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Fiber Optics
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 lightsignals over long distances.
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
Fiber Optics
plastic jacketglass or plasticcladdingfiber core
TOTAL INTERNAL REFLECTION
What are Fiber Optics?
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.
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:
The angle of acceptance is twice that given by the numerical aperture
2
2
2
1.. nnAN
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.
How Does an Optical Fiber Transmit Light?
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
Types of optical fibres:
Glass core with plastic cladding(PCS)
Glass core with glass cladding(SCS)
Plastic core with plastic cladding
Mode of propagation
Singlemode fibre
Multimode fibre
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
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 doenthe cable.
Fabrication is difficult,so high production cost.
Used in long haul communications.
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
Advantages of Fiber Optics
Less expensive .
Thinner
Higher carrying capacity
Less signal degradation Light signals Low powerDigital 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
