Topic – 4 Laser and Fibre Optics
What are the conditions for the total internal reflection to take place?
Angle of incidence in the optically denser medium must be greater than the critical
angle. Light ray must travel from an optically denser medium to an optically less dense
medium.
What is laser? What are the important characteristics of laser?
The word ‘Laser’ is an acronym for Light Amplification by Stimulated Emission of
Radiation. The laser beam (i) is monochromatic. (ii) is coherent, with the waves, all exactly
in phase with one another, (iii) does not diverge at all and (iv) is extremely intense
What are the laws of reflection?
1. The incident ray, the reflected ray and the normal drawn to the reflecting surface
at the point of incidence, all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection (i.e.) i = r
Define refractive index.
The ratio of the velocity of light c in a vacuum to its velocity v in a specified medium.
𝑐
𝜇=
𝑣
.
Explain the block diagram of fibre optic communication system?
Fiber optical communication
Coherent light can be generated with laser or light emitting diodes and may be
detected by photo-diodes. Optical fibers are used for transmission of light. An optical
fiber is a thin transparent rod, usually made of glass or plastic, through which light
can propagate. The light signals travel through the rod from the transmitter to the
receiver and can be easily detected at the receiving end of the optical fiber. The
Related to Unit – V Photonics and Fibre optics
Engineering Physics - I
Topic – 4 Laser and Fibre Optics
principle of total internal reflection is used for the transmission of light signals
through the optical fiber.
Advantages
(i) Transmission loss is low.
(ii) Fiber is lighter and less bulky than equivalent copper cable.
(iii) More information can be carried by each fiber than by equivalent copper cables.
(iv) There is no interference in the transmission of light from electrical disturbances
or electrical noise.
Applications
The various applications of fiber in communication area are, voice telephones, video
phones, message services, data network etc.
Fibre optics Communication Block Diagram
Explain the principle of propagation of light in an optical fibre.
The total internal reflection is the basic
principle of optical fibre. An optical fibre is a
very thin fibre made of glass or quartz
having radius of the order of micrometer
(10–6m). A bundle, of such thin fibres forms
a ‘light pipe’ shows the principle of light
transmission inside an optical fibre. The
refractive index of the material of the core is
higher than that of the cladding. When the
light is incident at one end of the fibreat a
small angle, the light passes inside,
undergoes repeated total internal reflections
along the fibre and finally comes out. The
angle of incidence is always larger than the
critical angle of the core material with
Related to Unit – V Photonics and Fibre optics
Engineering Physics - I
Topic – 4 Laser and Fibre Optics
respect to its cladding. Even if the fibre is bent or twisted, the light can easily travel
through the fibre.
Light pipes are used in medical and optical examination. They are also used to
transmit communication signals.
List out the various applications of laser.
Medical Uses of Lasers
The highly collimated beam of a laser can be further focused to a microscopic
dot of extremely high energy density. This makes it useful as a cutting and cauterizing
instrument. Lasers are used for photocoagulation of the retina to halt retinal
hemorrhaging and for the tacking of retinal tears. Higher power lasers are used after
cataract surgery if the supportive membrane surrounding the implanted lens becomes
milky.
Welding and Cutting
The automobile industry makes extensive use of carbon dioxide lasers with
powers up to several kilowatts for computer controlled welding on auto assembly lines.
Surveying and Ranging
Helium-neon and semiconductor lasers have become standard parts of the field
surveyor's equipment. A fast laser pulse is sent to a corner reflector at the point to be
measured and the time of reflection is measured to get the distance.
Lasers in the Garment Industry
The programmed cutter can cut dozens to hundreds of thicknesses of cloth, and can
cut out every piece of the garment in a single run.
Lasers in Communication
Fiber optic cables are a major mode of communication partly because multiple signals
can be sent with high quality and low loss by light propagating along the fibers. The light
signals can be modulated with the information to be sent by either light emitting diodes or
lasers.
Heat Treatment
Heat treatments for hardening or annealing have been long practiced in metallurgy.
For example, lasers can provide localized heat treatments such as the hardening of the
surfaces of automobile camshafts.
Barcode Scanners
Supermarket scanners typically use helium-neon lasers to scan the universal barcodes
to identify products.
Related to Unit – V Photonics and Fibre optics
Engineering Physics - I