What is the orbital angular momentum of a light beam?

Question

Physics

I accidentally came across an article on lenta.ru The twisting of laser beams accelerated the transfer of information .

The article says that the transmission rate of 2.56 terabits per second was achieved due to the twisting of the beam phase. Quote:

Light is called swirling, which does not have polarization (flat or circular), but the so-called orbital angular momentum.

Ordinary light in a plane that is perpendicular to the direction of the beam has the same phase. In swirling light, the set of waves with one phase is not a plane, but a spiral twisted along the beam. This spiral can be twisted with different periods (stronger or weaker). Light swirling with different periods can propagate in the same beam without interfering with each other. This, as the authors have shown, can be used to create separate information channels in one beam.

Unfortunately, my knowledge of physics is not enough to imagine a device that can twist a wave into a spiral. An internet search didn't help much.

Actually questions:

• What would a device that would give photons "orbital angular momentum" look like?
• Why not scatter the channels across the spectrum?
• Why can't the channels be separated by light polarization?
• Why is amplitude modulation not enough to pack 32 information channels in one beam?

Answer 1

There are different, in general, very simple ways to modulate the phase of a light beam.
For example, if light is passed through a crystal placed in an electric field, then the phase shift will be proportional to the field strength or the square of the field strength (Pockels effect or Kerr effect - in fact, a change in the refractive index of the medium). Accordingly, if the field is made inhomogeneous in space, then the beam can also be modulated in a very tricky way.
In general, a brief description of the different methods is given, for example, in the English Wikipedia .
And in the same article from the tape there is a link to Igor Ivanov's blog (for me, an excellent modern popularizer) with a description of the concept of the angular momentum of a photon.

Answer 2

It may look simple: a prism is supplied with light from several sources, the whole system rotates around an axis coinciding with the beam.
Do not scatter across the spectra, because it may have already been scattered or a suitable band is not so wide.
Why it is impossible to divide - well, because there are N types of polarizations, and here, roughly speaking, on a spiral you get 360*N types.
Amplitude modulation, I think, is generally not suitable in this case due to fluctuations in the atmosphere: UFOs fly by, all sorts of clouds ...

Answer 3

Chota laugh