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Author Topic: can some explain what a laser beam is ?  (Read 5311 times)

Offline percepts

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can some explain what a laser beam is ?
« on: 11/07/2014 14:12:37 »
what is the difference between a laser beam and a normal light wave ?  Why don't laser beams spread out like light waves?


 

Offline chiralSPO

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Re: can some explain what a laser beam is ?
« Reply #1 on: 11/07/2014 16:56:06 »
I'm sure someone else can explain better than I, but I'll take a stab at it:

Normal light, say from an incandescent bulb (white hot filament) is randomly distributed in terms of phase and polarization, and there is a broad distribution of frequencies of the light. That is, if you picture a sinusoidal wave representing the electric fields of the different photons emitted, they are not all in the same plane (looking head on you would see \, |, /, and others)--this is the polarization. The different phases, you can think of as sin(t+x), where t is time and x is some arbitrary value that is not necessarily the same for each photon.

Laser light is special. All of the photons are necessarily in phase (that x term is the same for every one), and usually the frequency of light emitted is very close to one value. Lasers aren't necessarily polarized, but can be.

It is the fact that the photons are all in the same phase (they are called "coherent") that allows the laser light to spread out so much more slowly than normal light.
 

Offline JP

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Re: can some explain what a laser beam is ?
« Reply #2 on: 11/07/2014 23:41:17 »
As chiralSPO's answer said, lasers are coherent, which is a requirement for them to not spread out. 

Light is waves and like all waves, light can interfere with itself to create bright spots and dark spots (essentially the waves reinforce each other or cancel each other out).  Coherence of a laser beam means that light from different points in space (or time) from that laser can interfere with other light from the laser.  This allows the laser to travel roughly in a straight line because the light that tries to go out to the side, away from the beam ends up being cancelled out by interference with other light from the beam. 

Incoherence means that light from different points in space and time doesn't interfere, as is the case with light from most light bulbs, the sun--indeed, most light sources we deal with in everyday life.  This light spreads out in all directions because it doesn't interfere and so there's nothing to cancel out light that goes in any particular direction.

You're probably familiar with spotlights and other light sources that go somewhat in a nice beam (though not as nice as a laser beam).  This is because all sources in reality are somewhere between the idealized fully coherent and fully incoherent cases and therefore can interfere with themselves somewhat and go in somewhat of a beam.  Laser beams are very coherent while the sun is very incoherent and spotlights using incandescent bulbs are somewhere in between.
 

Offline UltimateTheory

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Re: can some explain what a laser beam is ?
« Reply #3 on: 12/07/2014 00:29:36 »
what is the difference between a laser beam and a normal light wave ?  Why don't laser beams spread out like light waves?

Photons from Sun or lightbulb are obeying inverse square law.
http://en.wikipedia.org/wiki/Inverse-square_law
Intensity per m^2 = Initial intensity / 4*PI*r^2, where r  is distance from source.
They're typically emitted in the whole visible spectrum (pretty continuous frequency range), therefor white light.

Typical laser has wavelength +- some little tolerance. Like f.e. 532 nm +- 10 nm.
 

Online PmbPhy

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Re: can some explain what a laser beam is ?
« Reply #4 on: 12/07/2014 01:29:22 »
Quote from: JP
As chiralSPO's answer said, lasers are coherent, which is a requirement for them to not spread out. 
Actually that's not what chiralSPO said, exactly. He said that laser light spreads out much more slowly. By the time that the beam gets to, say, the moon, it's spread ot by miles, perhaps by hundreds of miles even.
 

Offline evan_au

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Re: can some explain what a laser beam is ?
« Reply #5 on: 12/07/2014 03:51:05 »
Lasers achieve their unique properties because within the laser cavity, a "population inversion" ensures that most of the electrons are in the same excited state. A photon will trigger another photon of the same phase, frequency and direction. Mirrors on each end of the cavity allows these photons to trigger yet other photons, resulting in a coherent beam of a common frequency, phase and low divergence.

Conventional light sources largely rely on the independent emission of photons from individual atoms with independent quantum states, which don't directly affect other atoms. This results in a non-coherent light source with photons of indepenendent frequency, phase and direction.

Quote
spotlights and other light sources that go somewhat in a nice beam
As I understand it, spotlights work as well as they do because the light source is pretty much a point source, compared to the diameter of the parabolic mirror. The parabola can focus this (almost) point source into an (almost) parallel beam. Imperfections in the mirror, dust in the atmosphere, and the wavelength of light relative to the mirror size affect the divergence.

Common semiconductor lasers are very close to a point source, only micrometers wide. Because of the parallel mirrors, the laser light is emitted with a fairly low divergence, which is dominated by the wavelength of light relative to the (very small) emission region. Because it is monochromatic, a simple lens can reduce this divergence to a (nearly) parallel beam.

Light from the Sun, a spotlight and a laser all follow the inverse square law. It's just that the divergence is lower in the latter two, so you have to pace a lot further before the beam width has doubled, and the intensity has reduced by a factor of 4.
 

Offline JP

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Re: can some explain what a laser beam is ?
« Reply #6 on: 12/07/2014 14:14:02 »
Quote from: JP
As chiralSPO's answer said, lasers are coherent, which is a requirement for them to not spread out. 
Actually that's not what chiralSPO said, exactly. He said that laser light spreads out much more slowly. By the time that the beam gets to, say, the moon, it's spread ot by miles, perhaps by hundreds of miles even.

True, not sure why I didn't type that more clearly.  Coherent light can spread the least of any light, but it still spreads.
 

Offline jccc

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Re: can some explain what a laser beam is ?
« Reply #7 on: 13/07/2014 18:39:13 »
Maybe laser is standing wave and light is random wave?


 

Offline chiralSPO

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Re: can some explain what a laser beam is ?
« Reply #8 on: 16/07/2014 17:02:17 »
Maybe laser is standing wave and light is random wave?

No a laser is not a standing wave. The wave moves at the speed of light. It's just that all of the light from a laser is in phase.
 

Offline alancalverd

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Re: can some explain what a laser beam is ?
« Reply #9 on: 16/07/2014 17:56:33 »
You can make laser light spread out, or focus it, just as with any other light. It just happens to be easier to control and less prone to dispersion because it has a very narrow bandwidth and a long coherence length. In most cases it is generated inherently almost parallel, unlike other light sources which are inherently radial in nature, but neither statement is absolute. 
 

Offline percepts

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Re: can some explain what a laser beam is ?
« Reply #10 on: 16/07/2014 18:57:36 »
Thanks for replies. All being read and absorbed.
 

Offline chiralSPO

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Re: can some explain what a laser beam is ?
« Reply #11 on: 16/07/2014 20:58:25 »
You can make laser light spread out, or focus it, just as with any other light. It just happens to be easier to control and less prone to dispersion because it has a very narrow bandwidth and a long coherence length. In most cases it is generated inherently almost parallel, unlike other light sources which are inherently radial in nature, but neither statement is absolute.

I will also point out that there is some interesting research going on with not-so-coherent laser light. The broader the spectrum of emitted light, the more quickly coherence is lost. The uncertainty principle allows broad spectrum light to come from extremely short laser pulses (femtosecond = 1015 seconds). I don't really understand it that well, but apparently there are some useful applications of lasers with low coherence (and not just because of their broad spectrum)
 

Offline evan_au

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Re: can some explain what a laser beam is ?
« Reply #12 on: 16/07/2014 21:28:50 »
Quote from: jccc
Maybe laser is standing wave?
Within the laser cavity, a laser can be considered a coherent standing wave (like a plucked guitar string becomes a coherent standing wave after a few seconds).
Frequencies which are a submultiple of the cavity length can continue and grow, while all other frequencies self-interfere and are suppressed.

If the cavity is long compared to the wavelength of light, this potentially allows several specific frequencies to propagate within the cavity, forming a multi-mode laser, which is fine for many applications.

For more demanding applications, a second cavity can be used to select a specific wavelength and produce a single-frequency laser.

Once laser light leaves the laser cavity, it propagates through free space as separate photons, and eventually loses coherence.

Quote from: chiralSPO
broad spectrum light to come from extremely short laser pulses
A sine wave has a constant amplitude forever, and a bandwidth of 0 Hz (at least in theory).

If you take a sine wave from 0 amplitude to maximum amplitude and back to 0 in a femtosecond (1015 seconds), this is no longer a pure sine wave, and the bandwidth is expanded to somewhere around 1015 Hz.

This wide bandwidth is effective at triggering a variety of chemical reactions, regardless of the velocity of the individual molecules.
The very tight control on timing also allows experimenters to probe the state of the various intermediate products at closely-spaced times during the various stages of the chemical reaction, producing a high-resolution "movie" of the steps within a chemical reaction, allowing the reaction to be optimised.
« Last Edit: 16/07/2014 21:31:06 by evan_au »
 

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Re: can some explain what a laser beam is ?
« Reply #12 on: 16/07/2014 21:28:50 »

 

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