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Old 09-13-2008, 06:08 PM #17
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Default Re: what is coherent light

time to break out my physics book!

* * * * *"To understand how laser light is different from conventional light, consider the light produced by an incandescent light bulb. When electric charges move through the filament, electromagnetic waves are emitted in the form of visible light. In a typical light bulb, there are variations in the structure of the filament and in the way charges move through it. As a result, electromagnetic waves are emitted at different times from different parts of the filament. These waves have different intensities and move in different directions. The light also covers a wide range of the electromagnetic spectrum because it includes light of different wavelengths. Because so many different wavelengths exist, and because the light is changing almost consistently, the light produced is incoherent. That is, the component waves do not maintain a constant phase difference at all times. The wave fronts of incoherent light are like the wave fronts that result when rain falls on the surface of a pond. No two wave fonts are caused by the same event, and they therefore do not produce a stable interference pattern."

* * * * *"Lasers on the other hand , typically produce a narrow beam of coherent light. The waves emitted by a laser are in phase, and they do not shift relative to each other as time progresses. Because all the waves are in a phase, they interfere constructively at all points. The individual waves effectively *behave like a single wave with a very large amplitude. In addition, the light produced by a laser is monochromatic, so all the waves have exactly the same wavelength. As a result of these properties, the intensity, or brightness, of a laser light can be made much greater than that of incoherent light. For light, intensity is a measure of the energy transferred per unit time over a given area."

* * * * *"A laser is a device that converts light, electrical energy, or chemical energy into coherent light. There are a variety of different types of lasers, but they all have some common features. They all use a substance called the active medium to which energy is added to produce coherent light. The active medium can be a solid, liquid, or gas. The composition of the active medium determines the wavelength of the light produced by the laser."

* * * * *Physics by Holt, copyright 2006

*


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Old 09-13-2008, 06:13 PM #18
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Default Re: what is coherent light

Incoherent light is people dancing at a Rave

Coherent light is soldiers marching in a line

Peace,
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Old 09-14-2008, 08:16 AM #19
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Default Re: what is coherent light

Quote:
Originally Posted by daguin
Incoherent light is people dancing at a Rave

Coherent light is soldiers marching in a line

Peace,
dave
LOL ;D Now i get it!!!!!!!!!!!!!!!!!!!!!!!!! lol
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Old 10-01-2008, 12:15 PM #20
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Default Re: what is coherent light

when you blow an LD, and it is known to be a 'very expensive LED'.
Is the light emmiting from the LD now, incoherent.
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Old 10-01-2008, 05:29 PM #21
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Default Re: what is coherent light

Quote:
Originally Posted by caleb
when you blow an LD, and it is known to be a 'very expensive LED'.
Is the light emmiting from the LD now, incoherent.
Yes. A laser diode, working properly, is emitting its light through stimulated emission. Stimulated emission results in coherent light. When an LD suffers COD (catastrophic optical damage), stimulated emission ceases (well, is really severely decreased) and spontaneous emission becomes the dominant mode of light emission. Spontaneous emission, is the same type that LEDs use, and the light produced by spontaneous emission is, by nature, not coherent.

COD happens when the mirrors at the ends of the "cavity" are damaged and no longer function as mirrors. Without the mirrors bouncing light back and forth, you get no feedback and no reflected photons to stimulate emission, so no gain and no laser. There will still be a tiny bit of stimulated emission, as the photons that are spontaneously emitted make their once-through trip through the laser, as opposed to the many trips that a photon would make in a functioning laser.
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Old 10-02-2008, 11:40 AM #22
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Default Re: what is coherent light

Quote:
Originally Posted by daguin
Incoherent light is people dancing at a Rave

Coherent light is soldiers marching in a line

Peace,
dave
Nice ;D

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Old 01-29-2009, 10:16 PM #23
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Default Re: what is coherent light

Quote:
Originally Posted by Justin
Light needs to be monochromatic in order to stay coherent over a distance, but the property of coherence basically just means that the waves of light are in phase with one another and travelling in the same direction. For this reason the coherence length of a laser is dependent on the spectral linewidth (the amount of variation in wavelength within a laser beam), among other things.
What do you mean? Why does light need to be monochromatic in order to stay coherent? Does this mean that you couldn't combine the beams of red, green, and blue lasers to create a white laser, because white light is the sum of all wavelengths of light?
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Old 01-30-2009, 12:42 AM #24
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Default Re: what is coherent light

Coherent light must be monochromatic. Coherent means that the wave fronts of all the fotons are at the same point. Consider waves in the ocean, of which the tops form lines paralel to the beach. If part of the wave moved faster than another, those lines would break up, and it would no longer be a wave as you know them.

A white beam can consist of a mix of 3 coherent beams, but in itself is not coherent. Sometimes this is easy to observe: while the beam appears white, the speckle it creates when hitting a rough surface seems to consist of red, green and blue dots.
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Old 01-30-2009, 12:52 AM #25
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Default Re: what is coherent light

Quote:
Originally Posted by elephant1292
[quote author=Justin link=1212351589/0#15 date=1219943424]Light needs to be monochromatic in order to stay coherent over a distance, but the property of coherence basically just means that the waves of light are in phase with one another and travelling in the same direction. For this reason the coherence length of a laser is dependent on the spectral linewidth (the amount of variation in wavelength within a laser beam), among other things.
What do you mean? Why does light need to be monochromatic in order to stay coherent? Does this mean that you couldn't combine the beams of red, green, and blue lasers to create a white laser, because white light is the sum of all wavelengths of light?[/quote]
Well, all the waves will be different lengths, and traveling at different speeds, therefore not being able to fully combine.
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Old 01-30-2009, 02:09 AM #26
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Default Re: what is coherent light

I see. I forgot that the shape of an electromagnetic wave depends on its wavelength as well. Thanks a lot!



Although, you could maintain coherency while combining the beams of lasers of the same wavelength, right?
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Old 01-30-2009, 02:41 AM #27
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Default Re: what is coherent light

Quote:
Originally Posted by Spyderz20x6
and traveling at different speeds, therefore not being able to fully combine.
Minor nitpick but light always travels at the same speed in a given medium.

You may be able to maintain coherency when combining beams of the same wavelength, but I would be skeptical. To begin with, even a fraction of a nm difference in wavelength would add up very quickly and break coherency. After that, you would need to line up the peaks of the waves which would mean aligning the lasers on the level of +- 20 or so nm. There is probbably better ways to combine beams and maintain coherency such as using them to pump a second laser.
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Old 01-30-2009, 06:53 AM #28
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Default Re: what is coherent light

Quote:
Originally Posted by 691175002
[quote author=Spyderz20x6 link=1212351589/20#24 date=1233280370]and traveling at different speeds, therefore not being able to fully combine.
Minor nitpick but light always travels at the same speed in a given medium.
[/quote]


Minor nitpick, but not always. *Case in point (with the shout out to Pink Floyd):

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Old 01-30-2009, 08:37 AM #29
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Default Re: what is coherent light

You are correct, I forgot about dispersion.
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Old 01-30-2009, 03:00 PM #30
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Default Re: what is coherent light

Quote:
Originally Posted by elephant1292
Although, you could maintain coherency while combining the beams of lasers of the same wavelength, right?
You could, though it is not that easy. If you 'just' combine 2 beams of the same wavelength, there is no way to make sure the wavefronts of both beams will be in sync.

However, since coupling typically also makes the lasers illuminate eachother to some degree, it is possible to lock them onto eachother by that mechanism. This could work using a combing cube, but will not work if you just shine 2 beams onto the same spot, even if the wavelengths are very very close (2 HeNe lasers, for example).
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