Construction of optical fiber cable

Construction of optical fiber cable

An optical fiber is a very thin strand of silica glass in geometry quite like a human hair. In reality it is a very narrow, very long glass cylinder with special characteristics. When light enters one end of the fiber it travels until it leaves the fiber at the other end. An optical fiber consists of two parts: the core and the cladding. The core is a narrow cylindrical strand of glass and the cladding is a tubular jacket surrounding it. The core has a (slightly) higher refractive index than the cladding. Light travelling along the core is confined by the mirror to stay within it even when the fiber bends around a corner.

A fiber optic cable has an additional coating around the cladding called the jacket. The jacket usually consists of one or more layers of polymer. Its role is to protect the core and cladding from shocks that might affect their optical or physical properties. It acts as a shock 14

absorber. The jacket also provides protection from abrasions, solvents and other contaminants. The jacket does not have any optical properties that might affect the propagation of light within the fiber optic cable.

1.Guiding mechanism in optical fiber

Light ray is injected into the fiber optic cable on the right. If the light ray is injected and strikes the core-to-cladding interface at an angle greater than an entity called the critical angle then it is reflected back into the core. Since the angle of incidence is always equal to the angle of reflection the reflected light will again be reflected. The light ray will then continue this bouncing path down the length of the fiber optic cable. If the light ray strikes the core-to-cladding interface at an angle less than the critical angle then it passes into the cladding where it is attenuated very rapidly with propagation distance. Light can be guided down the fiber optic cable if it enters at less than the critical angle. This angle is fixed by the indices of refraction of the core and cladding and is given by the formula

The critical angle is measured from the cylindrical axis of the core. By way of example, if n1 = 1.446 and n2 = 1.430 then a quick computation will show that the critical angle is 8.53 degrees, a fairly small angle.

Of course, it be noted that a light ray enters the core from the air outside, to the left of Figure. The refractive index of the air must be taken into account in order to assure that a light ray in the core will be at an angle less than the critical angle. This can be done fairly simply. Suppose a

light ray enters the core from the air at an angle less than an entity called the external acceptance angle It will be guided down the core.

2. Basic component of optical fiber communication

1           Transmitters - Fiber optic transmitters are devices that include an LED or laser source, and signal conditioning electronics, to inject a signal into fiber. The modulated light may be turned on or off, or may be linearly varied in intensity between two predetermined levels.

2           Fiber – It is the medium to guide the light from the transmitter to receiver.

3           Receivers – Fiber optic receivers are instruments that convert light into electrical signals. They contain a photodiode semiconductor, signal conditioning circuitry, and an amplifier at the receiver end.

Process of Optical Fiber Communication –

A serial bit stream in electrical form is presented to a modulator, which encodes the data appropriately for fiber transmission.

A light source (laser or Light Emitting Diode - LED) is driven by the modulator and the light focused into the fiber.

The light travels down the fiber (during which time it may experience dispersion and loss of strength).

At the receiver end the light is fed to a detector and converted to electrical form.

The signal is then amplified and fed to another detector, which isolates the individual state changes and their timing. It then decodes the sequence of state changes and reconstructs the original bit stream.

The timed bit stream so received may then be fed to a using device