
dL
P
dP
out
in
0



























in
out
in
out
L
P
P
P
P
L
L
P
P
L
P
out
in
ln
1
ln
ln
0



km
dB
P
P
L
unitlength
Bel
P
P
L
in
out
in
out
/
log
10
/
log
1
10
10























Point to point communication system





Sensing device




Pressure, voltage and current


Data link


Local area network




Large band width and can carry large amount of
information


Low cost of production


No corrosion and rusting of
OF


Superior than metallic cable


No interference of signal between different communication
channels


Lightning or sparking tend to cause disturbance in metallic
wires and no such disturbance in OF


No requirement of common ground


No energy radiation from a fiber and possibility of
information tapping is ruled out.


Due to superior attenuation characteristics transmission of
signal to longer distance can be achieved


OF communications systems are simple and cost effective.


Splicing is skillful job, requires
precission



Difficult to Detect the line break due to
accidents


maintenance cost is more


Loss become considerable if the fiber is bent
to certain angle.


Fibers undergo thermal expansion due to
environmental temperature changes
-

lead to
loss of signal power.


Calculate the numerical aperture and angle of
acceptance for an optical fiber having
refractive indices 1.6 and 1.48 for the core
and cladding respectively



An optic glass fiber of refractive index 1.5 is
to be clad with another glass to ensure total
internal reflection that will contain light
travelling within 10
o

of the fiber axis. What
maximum index of refraction is allowed for
the cladding.