Does twisted light violate Fermat's principle?

[fxdung]

The twisted light propagate in curved line, then does twisted light violate Ferma's priciple?And then it violate refraction and refractive laws?

 

[A.T.]

The twisted light propagate in curved line, ...

What do you mean? Please provide a reference for the "propagating in curved line" part.

 

[Drakkith]

The twisted light propagate in curved line, then does twisted light violate Ferma's priciple?And then it violate refraction and refractive laws?

Don't mistake the behavior of the field vectors for the direction of travel.

 

[fxdung]

In Optics by Hecht, he say the trajectory of phase of EM wave of twisted light is helix.

 

[jbriggs444]

In Optics by Hecht, he say the trajectory of phase of EM wave of twisted light is helix.

An exact quote would be helpful.

It sounds like he is speaking of a circularly polarized light beam and is depicting the direction and magnitude of the electric or magnetic field vectors at each point on the beam as if they were physical displacements of the beam.

As @Drakkith points out, those lateral vectors are not displacements (e.g. in centimeters) but are field strengths (e.g. in volts/meter).

 

[Andy Resnick]

The twisted light propagate in curved line, then does twisted light violate Ferma's priciple?And then it violate refraction and refractive laws?

Are you actually referring to accelerated light?

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-5-6-678&id=389822

I'm not that familiar with the details, but no physical principles are violated.

 

[A.T.]

In Optics by Hecht, he say the trajectory of phase of EM wave of twisted light is helix.

Trajectory of phase is not the propagation trajectory. The propagation direction is the straight green axis below.

From:
https://en.wikipedia.org/wiki/Orbital_angular_momentum_of_light

 

[fxdung]

The phase velocity is greater c if propagation velocity is c?

 

[Cthugha]

No, actually you get a local modification of the Poynting vector. The propagation velocity along the green axis will be slightly less than c because the local Poynting vector is slightly tilted away from the green axis at every point. See the following image:

This image was shamelessly stolen from the following talk slides by Miles Padgett: https://www.gla.ac.uk/media/Media_263851_smxx.pdf

And just for the record as people often confuse that: Twisted light is not circularly polarized light, but light with a spatially varying phase front, usually a helical gradient of multiples of 2 pi.

 

[tech99]

I think this only happens close to the source, within its radiation near zone, or within a waveguide.

 

[fxdung]

So, is there any difference between accelerated light and twisted light?If accelerated light propagate on curved trajectory, then does it violate Ferma's principle?

 

[Andy Resnick]

So, is there any difference between accelerated light and twisted light?If accelerated light propagate on curved trajectory, then does it violate Ferma's principle?

Twisted and accelerated light are totally different things. Ray trajectories associated with accelerated light do not violate Fermat's principle.