E-field polarization in the direction of propagation, TEM, TE, TM

[olgar]

I read that for a plane wave E, B and k are always perpendicular to each other but then in a TM mode I have electric field in the direction of propagation. This means this TM and TE modes I cannot have in free space? Can I have them in a laser? If I can have some E-field in the direction of propagation why I cannot have it all along the direction of propagation, or most of it?

Thanks!

 

[toneboy1]

I wish I could help but my knowledge of QM is limited to potential wells. What are TEM, TE and TM modes? Also k is the propergation vector isn't it? If so I would assume they were always perpendicular. (I actually came on here to see if anyone was giving an explanation of how circular polarisation works because I don't get it)

 

[wukunlin]

when looking at a waveguide which electromagnetic waves travels in, the wave equation actually has the same form as the Schrodinger's Equation so we tend to use their solutions and just replace with relevant variable. The "potential" of our "wells" are determined by the refractive indices of our waveguide and surrounding materials.

TE modes and TM modes, while the typical interpretation refers to whether there are E field or B field in the direction of propagation, its not really something easy to get your head around (took me a while anyway). They are really just the solution of the equation using different orders of all those different bessel functions. In my optical communications class we used Yariv's "Optical Electronics in Modern Communications" they are briefly described them with some maths shown. Most of the maths about it are from my lecturer's slides I will have to dig through them again to see if they cited where they got those derivations from :D

 

[toneboy1]

TE modes and TM modes, while the typical interpretation refers to whether there are E field or B field in the direction of propagation

you mean along the waveguide? Perhaps a picture drawn in MS paint would help me understand.

When you said "The "potential" of our "wells" are determined by the refractive indices of our waveguide and surrounding materials."

do you mean like the waveguide is like one big Well and the sides of the waveguide is like the potential barrier?

Finally, do you know anything about circular polarisation compared to horizontal or vertical?

Cheers

 

[the_emi_guy]

Electromagnetic waves in free space travel in TEM mode. TE and TM modes are found when waves are guided in a microwave waveguide or fiberoptic cable. In such a waveguide you have the equivalent of two TEM mode waves traveling at an angle to each other and each reflecting off of opposite sides of the waveguide boundaries. Their superimposed amplitudes of these zig-zagging waves form the TE or TM mode. Which mode (i.e. TE10, TE20) has to do with how many cycles of the wave occur between reflections off of the guide boundaries.

You will usually read that only TE and TM modes are possible within microwave waveguide. However this does assume that you have this pair of zig-zagging waves. If I have a huge WR2300 waveguide and I shine a laser straight down the middle of it, the laser light will still be TEM mode since it is not interacting with the guide walls.

Also, it is possible to guide an electromagnetic wave and still have it be TEM if there are more than one conductor involved. An example is a stripline.

 

[toneboy1]

Very interesting.