INTRODUCTION
1. Write the expression for the refractive
index in graded index fibers.
n(r)=
n1[1-2∆(r/a)α]1/2 for 0<=r<=a
n1(1-2∆)1/2
~ n1(1-∆) =n2 for r>=a
r radial
distance from fiber axis a core radius
n1 refractive index at the core
n2 refractive index at the cladding
α shape of
the index profile
∆ index
difference
2.
Define
Mode-field diameter.
The
fundamental parameter of a single mode fiber is the mode-field diameter. This
can be determined from the mode field distribution of the fundamental LPo1
mode.
3. Give the expression for linearly polarized
waves.
The
electric or magnetic field of a train of plane polarized waves travelling in a
direction k can be represented in the general form
A(x,t) =
eiAoexp[j(wt-k.x)]
With
x=xex+yey+zez representing a general position vector and k=kxex+kyey+kzez
representing the wave propagation vector.
4. What is Snell’s law?
The
relationship at the interface is known as Snell’s law and is given by
n1sinΦ1=n2
sinΦ2
5.
What is the
necessity of cladding for an optical fiber?
a) To
provide proper light guidance inside the core
b) To avoid
leakage of light from the fiber
c) To avoid
mechanical strength for the fiber
d) To
protect the core from scratches and other mechanical damages
6.
What are
the uses of optical fibers?
a) To
transmit the information which are in the form of coded signals of the
telephone communication, computer data, etc.
b) To
transmit the optical images (Example : Endoscopy)
c) To act as
a light source at the inaccessible places.
d) To act as
sensors to do mechanical, electrical and magnetic measurements.
7.
What is
the principle used in the working of fibers as light guides?
The
phenomenon of total internal reflection is used to guide the light in the
optical fiber. To get total internal reflection, the ray should travel from
denser to rarer i.e. from core to clad region of the fiber and the angle of
incidence in the denser medium should be greater than the critical angle of
that medium.
8. What are step index and graded index fibers?
In the
case of graded index fiber, the refractive index of a core is a constant and is
larger than the refractive index of the cladding. The light propagation is
mainly by meridional rays. In the case of graded index fiber (GRIN fiber) the
refractive index of the core varies parabolically from the center of the core
having maximum refractive index to the core-cladding interface having constant
minimum refractive index. Here the light propagation is by skew rays.
9. Why do we prefer step index single mode
fiber for long distance communication?
Step
index single mode fiber has
a) Low
attenuation due to smaller core diameter
b) Higher
bandwidth and
c) Very low
dispersion.
10.
Define
relative refractive index difference.
Thus
relative refractive index difference is the ratio between the refractive index
difference (of core and cladding) and refractive index of core.
11. What are meridional rays?
Meridional
rays are the rays following Zig Zag path when they travel through fiber and for
every reflection it will cross the fiber axis.
12. What are skew rays?
Skew rays
are the rays following the helical path around the fiber axis when they travel
through the fiber and they would not cross the fiber axis at any time.
13. What is V number of fiber or normalized
frequency of fiber?
V number
of fiber or normalized frequency of fiber is used to find the number of
propagating modes through the fiber.
V= 2∏a
(N.A) / λ
In step
index fiber number of modes propagating through the fiber=V2/2
Taking
the two possible polarizations, total number of possible modes propagating
through the fiber = [
14.
What are
the conditions for total internal reflection?
a) Light
should travel from denser medium to rarer medium.
b) The angle
of incidence should be greater than the critical angle of the denser medium.
15.
Give the
relation between numerical aperture of skew rays and meridional rays.
(N.A)skew
= cos γ(N.A)meridional when the fiber is placed in air. Here γ is the half of
the angular change in every reflection.
16.
State Goos-Haenchen effect.
Goos-Haenchen
effect states that there is a lateral shift of the reflected ray at the point
of incidence at the core-cladding interface. This lateral shift is called the
Goos-Haenchen shift.
17. When do you have phase shift during total internal reflection
of light?
When the
light ray travels from denser medium to rarer medium, if the angle of incidence
is greater than the critical angle of core medium, there is a phase shift for
both TE and TM waves.
18. What are hybrid modes? Give two examples.
Hybrid
modes are the mixture of TE and TM modes that can be traveled through the
optical fiber.
Examples:
1. HE1m
modes in which |Ez|>|Hz|
2. EH1m
modes in which |Hz|>|Ez|
19.
Define
cutoff wavelength of the fiber.
The
cutoff wavelength is defined as the minimum value of wavelength that can be
transmitted through the fiber. The wavelengths greater than the cutoff
wavelength can be transmitted.
λ cutoff
= 2∏a (N.A) / V
20. Mention the rule distinguishing ‘mode’ and
‘order’.
The rule
states that the smaller the modes propagating angle, the lower the order of the
mode. Hus the mode traveling precisely along the fiber’s central axis is zero
mode.
21. What is fiber birefringence?
Imperfections
in the fiber are common such as asymmetrical lateral stress, non-circular
imperfect variations of refractive index profile. These imperfections break the
circular symmetry of ideal fiber and mode propagate with different phase
velocity and the difference between their refractive index is called fiber
birefringence.
B=ko(ny-nx)
22. Give the expression for numerical aperture in
graded index fibers.
N.A(r)=N.A.(0)
(1-(r/a)α)1/2 for
r<=a
where
N.A(0) = axial numerical aperture = (n12-n22)1/2 a is
core radius and α is the refractive index profile.
Glossary
Mode-field diameter.
The
fundamental parameter of a single mode fiber is the mode-field diameter. This
can be determined from the mode field distribution of the fundamental LPo1
mode.
2. Snell’s law.
The
relationship at the interface is known as Snell’s law and is given by
n1sinΦ1=n2
sinΦ2
3.
cladding
for an optical fiber
a) To
provide proper light guidance inside the core
b) To avoid
leakage of light from the fiber
c) To avoid
mechanical strength for the fiber
d) To
protect the core from scratches and other mechanical damages
4.
Step
index and graded index fibers.
In the
case of graded index fiber, the refractive index of a core is a constant and is
larger than the refractive index of the cladding. The light propagation is
mainly by meridional rays. In the case of graded index fiber (GRIN fiber) the
refractive index of the core varies parabolically from the center of the core having
maximum refractive index to the core-cladding interface having constant minimum
refractive index. Here the light propagation is by skew rays.
5. Relative refractive index difference.
Thus
relative refractive index difference is the ratio between the refractive index
difference (of core and cladding) and refractive index of core.
6. Meridional rays.
Meridional
rays are the rays following ZigZag path when they travel through fiber and for
every reflection it will cross the fiber axis.
skew rays.
Skew rays
are the rays following the helical path around the fiber axis when they travel
through the fiber and they would not cross the fiber axis at any time.
8. V number of fiber or normalized frequency
of fiber.
V number
of fiber or normalized frequency of fiber is used to find the number of
propagating modes through the fiber.
V= [ 2∏a/
λ ] (N.A)
9. Goos-Haenchen effect.
Goos-Haenchen
effect states that there is a lateral shift of the reflected ray at the point
of incidence at the core-cladding interface. This lateral shift is called the
Goos-Haenchen shift.
10. Hybrid modes.
Hybrid
modes are the mixture of TE and TM modes that can be traveled through the
optical fiber.
Examples:
1. HE1m
modes in which |Ez|>|Hz|
2. EH1m
modes in which |Hz|>|Ez|
11.
Cutoff
wavelength of the fiber.
The
cutoff wavelength is defined as the minimum value of wavelength that can be
transmitted through the fiber. The wavelengths greater than the cutoff
wavelength can be transmitted.
λ cutoff
= [2∏a/ V] (N.A)
12. fiber birefringence.
Imperfections
in the fiber are common such as asymmetrical lateral stress, non-circular
imperfect variations of refractive index profile. These imperfections break the
circular symmetry of ideal fiber and mode propagate with different phase
velocity and the difference between their refractive index is called fiber
birefringence.
B=ko(ny-nx)
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