What Exactly is an Optical Mode in a Waveguide?

In summary, the different modes of electromagnetic radiation have different wavelengths, which can be explained by pressure waves in an idealized 3D box.
  • #1
The Head
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I have been working with equations throughout the semester and using them to find cutoff frequencies and such, but when you say something like TE10, TE20, TE21, etc., I am realizing that I am having trouble getting a physical idea of what exactly these things are. Of course if you transmit at a frequency below the cutoff frequency of TE21, it will not propagate, but maybe TE10 modes can.

But this is what confuses me. Let's say you are transmitting at a frequency greater than the TE21 cutoff frequency. That means you can have TE10 & TE21, among other possibilities. It seems to be in this case that both of these modes are transmitting a beam of light with the same wavelength. If that is correct, then what exactly is different about these modes? The QM I remember makes me want to say that one is in an excited state, but I wouldn't know what that really means, even if true. So what different properties would two waves (of the same wavelength, if that is true) from different modes have exactly?

Finally, I am struggling to see how this applies to multimode fibers too. I heard that you can separate different modes at the end of a fiber with a grating, which makes sense if the wavelengths are indeed different. So if different modes = different wavelengths, well I am not exactly sure how these different modes run in different wavelengths anyhow...

Thanks for any of these concepts you might be able to elucidate.
 
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  • #2
Optical mode: A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam. ... it's like the modes of vibration on a string only for the electromagnetic field.
 
  • #3
With the string, it is a physical object already, so it's wavelength is twice as long in the n=2 vs n=1 mode. , and if it's a guitar, the pitch will be different. With an electromagnetic radiation, I understand wavelength in the direction of propagation of course, but it is a little tricky for me to think of what it means for the electromagnetic field to have a different "wavelength," sort of. And it leaves me with three questions:

1) If the field has these different patterns based on what mode it is in, what physical difference will there be between these propagating waves? For example, the energy should be identical, but what physical aspect isn't?

2) Why would a higher mode (in the transverse direction) not be able to propagate in the z-direction at certain (lower) frequencies?

3) What causes a wave to be in one mode or another?

Thanks!
 
  • #4
It may be easier for you to think of it like pressure waves in a tube.

Solve the wave equations for an EM field in an idealized 3D box, dimensions LxWxD, with perfectly reflecting sides. You are going to get standing wave solutions in each dimension - which will have different wavelengths (there have to be nodes at each wall).

A waveguide is like one of these boxes, with one dimension very much longer than the others.
say: W=D=2R and L>>R.

IRL. the walls are not perfectly reflecting either - which gives you all the wrinkles you've probably been learning about.

Aside:
For a pressure wave in a tube, you will have seen the approximation for a long thin tube so only the "length" is considered. But the width of the tube also affects things. You can understand that the air pressure by the walls of the tube will be different from in the middle - any perpendicular motion in the molecules must be a minimum there. You can see the effect if you upend the tube, replace the air with water - and tap the "top" ... see the waves ripple out to the edges, as well as travel down the length of the tube?
 
  • #5


An optical mode in a waveguide refers to a specific pattern of electromagnetic waves that can propagate through the waveguide. In simple terms, it is the way in which light travels through the waveguide.

The TE (transverse electric) modes you mentioned are a type of optical mode that have a perpendicular electric field to the direction of propagation. The subscript numbers, such as TE10, refer to the number of nodes in the transverse electric field. For example, TE10 has one node, while TE21 has two nodes.

The different modes have different properties, such as the intensity and distribution of the electric and magnetic fields, as well as the direction and shape of the beam. This is due to the different ways in which the waves propagate through the waveguide.

In a multimode fiber, there are multiple possible modes that can propagate through the fiber. This means that different parts of the light beam can travel along different paths within the fiber, resulting in different modes. The separation of these modes at the end of the fiber with a grating is due to the fact that each mode has a slightly different wavelength, which can be separated by the grating. This is known as mode dispersion.

It is important to consider the different modes in waveguides and fibers because they can affect the transmission of light and the overall performance of the system. Understanding and controlling these modes is crucial in designing and optimizing optical systems.

I hope this explanation helps to clarify the concept of optical modes in waveguides and their relevance in multimode fibers. If you have any further questions, please don't hesitate to ask.
 

1. What is an optical mode in a waveguide?

An optical mode in a waveguide refers to the pattern of electromagnetic field that travels through the waveguide, allowing the transmission of light. It is the specific spatial distribution of the electric and magnetic fields that make up the light wave inside the waveguide.

2. How is an optical mode different from a normal electromagnetic wave?

An optical mode is confined to a specific region within the waveguide, while a normal electromagnetic wave can spread out in space. This confinement is due to the boundaries of the waveguide, which reflect the light back into the waveguide, causing interference and resulting in a specific mode shape.

3. What factors affect the properties of an optical mode in a waveguide?

The properties of an optical mode in a waveguide are affected by the waveguide's dimensions, material properties, and the wavelength of light that is being transmitted. The shape and size of the waveguide determine the number and type of modes that can exist, while the material properties affect the speed and direction of the mode.

4. How are optical modes measured and analyzed in a waveguide?

Optical modes in a waveguide can be measured and analyzed using techniques such as interferometry, near-field scanning, and mode spectroscopy. These methods involve creating and visualizing the interference patterns of the modes, measuring the near-field intensity distribution, and studying the spectral properties of the modes, respectively.

5. What are the applications of optical modes in waveguides?

Optical modes in waveguides have various applications in the field of optics, such as in optical communication networks, optical sensors, and photonic integrated circuits. They allow for the efficient and controlled transmission of light, making them essential for the development of advanced optical devices and technologies.

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