Why does a (laser) light beam not disperse?

In summary, The conversation discusses the dispersion of light beams and specifically compares the dispersion of laser light to that of "regular" light. The expert states that laser beams have less dispersion because they are a linear superposition of frequencies and have a shorter range of frequencies. They also mention that laser light can be described classically and suggest searching for "gaussian beam propagation" or "abcd matrix" for more information. The conversation also briefly touches on diffraction and dispersion being different concepts. Finally, a website is recommended as a valuable resource for information on lasers.
  • #1
Gerenuk
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Does anyone know where I can find a derivation how/why a light beam disperses? I'd like to see some sort of solution to Maxwell's equations for a finite width EM wave in free space.

If the beam is a linear superposition of frequencies and Maxwell's equations are linear, does a laser beam disperse less than an equivalent beam of normal light?
 
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  • #2
First, a laser beam does disperse, just not as much as "regular" lights. Laser beams are very closely 'collimated', much like a search light so it doesn't disperse very much to begin with. But the key to laser light is that it is NOT a "linear superposition of frequencies". The difference between laser light and "regular" light is that the laser's light is restricted to a very short range of frequencies (the ideal would be one frequency but that, strictly speaking, is not possible).
 
  • #3
So can laser light be described by a classical EM wave of a single frequency or do you require quantum mechanics?
I'd like to calculate this in detail with actually solutions to Maxwell's equation. From undergrad studies I know calculations for waves in free space and wave guides. Not sure what's the easiest way to consider a finite width beam in free space. Any references?

Why should a mixture of frequencies disperse more than each of them individually?
 
  • #4
The lowest mode laser beam is well described as a Gaussian. A google search for "gaussian beam propogation" or "abcd matrix" should pull up something useful.

Laser light is usually well described classically.
 
  • #5
Just a question, aren't we really talking about diffraction in this thread? Dispersion is what results from a variation in speed (or equivalently, refractive index) with wavelength.
 
  • #6
Good catch- yes, diffraction and dispersion are entirely different.

Laser light, being nearly monochromatic, displays significantly less dispersion in optical systems than braodband radiation.
 
  • #7

1. Why does a laser light beam not disperse?

A laser light beam does not disperse because it is a highly focused and coherent beam of light. This means that all the photons in the beam are traveling in the same direction and have the same wavelength, resulting in minimal spreading of the beam.

2. How does a laser maintain its beam coherence?

A laser maintains its beam coherence through a process called stimulated emission. This occurs when excited atoms in the laser medium release photons that are in phase with the original photon, resulting in a coherent beam of light.

3. What factors affect the coherence of a laser beam?

The coherence of a laser beam can be affected by several factors, including the quality of the laser cavity, the stability of the laser medium, and external environmental factors such as temperature and vibrations.

4. Can a laser beam disperse over long distances?

Yes, a laser beam can disperse over long distances due to atmospheric effects such as scattering and absorption. However, these effects are minimal compared to other light sources, allowing a laser beam to maintain its coherence over longer distances.

5. Why are laser beams often used for precise measurements and applications?

Laser beams are often used for precise measurements and applications because of their high coherence and focus. This allows for accurate and concentrated delivery of energy, making them useful in fields such as medicine, manufacturing, and research.

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