What are the physical significance of TE and TM modes in an multi and single mode optical fiber?
Dear Hariharan,
The word transverse indicates its absence in the same direction or in other words perpendicular to some reference. Here, for example transverse electric means "Electric field does not exist in the direction of propagation. Say the wave is propagating in the z-direction, then in TE mode, E-field in the z-directions is absent (Ez =0; but Hz is not equal to zero!). Similarly TM mode has no magnetic field in the direction of propagation. Also, in TEM mode both E- and H- field remain absent in the direction of propagation.
When it comes to optical fiber, as there exists no perfect metal boundary (E-filed does not vanish at the surface or boundary), hybrid modes get excited i., HE or EH modes exist.
Dear Hariharan,
The word transverse indicates its absence in the same direction or in other words perpendicular to some reference. Here, for example transverse electric means "Electric field does not exist in the direction of propagation. Say the wave is propagating in the z-direction, then in TE mode, E-field in the z-directions is absent (Ez =0; but Hz is not equal to zero!). Similarly TM mode has no magnetic field in the direction of propagation. Also, in TEM mode both E- and H- field remain absent in the direction of propagation.
When it comes to optical fiber, as there exists no perfect metal boundary (E-filed does not vanish at the surface or boundary), hybrid modes get excited i., HE or EH modes exist.
For a waveguide to truly support TE and TM, it must be: (1) an enclosed metal waveguide, and (2) a homogeneous dielectric fill inside the waveguide. In this case, it is possible to rigorously manipulate Maxwell's equations into two different wave equations. One wave equation is in terms of Ez and the other in terms of Hz, assuming the waveguide extends in the z direction. This means they are separate problems. We are free to set Ez=0 and solve for Hz. Since we set Ez=0, the E field is completely transverse to the direction of propagation and we call it a TE mode. We are also free to set Hz=0 and solve for Ez. Since the H field is completely transverse to the direction of propagation, we call it a TM mode.
I think you will notice that the lower-order modes in a dielectric waveguide are nearly TE and TM. If you analyze them as such, you can get a pretty accurate answer and these are usually called the quasi-TE and quasi-TM modes so that we do not confuse them for real TE and TM modes.
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