Module 2 : Ray Model
Lecture : Ray Model
  The lowest value of corresponds to the ray traveling along the axis of the fiber. In fact this ray does not have any constraint on the size of the fiber etc, as it does not really go through the total internal reflection at the core cladding boundary. This ray therefore always propagates.
 

The optical fiber in which only one ray travels along the axis of fiber is called the single mode optical fiber .

Single mode optical fiber is the best amongst the three types of fibers, namely the step index fiber, GI fiber and the single mode fiber.

In a long distance communication, we use single mode optical fiber, whereas in LANs we generally use graded index optical fiber.

Note: For single mode optical fiber however we have to use a source like laser because the diameter of the fiber is very small and without a highly collimated beam, sufficient light can not be launched inside the fiber.

The three types of fibers have typical diameters as follows:

 

OPTICAL FIBERS CORE DIAMETER.

SM                           
GRADED INDEX  
STEP INDEX         
Note: The Cladding Diameter for all types of fibers has been standardized to

   
 

Limitations of the Ray-model

(1)
 The ray model gives an impression that during total internal reflection the energy is confined to the core only. However,

it is not so. In reality the optical energy spreads in cladding also.

(2)
The ray model does not speak of the discrete field patterns for propagation inside a fiber.
(3)
 The ray model breaks down when the core size becomes comparable to the wavelength of  light. The ray model
  therefore is not quite justified for a SM fiber.
  The limitations of the Ray model are overcome in the wave model discussed in the next module.
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