# Hyugens principle and laser wave dispersion

Claude Bile
I'd like to know if radiowave EM beams can be made as directional as a light beam.
In principle, yes.
Does light make more directional beams than microwaves?
When a wave hits an obstacle or aperture, the degree to which the beam spreads depends on $\lambda/a$ where $\lambda$ is the wavelength and $a$ is the size of the obstacle or aperture.

In other words, a 100 m wavelength beam through a 100 m aperture will spread the same amount as a 500 nm wavelength beam through a 500 nm aperture.

So for this reason, it is far easier to keep an optical wave directional than a radio wave in most circumstances.

Also, is laser the most directional beam we can send?
No, there are several types of waves that are more directional than a standard laser beam.

Bessel beams (whose transverse cross section is a Bessel function - just as Gaussian beams have a Gaussian transverse cross section) are somewhat unique in that they do not spread out as they propagate.

Spatial solitons are another form of waves that do not spread out as they propagate, due to some form of nonlinear response in the medium they propagate through. You can't get a spatial soliton in a vacuum like you can with Bessel beams.

Finally, there are laser filaments. Laser filaments arise when you have very high intensity pulsed lasers - the intensity is so great that you get a combination of nonlinear Kerr self-focusing (which is the phenomenon whereby a medium possesses an intensity dependent refractive index, which causes regions of high intensity to act like a converging lens, hence the term self-focusing) and defocusing caused by plasma generation through ionisation of the surrounding medium. The result is that the laser beam remains tightly confined over long distances. Laser filaments are particularly useful for LIDAR (light radar) applications.

Claude.

Thank you very much, this post is very interesting.

This Wikipedia article shows a couple of examples of laser beam intensity profiles.
http://sensoria.in/TU/knowledge-pool/34-electronics-and-communication/78-laser-principles.html" [Broken]

Last edited by a moderator: