Talk:Transverse mode

TEoi* modes...

The article states

"The TEM0i* mode, the so-called doughnut mode, is a special case consisting of a superposition of two TEM0i modes (i=1,2,3), rotated 360°/4i with respect to one another."

Is this referring to two instances of the same mode, one being rotated then both being combined?

Or does it mean "TEM01 and TEM02 (for example) are rotated relative to each other and combined"?

It does not appear to form a 'doughnut' in the second case - could the wording on this be improved if the meaning is the former?

Assessment comment

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It says that TEM modes can exist in a waveguide. When Ez and Hz equal 0 this makes all other field components equal to zero, hence a TEM wave cannot exist inside a waveguide. Candy Coloured Clown 15:51, 26 February 2007 (UTC)

Last edited at 15:51, 26 February 2007 (UTC). Substituted at 09:11, 30 April 2016 (UTC)

What we call TE and TM in semiconductor labs

In the semiconductor field :

TE mode : mode with neither electric nor magnetic field in the direction of propagation AND electric field in the plane of the wafer

TM mode : mode with neither electric nor magnetic field in the direction of propagation AND magnetic field in the plane of the wafer

So if x is the direction of propagation and z the direction perpendicular to the wafer plane :

TE mode : E=Ey, H=Hz

TM mode : E=Ez, H=Hy — Preceding unsigned comment added by Ianniss (talk (talk • contribs) 14:55, 12 February 2017 (UTC)

Isn't that just two different orientations of TEM? SpinningSpark 21:12, 12 February 2017 (UTC)

He described it wrong. A TE mode has a magnetic field component in the direction of propagation. A TM mode has an electromagnetic component in the direction of propagation.--Srleffler (talk) 05:53, 17 February 2017 (UTC)

That's my understanding of TE and TM as well, but Ianniss seems to be saying that in his field it means something different. I don't think he has simply misstated it. As always, we need a source on that before putting anything in the article. SpinningSpark 12:43, 17 February 2017 (UTC)

I'm pretty sure he is mistaken, particularly with reference to modes in semiconductor waveguides. Whether he simply misstated it or has misunderstood it, I cannot say.--Srleffler (talk) 06:07, 18 February 2017 (UTC)

Yes in semiconductor field we always use TEM modes but we TE and TM words to call two different orientation of the TEM mode. I know it's confusing but it's how people name in this field in all papers and all books... — Preceding unsigned comment added by 81.194.35.225 (talk) 15:05, 20 February 2017 (UTC)

That's confirmed in books accessible in gbooks [1][2][3] SpinningSpark 21:00, 20 February 2017 (UTC)

Are transverse modes good, bad or indifferent?

Our coaxial cable article says that having multiple TM modes is "undesirable" since they can interfere with each other, but it doesn't say why that's a problem. I'm guessing that you might get fluctuating amplitude at the load end of the cable. If we can answer this question for the coax article, the same answer should apply to this article. --Heron (talk) 11:22, 26 May 2020 (UTC)

@Hwron: In general, any mode other than the design mode is undesirable. Coaxial cable (in the vast majority of cases) is operated in the TEM mode, so yes, TM modes are undesirable. It is not the mode per se that is undesirable, it is multiple modes existing simultaneously that is undesirable. The coax article already says that. It also already says that different modes have different phase velocities. The reason this is a problem is that it causes dispersion which distorts the wave shape and corrupts data. SpinningSpark 23:08, 26 May 2020 (UTC)

@Heron: pinging again with correct spelling this time. SpinningSpark 23:10, 26 May 2020 (UTC)

Just to expand on that a bit, having multiple modes is a problem only where that isn't part of the design. For example, multi-mode optical fiber is commonly used for short-distance communications, while longer distances are spanned by single-mode optical fiber, which avoids the dispersion due to different modes propagating at different speeds. Multi-mode fiber systems are significantly cheaper. Multi-mode fiber can also carry significantly higher optical power than single-mode fiber can.--Srleffler (talk) 23:29, 30 May 2020 (UTC)

Multi-mode optical fibre isn't exactly desirable, it's just that it's tolerated (over short distances) for the sake of economy. However, there are some equipment designs that exploit a positive advantage from multi-mode operation. See Waveguide filter#Dual-mode filter for instance. SpinningSpark 00:29, 31 May 2020 (UTC)

Engineering is always about optimizing a technological solution to fit the constraints of the problem, and cost is frequently one of those constraints. A cheaper solution for a problem that does not require a more expensive solution is the right solution to that problem. Besides that, multi-mode propagation is certainly desirable when high power needs to be carried over a short distance, such as in a laser welding system.--Srleffler (talk) 00:51, 31 May 2020 (UTC)