Total Internal Reflection

Total internal reflection is an optical phenomenon that happens when a ray of light strikes a
medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can
pass through and all of the light is reflected. The critical angle is the angle of incidence above
 which the total internal reflection occurs.

 

When light crosses a boundary between materials with different refractive induces, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the ray is closer to being parallel to the boundary) than the
critical angle – the angle of incidence at which light is refracted such that it travels along the
boundary – then the light will stop crossing the boundary altogether and instead be totally reflected back internally. This can only occur where light travels from a medium with a higher
[n1=higher refractive index] to one with a lower refractive index [n2=lower refractive index].
For example, it will occur when passing from glass to air, but not when passing from air to glass.


Critical Angle






                     The critical angle is the angle of incidence above which total internal reflection occurs.The angle of incidence is measured with respect to the normal at the refractive boundary.
The critical angle θ_c is given by:

where n2 is the refractive index of the less optically dense medium, and n1 is the refractive
 index of the more optically dense medium.

The derivation for this is simple, and can easily be demonstrated using Snell's Law:

Rearranging Snell's Law, we get sin θr = (n1/n2)sin θi .
To find the critical angle, we find a value for θi such that the left hand side of this
equation equals 1. The resulting value of θi is equal to the critical angle θc.

Refraction

 Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth. Refraction is described by Snell's law, which states that the angle of incidence θ1 is related to the angle of refraction θ2 by

              sinθ1/sinθ2 = v1/v2 =n2/n1

 where v1 and v2 are the wave velocities in the respective media, and n1 and n2 the refractive induces. In general, the incident wave is partially refracted and partially reflected; the details of this behavior are described by the Fresnel equations.

Laws Of Reflection

If the reflecting surface is very smooth, the reflection of light that occurs is called specular or
regular reflection. The laws of reflection are as follows:

• The incident ray, the reflected ray and the normal to the reflection surface at the point of  the   incidence lie in the same plane.
• The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

Here are descriptions for the terms in this diagram:

• The ray of light which strikes the surface is called the incident ray.
• The ray of light which leaves the surface is called the reflected ray.
• A line perpendicular to the surface is imagined at the point of reflection. This line is called a normal. In this context the word normal means perpendicular. In the above diagram the normal is colored blue.
• The angle between the incident ray and the normal is called the angle of incidence, or the incident angle.
•  The angle between the reflected ray and the normal is called the angle of reflection, or the reflected angle.

OPTICAL PHENOMENA

Reflection

Reflection of light is either specular  or diffuse depending on the nature of the interface.Furthermore, if the interface is between a
dielectric and a conductor,the phase of the reflected wave is retained, otherwise if the interface
is between two dielectrics, the phase may be retained or inverted, depending on the induces of
refraction.

A mirror provides the most common model for specular light reflection,and typically consists of a glass.sheet with a metallic coating where the reflection actually occurs.Reflection is enhanced in metals by.suppression of wave propagation beyond their skin depths. Reflection also occurs at the surface of.transparent media,such as water or glass.

                                 In fact,reflection of light may occur whenever light travels from a medium of a given refractive.index into a medium with a different refractive index.In the most general case,a certain fraction of the light is reflected from the interface,and the remainder is refracted.Solving Maxwell's equations for a light ray striking a boundary allows the derivation of the Fresnel equations,which can be used to predict how much of the light reflected, and how much is refracted in a given situation. Total internal reflection of light from a denser medium occurs if the angle of incidence is above the critical angle.

When light reflects off a material denser (with higher refractive index) than the external medium, it undergoes a polarity inversion. In contrast, a less dense, lower refractive index material will reflect light in phase. This is an important principle in the field of thin-film optics.

Advantages

Optical Fiber  Transmission Vs Copper Transmission

• Very low attenuation
• High available bandwidth
• Small size & weight
• High electrical isolation
• Higher immunity to noise & interference
• Higher security   

Fiber Types

Fiber Types

There are two type of fibers

1. Multimode Fiber
2. Monomode Fiber   

Multimode Fiber

 

                         Core:50μm                                    

Cladding:125μm   

 

                                                                           50/125μm

                                                                    Core:62.5μm
                                                                    Cladding:125μm

                                                                         62.5/125μm 

Multimode fiber has two types 

1.  Step Index
2. Graded Index

Graded Index
                       
                          In this type of fiber,there is no step change of refractive index between core and the cladding.The core has a refractive index that gradually decreases  from the center to out words  to outer surface of fiber.

                                                                                           
                            The rays close to the center are slowed down because they travel smaller distance.The rays traveling away from the axis are speed up using law refractive index than the center.There for they can catched up.Now all the rays will arrive at nearly same times at  the Rx end.There for law dispersion-end  high band width is achieved.Graded index just help to all the rays with entered to fiber with any angle through acceptance cone.


  Mono-mode Fiber
                  To minimize dispersion we can reduce core diameter to very small valuves such as 5-10 μm 

                         How ever cast of mono-mode fiber high due to manufacturing difficulties.For a multimode fiber 'NA' is between 0.2-0.3.But in a mono-mode fiber 'NA' is approximately 0.1 there for NA help us to identify the capability of the fiber.When the 'NA' is low the fiber is capable of transmitting higher bit rates.

                                                                       

Fiber Construction

                                     

These images showing the cross section of fiber cable. 













Fiber Optic Cable Manufacturing








Three types of fiber construction

• PCP:Plastic Cladding with Plastic Core
• PCS:Plastic Cladding with Glass Core
• SCS:Glass Cladding with Glass Core  

 Fiber Materials 
     
Core 

• Glass: Doped silica
• Plastic: Polystyrene  ,Polymethylmathaccrillic 

Cladding

• Glass:Doped silica
• Plastic:Silicon,Teflon

Introduction

In the past the light was used only to see things but not to communicate.Now we know that use of optical(light) waves can carry large amount of information.The air(Atmosphere) is not a very good transmission medium for optical communication.There are many reason for make signal attenuate..optical communication become a reality with the development of optical fibers.The optical fiber communication belongs to the category of wired communication.To carry this optical frequencies over long distance we need a guiding device.This device is the optical fiber.