Optical Fibres

1. Fiber Optics

2. What are Fiber Optics?
 Fiber optics (optical fibers) are long, thin
strands of very pure glass about the
diameter of a human hair.
 They are arranged in bundles called
optical cables and used to transmit light
signals over long distances.

3. What are Fiber Optics?
 If you look closely at a single optical fiber,
you will see that it has the following parts:
 Core - Thin glass center of the fiber where
the light travels
 Cladding - Outer optical material
surrounding the core that reflects the light
back into the core
 Buffer coating - Plastic coating that protects
the fiber from damage and moisture

4. Fiber Optics
glass or plastic plastic jacket
fiber core cladding
TOTAL INTERNAL REFLECTION

5. What are Fiber Optics?

6. Fiber Optic Cables
SOURCE: SURFNET.NL

7. What are Fiber Optics?
 Single-mode fibers have small cores (about 3.5 x 10-4
inches or 9 microns in diameter) and transmit infrared
laser light (wavelength = 1,300 to 1,550 nanometers).
 Multi-mode fibers have larger cores (about 2.5 x 10-3
inches or 62.5 microns in diameter) and transmit
infrared light (wavelength = 850 to 1,300 nm) from
light-emitting diodes (LEDs).
 Some optical fibers can be made from plastic. These
fibers have a large core (0.04 inches or 1 mm diameter)
and transmit visible red light (wavelength = 650 nm)
from LEDs.

8. NUMERICAL APERTURE
The numerical aperture of the fiber is closely related to
the critical angle and is often used in the specification for
optical fiber and the components that work with it.
The numerical aperture is given by the formula:
2
2
2
N.A.  n1  n
The angle of acceptance is twice that given by the
numerical aperture

9. How Does an Optical Fiber
Transmit Light?
 Suppose you want to shine a flashlight beam down a
long, straight hallway.
 Just point the beam straight down the hallway -- light
travels in straight lines, so it is no problem. What if the
hallway has a bend in it?
 You could place a mirror at the bend to reflect the light
beam around the corner.
 What if the hallway is very winding with multiple bends?
 You might line the walls with mirrors and angle the
beam so that it bounces from side-to-side all along the
hallway. This is exactly what happens in an optical fiber.

10. How Does an Optical Fiber
Transmit Light?

11. How Does an Optical Fiber
Transmit Light?
 The light in a fiber-optic cable travels through the core
(hallway) by constantly bouncing from the cladding
(mirror-lined walls), a principle called total internal
reflection.
 Because the cladding does not absorb any light from the
core, the light wave can travel great distances.
 However, some of the light signal degrades within the
fiber, mostly due to impurities in the glass. The extent
that the signal degrades depends on the purity of the
glass and the wavelength of the transmitted light

12. Types of optical fibres:
 Glass core with plastic cladding(PCS)
 Glass core with glass cladding(SCS)
 Plastic core with plastic cladding

13. Mode of propagation
 Singlemode fibre
 Multimode fibre

14. Modes and Materials
 Since optical fiber is a waveguide, light can propagate in a number
of modes
 If a fiber is of large diameter, light entering at different angles will
excite different modes while narrow fiber may only excite one mode
 Multimode propagation will cause dispersion, which results in the
spreading of pulses and limits the usable bandwidth
 Single-mode fiber has much less dispersion but is more expensive
to produce. Its small size, together with the fact that its numerical
aperture is smaller than that of multimode fiber, makes it more
difficult to couple to light sources

15. Fibre configurations
 Singlemode Step Index Fibre(SMSI fibre)
 A SMSI fibre has a central core material of uniform
refractive index value.
 Diameter is small(8-10 μm)
 So there is only one path for propagation of light doen
the cable.
 Fabrication is difficult,so high production cost.
 Used in long haul communications.

16.  Multimode StepIndex Fibre(MMSI)
 Diameter of core is more than SMSI(50-100μm)
 There are many paths for light to propagate
 Due to multimode propagation and material
scattering,there is signal degradation.
 Fabrication is easy,so low production cost.
 Used in LANs

17. Advantages of Fiber Optics
 Less expensive .
 Thinner
 Higher carrying capacity
 Less signal degradation Light signals Low power
Digital signals Non-flammable
 Lightweight
 Flexible Medical imaging
 - in bronchoscopes, endoscopes, laparoscopes
 Mechanical imaging - inspecting mechanical welds in pipes
and engines (in airplanes, rockets, space shuttles, cars)
 Plumbing - to inspect sewer lines