Fiber Optics in Brief


What is the Fiber Optics Communication System?

Fiber Optics:

Optical Fiber is new medium, in which information (voice, Data or Video) is transmitted through a glass or plastic fiber, in the form of light, following the transmission sequence give below :

(1) Information is encoded into electrical signals.

(2) Electrical signals are converted into light signals.

(3) Light travels down the fiber.

(4) A detector changes the light signals into electrical signals.

(5) Electrical signals are decoded into information.

ADVANTAGES OF FIBER OPTICS :

Fiber Optics has the following advantages :

(I) Optical Fibers are non conductive  (Dielectrics)

–  Grounding and surge suppression not required.

– Cables can be all dielectric.

(II) Electromagnetic Immunity :

– Immune to electromagnetic interference (EMI)

– No radiated energy.

– Unauthorised tapping difficult.

(III) Large Bandwidth (> 5.0 GHz for 1 km length)

– Future upgradability.

– Maximum utilization of cable right of way.

– One time cable installation costs.

(IV) Low Loss (5 dB/km to < 0.25 dB/km typical)

–  Loss is low and same at all operating speeds within the fiber’s specified bandwidth long, unrepeated links (>70km is operation).

(v) Small, Light weight cables.

–  Easy installation and Handling.

– Efficient use of space.

(vi) Available in Long lengths (> 12 kms)

–   Less splice points.

(vii) Security

– Extremely difficult to tap a fiber as it does not radiate energy that can be received by a nearby antenna.

– Highly secure transmission medium.

(viii) Security – Being a dielectric

–  It cannot cause fire.

–  Does not carry electricity.

–  Can be run through hazardous areas.

(ix) Universal medium

–  Serve all communication needs.

–  Non-obsolescence.

APPLICATION OF FIBER OPTICS IN COMMUNICATIONS :

–  Common carrier nationwide networks.

–  Telephone Inter-office Trunk lines.

–  Customer premise communication networks.

–  Undersea cables.

–  High EMI areas (Power lines, Rails, Roads).

–   Factory communication/ Automation.

–   Control systems.

–   Expensive environments.

– High lightening areas.

–   Military applications.

–   Classified (secure) communications.

 Transmission Sequence :

(1) Information is Encoded into Electrical Signals.

(2) Electrical Signals are Coverted into light Signals.

(3) Light Travels Down the Fiber.

(4) A Detector Changes the Light Signals into Electrical Signals.

(5) Electrical Signals are Decoded into Information.

–  Inexpensive light sources available.

–  Repeater spacing increases along with operating speeds because low loss fibers are used at high data rates.

Fiber Optics

Principle of Operation – Theory

  • Total Internal Reflection – The Reflection that Occurs when a Ligh Ray Travelling in One Material Hits a Different Material and Reflects Back into the Original Material without any Loss of Light.

Fiber Optics Principle of Operation

THEORY AND PRINCIPLE OF FIBER OPTICS

Speed of light is actually the velocity of electromagnetic energy in vacuum such as space. Light travels at slower velocities in other materials such as glass. Light travelling from one material to another changes speed, which results in light changing its direction of travel. This deflection of light is called Refraction.

The amount that a ray of light passing from a lower refractive index to a higher one is bent towards the normal. But light going from a higher index to a lower one refracting away from the normal, as shown in the figures.

As the angle of incidence increases, the angle of refraction approaches 90o  to the normal. The angle of incidence that yields an angle of refraction of 90o is the critical angle. If the angle of incidence increases amore than the critical angle, the light is totally reflected back into the first material so that it does not enter the second material. The angle of incidence and reflection are equal and it is called Total Internal Reflection.

PRINCIPLE OF FIBER OPTICS

PROPAGATION OF LIGHT THROUGH FIBER

The optical fiber has two concentric layers called the core and the cladding. The inner core is the light carrying part. The surrounding cladding provides the difference refractive index that allows total internal reflection of light through the core. The index of the cladding is less than 1%, lower than that of the core. Typical values for example are a core refractive index of 1.47 and a cladding index of 1.46. Fiber manufacturers control this difference to obtain desired optical fiber characteristics.

Most fibers have an additional coating around the cladding. This buffer coating is a shock absorber and has no optical properties affecting the propagation of light within the fiber.

Figure shows the idea of light travelling through a fiber. Light injected into the fiber and striking core to cladding interface at grater than the critical angle, reflects back into core, since the angle of incidence and reflection are equal, the reflected light will again be reflected. The light will continue zigzagging down the length of the fiber.

Light striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly. Propagation of light through fiber is governed by the indices of the core and cladding by Snell’s law.

Such total internal reflection forms the basis of light propagation through a optical fiber. This analysis consider only meridional rays- those that pass through the fiber axis each time, they are reflected. Other rays called Skew rays travel down the fiber without passing through the axis. The path of a skew ray is typically helical wrapping around and around the central axis. Fortunately skew rays are ignored in most fiber optics analysis.

The specific characteristics of light propagation through a fiber depends on many factors, including

– The size of the fiber.

– The composition of the fiber.

– The light injected into the fiber.

Total internal reflection in an optical fiber

FIBER GEOMETRY

An Optical fiber consists of a core of optically transparent material usually silica or borosilicate glass surrounded by a cladding of the same material but a slightly lower refractive index.

Fiber themselves have exceedingly small diameters. Figure shows cross section of the core and cladding diameters of commonly used fibers. The diameters of the core and cladding are as follows.

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