Module 3 : Wave Model
Lecture   : Wave Model
3. Modes of optical fiber
1. A mode can be characterized by two parameters, and the solution number.
   
2 (a) If we take , all field components are expressed in terms of and whatever fields we get, they do not have any
 

magnetic field component in the direction of propagation. We call this mode the Transverse Magnetic mode (TM mode).

Similarly if , the mode is called the Transverse Electric mode (TE mode).

(b) If both the longitudinal components of the fields ( and ) are non-zero then we call the mode the Hybrid mode.
  This mode is a combination of TE and TM modes.
(c) So inside an optical fiber we have three types of modes
1.  TE modes
2.  TM modes
3.  Hybrid modes
(d) For a hybrid mode, if we calculate the contribution by and to the transverse fields, one of them i.e. or would
 

dominate. Depending upon which of them contributes more, we can sub-classify the Hybrid modes.
If Dominates EH mode
If Dominates HE mode

   
4. Each of the above three modes are characterized by two indices, and (solution number). The mode are therefore
  designated as &
   
5.

What do these two indices physically mean?

  In a rectangular wave guide, they represent the number of half cycle variations and the number of zero crossings in the x direction and in y directions. The same thing is applicable in this case also.
(a) If , the function is constant and the field distribution is circularly symmetric.
(b) For we get the one cycle variation in direction, if we get two cycle variation in the direction, and so on.
(c)

The first index of the mode therefore gives the zero crossings in the azimuth of the cross section of the optical fiber.

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