1. Mid semester project
Optical fibers
Shubham Patel
17PH40035
IIT KHARAGPUR
Department Of Physics
Optics and EM Lab
2. Introduction
An optical fiber or optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to
a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to
transmit light between the two ends of the fiber and find wide usage in fiber optic communications. The
main use of optical fiber is in long distance communication (telecommunication). Since the light does not
leak out of the fiber much as it travels, the light can go a long distance before the signal gets too weak.
This is used to send telephone and internet signals between cities.
3. Structure
Optical fibers typically include
a core surrounded by a
transparent cladding material with a
lower index of refraction. Light is kept in the
core by the phenomenon of total internal
reflection which causes the fiber to act as
a waveguide. A waveguide is a structure that
guides waves, such as electromagnetic
waves or sound, with minimal loss of energy.
4. Optical waveguide
An optical wave guide is a structure that
"guides" a light wave by constraining it to
travel along a certain desired path. If the
transverse dimensions of the guide are
much larger than the wavelength of the
guided light, then we can explain how the
optical waveguide works using geometrical
optics and total internal reflection (TIR).
A wave guide traps light by surrounding a
guiding region, called the core, made from
a material with index of refraction ncore, with
a material called the cladding, made from a
material with index of refraction ncladding <
ncore. Light entering is trapped as long as
sinθ > ncladding/nncore.
Light can be guided by planar
or rectangular wave guides, or
by optical fibers.
5. Optical fiber waveguide
An optical fiber consists of three concentric
elements, the core, the cladding and the outer
coating, often called the buffer.
Core : The core is usually made of glass or
plastic. The core is the light-carrying portion of
the fiber.
Cladding : The cladding surrounds the
core. The cladding is made of a material with a
slightly lower index of refraction than the
core. This difference in the indices causes total
internal reflection to occur at the core-cladding
boundary along the length of the fiber.
Buffer: The outer layer which serve as a ‘shock
absorber’ to protect the core and cladding from
damage.
6. Wave guide modes
Transverse electromagnetic (TEM) modes: neither electric nor magnetic
field in the direction of propagation.
Transverse electric (TE) modes: no electric field in the direction of
propagation. These are sometimes called H modes because there is only
a magnetic field along the direction of propagation (H is the conventional
symbol for magnetic field).
Transverse magnetic (TM) modes: no magnetic field in the direction of
propagation. These are sometimes called E modes because there is only
an electric field along the direction of propagation.
Hybrid modes: non-zero electric and magnetic fields in the direction of
propagation.
8. classification
There are two types of Optical fibers:
(1) Single mode optical fiber: As the name
suggests, this type of optical fiber transmits only one
mode of the light. To put it another way, it can carry
only one wavelength of light across its length.(1310nm
or 1550nm)
(2) Multimode optical fiber: These types of optical
fibers allow multiple modes of light to travel along
their axis. To explain physically, they can do this by
having a thicker core diameter.(850 to 1300 nm)
9. project
Study of total internal reflection and calculation
of refractive index of PMMA rod.
Determination of numerical aperture, bending
loss and Splice Loss of a given multimode
optical fiber.
Determination of numerical aperture, mode
field diameter and V number of a given single
mode fiber.
10. Single mode optical fiber
(refractive index)
Aim : we will find the refractive index of a
transparent solid using diode laser.
Total Internal Reflection:
Total internal reflection is an optical phenomenon
that happens when a ray of light strikes a medium
boundary at an angle larger than critical angle with
respect to the normal to the surface. If the
refractive index is lower on the other side of the
boundary and the incident angle is greater than the
critical angle, no light can pass through and all of
the light is reflected.
11. Snell’s law:
Refractive index is given by
n = sin i / sin r,
where i= incident angle
r= refractive angle
In the given figure,
r = tan-1 (b/a)
Measurement pictures:
(1) Adjusting at appropriate incident angle.
b b
Incident ray
Refracted ray
a
a
r
i
Transparent bar
12. (2) Noting down incident angle of laser beam and
distances a and b using meter scale
(3) We took 3 measurement.
Result : The refractive index of the PMMA rod n = 1.52.
Actual refractive index of a PMMA rod is 1.5184.
13. Numerical aperature
1. for Single mode:
A single mode optical fiber will
only propagate light that enters
the fiber within a certain cone,
known as the acceptance cone
of the fiber. The half-angle of
this cone is called the
acceptance angle, θa.
Acceptance angle
θa = tan-1 ( D/Z ).
D and Z are shown in figure.
Numerical aperature:
NA = Sin θa
θa
z
Fiber
5%Level
I()θ
D
15. Measurements: from graph following measurements observed→
1/e2 intensity level = 10.45mm.
Level of maximum intensity= 11.5mm ,
Diameter of far field intensity D = 11.5 – 10.45 = 1.2mm (1/e2 of maximum intensity)
Distance between the detector and fiber output end Z = 3.3mm
Acceptance angle, θa = tan-1 (D/Z) = 0.3487.
Then the Numerical aperture is given by,
NA = Sin θa = 0.3416.
V number is given by,
V = (2πa/ λ)NA
where, a = 4.5μm for this fiber and λ = 650nm
So, V = 14.829 ̴ 14.8.
Result: The Numerical Aperture of the given optical fiber, NA= 0.3416.
(2) Field diameter D = 1.2mm .
(3) V number = 14.8.
16. (2) For multi-mode
A multi-mode optical fiber will only propagate
light that enters the fiber within a certain cone,
known as the acceptance cone of the fiber. The
half-angle of this cone is called the acceptance
angle, θa.
As similar to single mode here also acceptance
angle is given by,
Acceptance angle
θa = tan-1 ( D/Z ).
Numerical aperature = Sin θa
18. Measurements: from graph following measurements
observed→
1/e2 intensity level = 11.71mm (1/e2 of maximum intensity)
Level of maximum intensity= 12.28mm
Diameter of far field intensity D = 12.28 – 11.71= 0.76mm
Distance between the detector and fiber output end Z = 3.0mm.
Acceptance angle, θa = tan-1 (D/Z) = 0.2166.
Then the Numerical aperture is given by,
NA = Sin θa = 0.2445.
Result : The Numerical Aperture of the given optical fiber, NA=
0.2445.
19. Bending loss
The bend of a fiber causes loss in
transmittance and increase in attenuation as
the angle of incidence decreases at the points
where curveted radius is too small and the
condition of total internal reflection is not
fulfilled.
And since diameter decreases, intensity will
also decrease.
The loss will increase with respect to the
decrease in diameter.
21. (1) For single mode
S.N
o.
Diameter(cm) No. of
turns
Output
current from
the detector
(μA)
1. 6.5 2 138.0
2. 5.5 2 134.3
3. 4.5 2 130.7
4. 3.5 2 126.9
23. conclusion for bending loss
From the above figures and tables we conclude that
with increasing the diameter the intensity will be
increased and since intensity increase loss will be
decreased.
24. Splice loss
Splice loss: Optical power loss at the splicing point of two ends of optical fiber is
known as splice loss.
Transverse offset Angular offset End separation
s
25. Graph for splice loss
In this graph energy loss is represented, with respect to transverse and longitudinal
offsets →
26. Loss= αL=10log(Pi /Po)
where, α= attenuation coefficient,
L = length of fiber
Pi = input (or maximum) intensity(current),
Po= output intensity
Result: →
→ Made comparative study of loss tolerance to various offsets between two fibers at a
joint.
→Observed that splice loss is most sensitive to transverse misalignments.
