10-11-2012, 11:54 AM
INTRODUCTION TO OPTICAL FIBRE COMMUNICATIONS
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Fiber-optic communication
is a method of transmitting information from one place to another by sending light through an optical fiber.
The light forms an electromagnetic carrier wave that is modulated to carry information.
The process of communicating using fiber-optics involves the following basic steps:
Creating the optical signal using a transmitter,
relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak,
and receiving the optical signal and converting it into an electrical signal.
OPTICAL FIBER
An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length.
An optical fiber is essentially a waveguide for light.
Light is kept in the "core" of the optical fiber by total internal reflection.
Critical angle, θc
The minimum angle of incidence at which a light ray strike the interface of two media and result in an angle of refraction of 90° or greater.
ACCEPTANCE ANGLE
The maximum angle in which external light rays may strike the air/glass interface and still propagate down the fiber.
Advantages of Optical Fibre
High bandwidth
Long distance signal transmission
Light weight and small diameter
Immune to EMI and RFI
Highly Secure
Due its non-electrical nature can be used in many industrial applications………………
INDEX PROFILE
The index profile of an optical fiber is a graphical representation of the magnitude of the refractive index across the fiber.
The refractive index is plotted on the horizontal axis, and the radial distance from the core axis is plotted on the vertical axis.
STEP-INDEX
A step-index fiber has a central core with a uniform refractive index. An outside cladding that also has a uniform refractive index surrounds the core;
however, the refractive index of the cladding is less than that of the central core.
GRADED-INDEX
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary.
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
Dispersion
Dispersion in fiber optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others
Single-mode fibers are relatively free from dispersion except for intramodal dispersion
Graded-index fibers reduce dispersion by taking advantage of higher-order modes
One form of intramodal dispersion is called material dispersion because it depends upon the material of the core
Another form of dispersion is called waveguide dispersion
Dispersion increases with the bandwidth of the light source
Single Mode Step Index Fiber
Single mode or mono mode step index fiber has a very thin core of about 5µm diameter and has a relatively larger cladding of glass or plastic as shown in the fig. It has a very thin core. A strong monochromatic light source i.e. a laser source is used to send light signals through it .It can carry more than 14 TV channels or 14000 phone calls.
Multimode Step Index Fiber
This type of fiber has a core of relatively larger diameter about 50μm.It is mostly used for carrying white light but due to dispersion effects, it is useful for a short distance only. The fiber core has a constant refractive index n1 (1.5) from center to the boundary on both sides .The cladding has a lower refractive index as 1.48 which remains constant throughout the cladding. This is called step –index multimode fiber, because the refractive index steps down from 1.52 to 1.48 at the boundary with the cladding.