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Old 06-29-2010, 07:37 AM #1
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Default What is the difference in beam diameter of red green and blue lasers?

i need to no what wavelength the lasers are and what is the diameter of the beam is being produced,

i am doing an assignment on lasers and i want proof that a smaller wavelength produces a more concentrated beam and if you could include the website so i can use it in my bibliography.

cheers.


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Old 06-29-2010, 08:28 AM #2
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Default Re: What is the difference in beam diameter of red green and blue lasers?

Quote:
i need to no what wavelength the lasers are
The wavelength is basically the color of the laser, and depends on the type of laser being used.

Common red lasers are often around 650-670nm, the more visible orange-red ones are 635nm
Green lasers are usually 532nm
BluRay lasers (actually violet, and almost UV) are around 405nm
The new blue laser diodes are 445nm
A medical Nd:YAG laser is often 1064nm (invisible IR)
The diode used to pump a green DPSS laser is 808nm (also IR)

There are all sorts of other lasers & wavelengths, covering the visible spectrum and beyond.

Quote:
and what is the diameter of the beam is being produced
That's like asking what's the normal beam width of a flashlight is. There is no standard width, it varies widely, depending on the exact model of laser, and the lenses being used.

Quote:
i am doing an assignment on lasers and i want proof that a smaller wavelength produces a more concentrated beam
UPDATE: Just noticed, you were asking about smaller wavelength, not beam width.

A shorter-wavelength blue laser does not automatically have a thinner beam width than a longer-wavelength red laser, if that is what you were asking.

A shorter wavelength means a higher frequency - the wave is more concentrated - it is moving up and down faster. A photon of shorter-wavelength light is more energetic, and requires more energy to produce, than a longer-wavelength one. Because it is more energetic, it can also do more damage.

This is one of the reasons UV light (or X-rays, for that matter), are more dangerous than visible light.

Last edited by seoguy; 06-29-2010 at 09:21 AM. Reason: update
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Old 06-29-2010, 09:12 PM #3
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Default Re: What is the difference in beam diameter of red green and blue lasers?

if this is a project then search. there is plenty of info here. either way you are reading. if you search at least you will actually learn. Also, you are in the wrong section. ask a mod to maybe move this to the appropriate section... that way you will get more traffic reading this.


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Old 06-30-2010, 01:29 AM #4
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Default Re: What is the difference in beam diameter of red green and blue lasers?

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Originally Posted by Braaksma View Post
i need to no what wavelength the lasers are and what is the diameter of the beam is being produced,

i am doing an assignment on lasers and i want proof that a smaller wavelength produces a more concentrated beam ...
You will find this a very hard hypothesis to prove using anything practical.

The limit on divergence and focusability of laser beams depends mostly on the actual laser source, not the wavelength. To give some examples:

- a red diode laser will have a worse beamwidth/divergence profile than the beam coming out of a long helium-neon laser tube, despite the similar wavelengh.

- well constructed green dpps lasers feature a narrow beam to start with, but can have excellend beam/divergence ratios too.

- bluray laser diodes are very similar in beam/div perfomance compared to red laser diodes, despite the much smaller wavelength.

Essentially, however, your assumption is correct - if all factors are equal, and only the wavelength is different, it is possible to collimate the light onto a smaller spot with a smaller wavelength.

For any practical comparison i would suggest comparing red and bluray diodes to eachother, since they have comparable emitting areas to start with, and are nice single-TEM diodes too.
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Old 06-30-2010, 01:46 AM #5
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Default Re: What is the difference in beam diameter of red green and blue lasers?

I think he's not getting his point across quite right, guys.

He's right, in that a smaller wavelength can be focused into a smaller pinpoint. That's not the "beam quality" that we all have come to know, though.

The reason blu-ray has a higher storage density is because it can be focused to a smaller spot; therefore smaller pits and lands, and higher data density. That's also why 650nm was used for DVD vs CD's 780nm. With each reduction in wavelength, we get a closer focal point or 'beam waist', and a smaller spot.

That's -probably- what he's getting at: "Why are shorter wavelengths used in blu-ray vs longer wavelengths in CD and DVD to get more storage density".. That's what it sounds like at least.

This isn't the same as "beam width at aperture" or DPSS vs diode beam profiles. (BenM, I know you're fully aware of that, i'm not really replying to you; i'm just trying to demonstrate how he may be able to show this.)

He is correct, it's just that it's not something that we ever tend to care about (minimum width of focal point) in our daily hobbyist laser lives.

Braaksma, a good way to prove this experimentally would be to use diffraction gratings. Smaller/shorter wavelengths (towards blue) should produce diffractive spots that are closer together, whereas longer wavelengths (towards red) will bend more, and cause the distance between dots to be greater; a spread out pattern.

Diffraction grating - Wikipedia, the free encyclopedia

Scroll down to "Gratings as Dispersive Elements".

Notice how in the picture here:



the shorter wavelengths (blue, violet) are on the inside of the zero mark (source) to either side, and the longer (orange, red) wavelengths appear at the outside of the span.

This shows that when being passed through an element, shorter wavelengths disperse less, allowing for a less spread-out focal point.

Another place to look for reasonable material on this would be articles on chromatic aberration.

http://en.wikipedia.org/wiki/Chromatic_aberration

This explains the tendency of different wavelengths to have different focal lengths after passing through an element; wavelengths towards blue (shorter) having a shorter focal length.



Whatever you do, don't use wikipedia as a source, though!!! Nothing else makes a professor or teacher angry so easily.
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Old 06-30-2010, 04:28 PM #6
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Default Re: What is the difference in beam diameter of red green and blue lasers?

why do red lasers tend to have larger beam diameters anyway? i can understand that with the same lense, to reduce the divergence into a beam would require the lense to be further away, hence making a thicker beam, but you could always just make a lense that converges light to a greater extent, and put it closer to the diode.
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