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Old 06-08-2013, 12:53 AM #17
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Default Re: Best RGB Laser Combination

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Originally Posted by EpicHam View Post
coherent white
These two words are actually contradictory.

Coherent means the waves are in phase. White has at least two different wavelengths, and it is a physical impossibility for waves of different wavelengths to be in phase.

Consider your nits picked


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Old 06-08-2013, 02:42 PM #18
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Default Re: Best RGB Laser Combination

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Originally Posted by Cyparagon View Post
These two words are actually contradictory.

Coherent means the waves are in phase. White has at least two different wavelengths, and it is a physical impossibility for waves of different wavelengths to be in phase.

Consider your nits picked
mode-locking (i work with this every day...I should say its awesome!



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Old 06-08-2013, 03:59 PM #19
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Default Re: Best RGB Laser Combination

I'm sure cyparagon will understand that article, but I don't get how it relates to the what he wrote about white coherency?? Care to explain


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Old 06-08-2013, 04:00 PM #20
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Default Re: Best RGB Laser Combination

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Originally Posted by LordKaramazov View Post
mode-locking (i work with this every day...I should say its awesome!
Consider your nits picked
Interesting,

The graphic shows the various frequency waves being in phase at one point in time (time zero) for a infinitesimal fraction of a wavelength.
Technically correct, but we're talking CW lasers here, so that is splitting hairs on nits

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Old 06-08-2013, 04:11 PM #21
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Default Re: Best RGB Laser Combination

even talking about CW or not, 2 different, single line lasers (although real CW doesnt exist, every laser has a finite bandwidth) can be coherent with each other or not.

In the case of the 3 different sources where you then combine them together, well, yeah those are incoherent with each other, but for example when you have your 532nm laser, the fundamental (1064nm) and the green are coherent with each other...although they have different wavelegnths

*EDIT*
Forgot about you Chipdouglas:
Well, what i meant to refer was that when you have a "white light" (i put it in "" because by that im just meaning a very broadband light source, be it in IR or VIS) laser, that is completely coherent, that (modelocking) is possible to do. While when you have a normal light source you cant (well, I guess that it might be possible to make everything coherent using slits and what not, but in the end your output will be negligible).

I meant that in order to have all the electric fields constructively and destructively interfering with each other (and hence producing those pulses), all the light components need to be coherent with each other. In other words, if they wouldnt be coherent, perhaps you would be able to create a pulse, but localized in time and space, as you dont have coherence to keep it going forever and everywhere.

To give an example of how awesome light is:
When you have a single line laser, you can perform the famous double slit experiment and see the interference pattern on the wall. That is a spatial interference. When you have a coherent broadband source, and you make them interfere, usually you look to the frequency domain instead and you are able to see a modulation of the spectrum, just like the fringes in the slit experiment, but in this case you see them in a spectrometer.
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Old 06-08-2013, 04:23 PM #22
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Default Re: Best RGB Laser Combination

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Originally Posted by LordKaramazov View Post
even talking about CW or not, 2 different, single line lasers (although real CW doesnt exist, every laser has a finite bandwidth) can be coherent with each other or not.
In the case of the 3 different sources where you then combine them together, well, yeah those are incoherent with each other, but for example when you have your 532nm laser, the fundamental (1064nm) and the green are coherent with each other...although they have different wavelegnths
I get some of that, (don't understand 'real CW doesn't exist') but the discussion was regarding white "coherence".
532 and 1064nm would not make white.
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Old 06-08-2013, 04:36 PM #23
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Default Re: Best RGB Laser Combination

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Originally Posted by Multimode View Post
I get some of that, (don't understand 'real CW doesn't exist') but the discussion was regarding white "coherence".
532 and 1064nm would not make white.
Well, i can give you a real example of single lines white light source (instead of broadband) where the light is coherent (so, the second harmonic generation is a non-linear optical effect of the second order, when you go to the 3rd order, there is a process called four-wave mixing) where you can create several lines in the visible and obtain white light. Then you can even use a prism to select just 3 (green, blue and red) and have a white light and those 3 lines are coherent

By "real CW doesnt exist" i mean that in a physics sense. For our purposes here, CW is something that is not pulsed and the small ripples in power/instantaneous wavelength and so on dont matter. The broad sense of CW. So you are right, CW lasers do exist

In the physical sense, true single line CW would be a laser that had a single wavelength and would exist infinitely in time and space. As your HeNe still has a slight bandwidth (for example 328.8nm+/-0.05nm...just inventing the number, im not sure whats the usual value) i mean that if you measure the exact wavelength at a point in time A and in a point in time B most probably they would differ for a little bit and so it was not a true CW laser. But thats impossible to achieve, only mathematically, so, oh well, doesnt matter xD
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Old 06-08-2013, 05:00 PM #24
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Default Re: Best RGB Laser Combination

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Originally Posted by LordKaramazov View Post
Then you can even use a prism to select just 3 (green, blue and red) and have a white light and those 3 lines are coherent
Now, that is cool

I consider the nit well and truly picked


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Old 06-08-2013, 05:23 PM #25
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Default Re: Best RGB Laser Combination

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Originally Posted by LordKaramazov View Post
Well, i can give you a real example of single lines white light source (instead of broadband) where the light is coherent
Just the red, just the green, and just the blue is coherent, but they cannot be coherent with eachother.

Quote:
modelocking
Wikipedia: "A wave with a longer coherence length is closer to a perfect sinusoidal wave."

This looks nothing like sine to me, so I would argue a mode locked laser isn't coherent.



Each of it's constituent frequencies are coherent, but not the final output. Just like white.
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Old 06-08-2013, 05:31 PM #26
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Default Re: Best RGB Laser Combination

i could also argue that if all those wavelenghts werent coherent with each other (in time and space) you wouldnt be able to see the intereference between them.

each wavelength there is coherent to itself and to the others. They change their phase properties at the same time, keeping them spectrally in phase for a long time and distance, hence obeying the wikipedia quote "When interfering, two waves can add together to create a wave of greater amplitude than either one (constructive interference) or subtract from each other to create a wave of lesser amplitude than either one (destructive interference), depending on their relative phase. Two waves are said to be coherent if they have a constant relative phase. The degree of coherence is measured by the interference visibility, a measure of how perfectly the waves can cancel due to destructive interference." (exactly what i said)

In the wikipedia quote you assume a single wave. The red mix in the image is the case i presented.

In a mode-locked laser you have a pulse train and its only possible if they are coherent
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