Why are there "TEM" modes

Just out of curiousity, why are there such things as "modes" for a laser?

Does it have to do with the way the laser light is reflected inside the cavity or something?

How come laser DIODE's are called multimode, but don't really have a TEM xx dot type?

What really causes difference mode types in the first place? Specifically Rectangular modes vs. circular?

I know a lot of questions, but I'm curious.

Re: Why is there "TEM" modes

Le Quack said:

Just out of curiousity, why are there such things as "modes" for a laser?

Does it have to do with the way the laser light is reflected inside the cavity or something?

How come laser DIODE's are called multimode, but don't really have a TEM xx dot type?

What really causes difference mode types in the first place? Specifically Rectangular modes vs. circular?

I know a lot of questions, but I'm curious.

Click to expand...

I do believe multimode is caused by the emitters. Rather than a single emitter, there's an emitter "bar" Or multiple emitters in a row within the cavity.

Re: Why is there "TEM" modes

No, there is a single emitter. There are diodes with multiple emitters, but that's in the >5W range and they're almost always IR.

As I understand it, modes other than TEM00 make use of more of the cavity. That makes them more efficient and for that reason it can be difficult to fight them. If the cavity is beyond a certain diameter, it is inevitable you will have many paths the light can lase in.

Re: Why is there "TEM" modes

I think it is because the cavity is wide enough that it can fit multiple "laser beams" side by side, and the gain gets concentrated in the middle of each of these laser beams. Not sure why the beam doesn't just fill the entire cavity, but instead decides to seperate into different modes. Probably something that someone smart can help fill us in on.

I think it's the same idea behind solid state TEM modes as with diodes, but instead of a cavity the size of the crystal set, it's only as big as the active region on the die. Thus, waveguides built into the die confine the beam, but if those waveguides have enough space between them for multiple modes, then those modes will appear.

Re: Why is there "TEM" modes

Higher TEM modes come about because the cavity length (NOT WIDTH!!!) is not exactly the correct size for the wavelength being produced, which results in multiple waves being formed. It only takes 1/10th of a wavelength change to cause a shift needed for mode change, and the wavelengths for light are very small distances. Each wave is slightly different in phase from the fundamental (TEM00) wave and thus you get interference boundaries in between them as they mix and exit the aperture. This is why a TEM30 beam looks like a large beam with many lobes instead of just a large blob.

The fact that more of the cavity's width gets used is just a side effect, and not the causative factor. When a laser mode hops what you are really seeing is the length of the cavity change in size/shape in relation to the wavelength, which changes the phase angle of the resulting waves, which creates lobes. So it isn't that there is more cavity so you get higher modes, as long as the active region is the right size the cavity could be HUGE if it is the right multiple of the wavelength.

Multimode emitters are different than mode hopping. Multimode emitters have a physical characteristic changed to emit more light (I suspect active region size).