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Why is Laser Light Different?

02/20/2009 4:55 AM

Hi All

In idle moments I often ponder how light apparently disappears, I'm an air con engineer and none to bright at that, it is way out of my field, so if my question is a dumb one apologies up front.

In a previous question I asked what happened to light in space , did it just carry on travelling etc. The answer as I recall was yes, which is accepted except for the fact that the sky is very dark if all the light ever created is still whizzing around out there where is it or why can I not see it.

I then got to thinking about LASER light and discovered that I know precious little about the qualities of LASER light, can someone enlighten me a little please.

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#1

Re: LASER, what is and why is LASER light different

02/20/2009 7:27 AM

LASER = Light Amplification by Stimulated Emission of Radiation.

I don't know how much detail you want.

Briefly speaking -normal light as we see is electromagnetic wave (just a tiny portion of it). In fact if you see the spectrum of any radiation this will be a combination of infinite number of different wave lengths. It is like a busy railway station where all type of persons of will be moving around.

Now let us say we cut off all of them except a single wave length (very difficult by normal filters) but let us say we do. Like we put some sort of gate - such that in a platform men of a particular height (no more no less) can only move in.

But still all these similar persons will be moving in all direction.

We arrange them so that they can only move together in steps (like military) in case of EM wave it is all lights in same phase ie they pass through zero simultaneously.

In the last we order them to move only in one direction.

And presto we have laser.

Thus in laser, we have light beams of same wavelength, all in phase moving in a single beam (in one direction).

Due to this property, the energy is hightly concentrated (since after scatter/ diffraction, no destructive interference will take place) and anyway the particle scattering them will be vapourised in an instant. And the divergence of the beam is negligible.

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#2
In reply to #1

Re: LASER, what is and why is LASER light different

02/20/2009 9:32 AM

Thanks SB,

Clarity and simplicity for my simple mind!

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#3
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Re: LASER, what is and why is LASER light different

02/20/2009 11:18 PM

sd,

I understand your desire to be simplistic, but you've definitely gone over the edge into error! Any later attempts to understand laser light will be incomprehensible if armed with your explanation.

No offence.

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#4
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Re: LASER, what is and why is LASER light different

02/21/2009 12:53 AM

errors may always be corrected.

However the OP wanted a simple explanation.

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#5
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Re: LASER, what is and why is LASER light different

02/21/2009 1:04 AM

And we call that Coherent light.

The description was so good you are ready for part two.

Brad

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#6
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Re: LASER, what is and why is LASER light different

02/21/2009 1:44 AM

Did I tell you that it is all in the family ?

You have the smallest baby as GASER (gamma ray) with maximium power and the family tree moves on to the XASER (X ray), MASER (Microwave) through to RASER (Radio Waves)

Now ERASER though except in primary classes and movies.

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#7
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Re: LASER, what is and why is LASER light different

02/21/2009 2:23 AM

I'm bias to U V lasers. Vaser any one. How about a millisecond tan but not two milliseconds.

But I'm still confused on how a Phaser works. Must be the unobtainium Dilithium crystals.

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#8
In reply to #7

Re: LASER, what is and why is LASER light different

02/21/2009 4:57 AM

Did you talk to Spock ? He is supposed to know.

And NO - his ears may be similar to mine but he is not even distant cousin of me. He is too human. (which one is the smiley of disgust ?)

BTW

What is tan = ?

is it

= 1/cot = √(sec2 -1) ?

But what ever may be the value of , if you put the UASER tangentially, it may not harm you- It infact may be used for depilation by

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#13
In reply to #8

Re: LASER, what is and why is LASER light different

02/22/2009 6:57 PM

I have to guess he is just a Tan Gent.

.

Would be a cooked gent.

and while Leonard Simon Nimoy is a fine actor (for a Vulcan) as a science officer he is a little light on application. He has more in common with me in studding the Kabbalah than your ears. As for mischief, if I catch your kin in my system again and my wifi isn't being fixed then it's 10 milliseconds fo U V for him. He will be red for weeks.

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#14
In reply to #13

Re: LASER, what is and why is LASER light different

02/23/2009 1:09 AM

what is wifi? is it a endearing term for wife ?

And why you want to fix her. I thought she has fixed you with the vows.

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#11
In reply to #6

Re: LASER, what is and why is LASER light different

02/21/2009 12:01 PM

I want to build a long range ERASER...

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#12
In reply to #11

Re: LASER, what is and why is LASER light different

02/21/2009 12:31 PM

I thought the Hungeryones believed they were well out of that race - but perhaps you are Iranian and masquerading?

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#15
In reply to #12

Re: LASER, what is and why is LASER light different

02/23/2009 8:04 AM

Hungeryones are shrewd, secrective, don't draw attention, all over the world where least expected, loved, simply the best.

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#16
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Re: LASER, what is and why is LASER light different

02/23/2009 9:10 AM

Hungeryones are shrewd, secrective, don't draw attention, all over the world where least expected , (.) loveds , simply the best (and the fattest).

(I think he meant Hungarians)

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#17
In reply to #16

Re: LASER, what is and why is LASER light different

02/24/2009 12:21 AM

hehehe. I know what he meant, I am one, proud of it, and not the fattest.

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#18
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Re: LASER, what is and why is LASER light different

02/24/2009 2:14 AM

Proud of being hungry-one .

Well though I will not fit your belly and infact you may not like things with my nature anywhere near it. Still I don't want to take any chance. Never tried the effect of dilute HCl on me. On somebody else yes and it was not pleasant to that machine (but it was to me)

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#9

Re: Why is Laser Light Different?

02/21/2009 10:08 AM

Regular light is like noise, laser light is like the sound of a tuning fork.

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#10

Re: Why is Laser Light Different?

02/21/2009 10:36 AM

First, to highlight why you should go further - here are some possible misconceptions you might arrive at from reading sb's "simplified" explanation?

"After scatter/diffraction, no destructive interference will take place"
. incomprehensible, or wrong, or both
. the most significant difference between laser light and "ordinary" light is quite simply that diffraction effects do not usually average out

"and anyway the particle scattering them will be vaporised in an instant"
. lasers are not necessarily particularly powerful, but
. yes they can focus as well as is possible for their frequency (wavelength) of light

What is correct is how laser light is produced - amplification of incoming light, and comments about the existence of coherence - but...

Now to look at coherence:
Let us first consider the focusing of light from everyday objects - usually, it will produce an image. If we block all the light except that starting out from a very small area, we will have an image of a point. The image will be larger than the original, because the wavelength of the light is not zero (its size is determined by interference effects - see Airy disc for some more detail). A beam that focuses to the minimum possible size is called "spatially coherent", and there are multiple different measures for the quality of this spatial coherence. Laser light is often spatially coherent as is theoretically possible; NB that you will not usually see the structure of the Airy disc if you focus all the light from a laser - this is suppressed by the gradual way that the intensity of the beam decays at its edges. . . So, lasers can be focused down to the smallest size that is practical for the wavelength.
Now let us think about colour, (or wavelength or frequency). Normal (even what appears to the eye as single-colour) light is built from a range of wavelengths. As nothing is ever perfect, this is even true of laser light. A measure of colour purity used for lasers is known as "temporal coherence"; paradoxically, this is usually measured as a length! This is based on the observation that if you split the beam into two paths and recombine them after they have travelled different distances you will sometimes still be able to see interference patterns. The difference in the path lengths for which interference remains visible is known as the "coherence length". For ordinary white light, this distance is the order of a micron (a millionth of a meter). For typical sodium street lighting it is a few mm. Mercury vapour lamps (still not lasers) can give half a metre. Conditioned lasers can give coherence lengths of millions of km. So you can see from this that, contrary to how you might have read sb's explanation, you are far more likely to get interference of any sort from a laser than from normal sources - and this would of course include destructive interference.

So, what does all this mean in practical terms: practical 'ordinary' lamps can give very high power, but this will not come from a single point - so we cannot focus this power into a very small area; all the power in a laser beam can be focused down to a diffraction-limited spot, so a powerful laser can be very good for cutting (by burning through a very small width). Another application of spatial coherence is that it allows us to create the most a tightly-defined parallel beam; this can be useful for surveying, or for free-space optical communications (to get the maximum practical power to a distant receiver.

The applications of temporal coherence are in general more subtle, so I'll come to those later. However, you can see the combined effects of temporal and spatial coherence in the speckle pattern that appears whenever you shine a laser onto a matte surface - though even here the spatial coherence is of more importance - a coherence length similar to the depth of the coat of paint (usually much less than a mm) will usually be adequate.

So, where is extreme temporal coherence important?
Coherence lengths of a few metres are necessary to get the best from room-temperature spectroscopy; longer coherence lengths can be exploited at lower temperatures; but possibly the largest benefit of lasers for this application is that they can be made tunable.
The interference between beams that have travelled different distances is useful in accurate determination of distance - the beneficial coherence length can be up to twice the distance to the object being measured [Note: even if the measurement error is many (even hundreds/thousands of) wavelengths, this can contribute to our ability to detect the returning beam].

I think that's more than enough for now...

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