Application of EM backward wave

[alexyan]

Hi, does anybody who knows the applications of backward type wave tell the detail of them? thank you very much!

 

[Tide]

I don't understand the question. Can you explain what you mean?

 

[alexyan]

the application of electromagnetic backward waves. thank you!

 

[Tide]

Do you mean reflected EM waves?

 

[ZapperZ]

the application of electromagnetic backward waves. thank you!

alexyan,

I am beginning to see a pattern in your questions. As in your posting asking for "plasmonic resonance", you seem to make the assumption that by throwing out a phrase or two, the question is clear. It isn't!

You need to put your question in an appropriate CONTEXT! In other words, either give an exact reference where you read it from, or be as specific as possible on the area of physics that term is being used.

If not, a lot of effort and time are wasted in just trying to decipher exactly what you mean. If you wish others to spend some effort in providing you with a complete answer, you should consider spending at least some effort in presenting a clear and unambiguous question.

Zz.

 

[alexyan]

thank you for your advice. I try to make clear questions.

 

[alexyan]

I have studied some kind of guides wave struxtrues. I know backward wave can exist in some structures. also I understand the concept of backward wave. but I do not know how to use this property, so could somebody know it tell me the applications?

 

[LURCH]

Alex,
I'm just guessing here, but would I be correct in assuming that English is not your first language? I think the confusion in this thread is the definition of the term "backward wave". This would appear to be some term that you are attempting to translate into English from some other language, but the translation you've come up with, "backward wave", is not a term with which our English-speaking members are familiar.

Could you describe exactly what a "backward wave" is? Do you mean a wave that has been reflected back towards its source? A wave pattern that is inverted from a previously existing wave? A negative energy wave? Also, could you please provide a link or reference to some source that talks about "backward waves"?

 

[Claude Bile]

When analysing waveguides and the mode coupling of the waveguide, the existing wave in the structure is often decomposed into two components - a forward propagating wave and a backward propagating wave.

There is a family of devices that couple light from the forward propagating wave to the backward propagating wave calling gratings, so called because they posess a periodic refractive index profile. Becuase the backward coupling is dependant on resonances within the structure, the coupling is highly wavelength dependant, making these structures ideal for filtering in optical networks.

Bragg gratings are the most common type of optical filter. Bragg gratings can reflect a very narrow bandwidth at a given design wavelength, the reflected (backward propagating) wave can then be routed elsewhere. Bragg gratings are a vital component of all optical demultiplexers.

For info on the physics behind this, look up 'Coupled-Mode Theory' or 'Bragg Gratings'.

Claude.

 

[lightgrav]

About 1% of light from a diode laser leaks out behind the diode -
it is usually used in a feedback loop for thermal stabilization, etc
but can be used for continuous intensity calibration.

 

[Loren Booda]

Alexyan,

Are you referring to negative indices of refraction, which I believe Claude suggests?

 

[Tide]

Claude,

That sounds like stimulated Brillouin scattering.

 

[Claude Bile]

Claude,

That sounds like stimulated Brillouin scattering.

It is the mechanism behind SBS, yes. For media where the refractive index is a function of the Intensity of the optical field, a suitably intense laser beam will form a temporary grating in the sense that it will vanish once the laser beam is removed.

Bragg gratings are permanent and are typically fabricated by exposing a piece of optic fibre with a photosensitive core to a UV laser. The required modulation is acheived by passing the laser beam through a slit and using the diffraction pattern, or by splitting the beam and creating an interference pattern

Claude.

 

[alexyan]

does only grating structure contain the backward wave?

 

[Claude Bile]

Light will couple from the forward traveling wave to the backward traveling wave only in the presence of the grating, but ONLY for particular wavelengths.

Claude.

 

[alexyan]

I do not sure what we do talk about same thing. the backward wave has negative group velocity compare to the phase velocity.

 

[Claude Bile]

When I refer to the backward propagating wave, I mean the component of the wave whose propagation constant is negative.

Claude.

 

[alexyan]

do we talk the same concept of "backward wave"? from my understanding, backward wave is that its phase velocity has a opposite sign of its group velocity which represents the power flux direction. here your explanation seems only talk about two forward waves traveling in opposite direaction.

 

[Claude Bile]

It would seem in that case that we are talking about different things. Sorry about the mix up!

Claude.