fiber optics pdf

2. Optical Fibre
• An optical fiber is a flexible, transparent fiber made by 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
• It has wide usage in fiber-optic communications, where it permit
transmission over longer distances and at higher bandwidths (data
rates) than electrical cables.

3. Fiber Optics Basics - Construction
• The optical fibre consists of two
layers—
1. the Core and
2. the Cladding

4. Fiber Optics Basics - Construction
The core:
It is the inner cylindrical structure of an optical
fiber. It is made of some optically pure
transparent material like glass, quartz, plastic,
etc.
The cladding:
It is the outer coaxial cylindrical jacket of the
optical fiber. It is made of some transparent
material having refractive index slightly lower
than that of the core material

5. • For a typical step-index optical fibre, the
refractive index of the core material, n1= 1.48.
And the refractive index of the cladding, n2= 1.46.
• The fibre is given an acrylic coating which serves
as a jacket. It is then enclosed in several other
coaxial layers, namely a buffer jacket, a sheath
and an outer jacket to provide strength and
protection to the cable.
• The diameter of an optical fibre varies from 5
microns (1 micron =10-6 metre) to several
hundreds of microns.

6. THE NECESSITY OF THE CLADDING
• The cladding is necessary because a long fibre transmitting light from
one place to another is to be supported along its length and thus the
fibre is likely to be distorted. This is avoided by using sufficiently thick
and hard cladding over the central core. Again, in a bundle of fibres,
the cladding also prevents leakage of light from one fibre to another

7. Principle of operation
1. Index of refraction:
• The refractive index of the core
must be greater than that of the
cladding.
2. Total internal reflection
• This effect is used in optical fibers
to confine light in the core. The
light incident at core and cladding
interface should must have angle
of incidence greater then the
critical angle.

8. Fiber Types:
There are basically three types of
optical fiber:
1. Single mode fiber
2. Multimode graded index fiber
3. Multimode step index fiber
• They are characterized by the way
light travels down the fiber and
depend on both the wavelength
of the light

9. 1. SINGLE-MODE FIBER
• SINGLE-MODE FIBER
• has a narrow core (eight microns or less), and the index of refraction between
the core and the cladding changes less than it does for multimode fibers.
Light thus travels parallel to the axis, creating little pulse dispersion.
Telephone and cable television networksinstall millions of kilometers of this
fiber every year

10. 2. GRADED-INDEX MULTIMODE FIBER
• GRADED-INDEX MULTIMODE FIBER
• contains a core in which the refractive index
diminishes gradually from the center axis out
toward the cladding.
• The higher refractive index at the center makes
the light rays moving down the axis advance more
slowly than those near the cladding.
• Also, rather than zigzagging off the cladding, light
in the core curves helically because of the graded
index, reducing its travel distance. The shortened
path and the higher speed allow light at the
periphery to arrive at a receiver at about the
same time as the slow but straight rays in the core
axis.
• The result: a digital pulse suffers less dispersion.

11. 3. STEP-INDEX MULTIMODE FIBER
STEP-INDEX MULTIMODE FIBER
• Has a large core, up to 100 microns
in diameter. As a result, 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.
• This type of fiber is best suited for
transmission over short distances, in
an endoscope, for instance

12. Advantages
1. Broad bandwidth: A single optical fiber can carry over
3,000,000 voice calls or 90,000 TV channels.
2. Immunity to electromagnetic interference: Light transmission
through optical fibers is unaffected by other electromagnetic
radiation nearby.
3. Low attenuation loss over long distances: Attenuation loss can be
as low as 0.2 dB/km in optical fiber cables, allowing transmission
over long distances without the need for repeaters.
4. Electrical insulator: Optical fibers do not conduct electricity,
preventing problems with ground loops and conduction
of lightning.

13. USESOF OPTICALFIBERS
1. Optical fibers are extensively used for communication
purposes.
2. It is also used for the purpose of medical investigations.
❖With the help of optical fibers it is possible to study the
interior of the lungs and other parts of human body that
cannot be viewed directly.
❖ It is also useful in the study of tissues and blood-vessels far
below the skin.
➢For this purpose a bundle of fibers is enclosed in a hypodermic
needle and pushed into the area under investigation.
❖A number of optical fibers may be grouped into a flexible
coherent bundle. Such an arrangement is called an
ENDOSCOPE which is employed to make observations of a
place that can be approached only along a curved path.