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Author Topic: Lazer pens  (Read 4703 times)

paul.fr

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« on: 10/05/2007 02:06:34 »
This is a lazer pen


But, is it really a lazer? if it is what type of range would it have, and wheather it is or not, are they dangerous?


 

another_someone

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« Reply #1 on: 10/05/2007 02:55:39 »
OK - firstly, I know you said your spelling was a bit off today - but it is laser (Light Amplification through Stimulated Emission of Radiation).

I cannot say whether that particular article is a true laser pen, but true laser pens do contain very low power lasers.

In theory, in clear air (or a vacuum) a laser should have almost infinite range.  Since air is never totally clear, one would expect some absorption of light in the atmosphere, but it would still be a very, very, long range.

Supposedly, the very low power of the laser should mean that it is safe, but I would still advise against shining it directly into someone's eye.
 

Offline lightarrow

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« Reply #2 on: 10/05/2007 12:03:52 »
This is a lazer pen


But, is it really a lazer? if it is what type of range would it have, and wheather it is or not, are they dangerous?
Yes, it's a real laser, a laser diode. Their power are of the order of 1 mW and they are not dangerous unless their direct or reflected beam (from a mirror or high reflective surface) hits the eyes.
If you can make or find a little screen with a very tiny hole in it, hit it with the laser beam and then observe in darkness the emerging light hitting a wall; you could see interference fringes.
About range, in about 200 metres the beam has diverged from a few millimetres to 20-40 centmetres, so its intensity is very reduced; I think you won't be able to see it after 1 km even in complete darkness.
 

another_someone

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« Reply #3 on: 10/05/2007 13:08:56 »
About range, in about 200 metres the beam has diverged from a few millimetres to 20-40 centmetres, so its intensity is very reduced; I think you won't be able to see it after 1 km even in complete darkness.

I am surprised at such a wide divergence from a laser beam - is this a deliberate safety measure, or just a limitation of the small size of the diode?
 

Offline Batroost

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« Reply #4 on: 10/05/2007 19:13:19 »
What makes a LASER a LASER is not lack of divergence but rather, being (close to) a single wavelength in output and being a coherent source of light i.e. photons of pretty much the same energy, being emitted in phase with one another (though this does tend to be limited to beam lengths of a few metres at a time).

The divergence qualities of the beam are associated with the design of the optics inside, especially the design of the aperture through which the beam emerges. Even if the aperture were unobstructed and made of a material with good transparency you'd still be up against the 'diffraction limit' - the beam would still diverge.
 

Offline Bored chemist

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« Reply #5 on: 10/05/2007 19:22:09 »
What makes a laser a laser is that it produces light by stimulated emision.

It is possible to, for example, make holograms without using a laser even though the production of holograms needs coherent light. (In fact the first holograms were made without lasers)
It is alo perfectly possible to make pulsed lasers with wide bandwidths and poor coherence.
 

Offline Batroost

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« Reply #6 on: 10/05/2007 19:54:56 »
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What makes a laser a laser is that it produces light by stimulated emision.

Agree that this is a feature of a LASER but Light is produced by stimulated emission all around us, all of the time; but not in any way you could call 'organised'. What makes a LASER a LASER is a deliberate design that encourages this process to dominate over other emission processes (at least for some of the time). It is the near-monochrome coherent nature that makes a LASER useful.
 

Offline Bored chemist

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« Reply #7 on: 10/05/2007 20:02:47 »
Surely most light is produced by spontaneous radiation? Even lasers produce quite a bit of this. OK, to be a laser it needs the A of "lAser" as well. There needs to be a gain medium through which the light travels "picking up reinforcements" by stimulated emission.
The most expensive, ultra-short pulse lasers are the least monochromatic, but they are still useful.
 

Offline Batroost

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« Reply #8 on: 10/05/2007 20:15:09 »
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Surely most light is produced by spontaneous radiation?

No, although this could be the result of a population inversion (i.e. light would be near-monochromatic) it wouldn't be coherent unless the majority of photons were produced through stimulated emission.

Remember that the word LASER was coined only as a corruptioon of MASER, and I think Microwave Amplification was what was being looked for by the original expermienters i.e. rather than coherence.

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The most expensive, ultra-short pulse lasers are the least monochromatic, but they are still useful

Very good point. Of course it depends what you want to use the LASER for. I'm coming from a background where monochomism and coherence was important rather than just power. If you're in the inertial fusion game you'll want it the other way around....
 

Offline Bored chemist

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« Reply #9 on: 10/05/2007 21:03:37 »
I meant that in all other sources most light is from spontaneous radiation so, overall, in the grand scheme of things, most light is spontaneous rather than, as you wrote "Light is produced by stimulated emission all around us, all of the time" I agree that some is, but it's very much the minority. Also, I'm quite sure that more light spontaneously leaves the sides of a HeNe laser than comes out as the beam so, even in a laser, most of the light is spontaneous.
The requirement for short pulses for fast kinetics experiments also produces broad bandwidths so it's not just a matter of power. The high power ones are just the most fun :-)
« Last Edit: 10/05/2007 21:05:32 by Bored chemist »
 

Offline Batroost

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« Reply #10 on: 10/05/2007 21:17:21 »
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I meant that in all other sources most light is from spontaneous radiation so, overall, in the grand scheme of things, most light is spontaneous rather than, as you wrote "Light is produced by stimulated emission all around us, all of the time" I agree that some is, but it's very much the minority.

Ah. Sorry, thought you were talking about the LASER there. My mistake. Agree that in the real world spontaneous emission is king but stimulated emission still occurs as a natural process.

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Also, I'm quite sure that more light spontaneously leaves the sides of a HeNe laser than comes out as the beam so, even in a laser, most of the light is spontaneous.

I'd disagree here. There will always be some spontaneously emitted photons produced from a populatoion inversion, and these will occur in random directions. You can think of these as 'seeding' the LASER though they will continue to be produced. Each is capable of producing stimulated emission photons from the atoms they pass as they travel. Of course this means that some light is emitted off-beam, though the majority is still due to stimulated emission. However, the optics of the LASER are designed to give multiple internal reflections along a single axis. This in turn gives a positive feedback mechanism i.e. more photons, gives stronger electromagnetic fields, gives more stimulated emission photons, gives more reflections etc... The result is that the makjority of photons are produced by stimulated emission and are coherent i.e. have the same phase, frequency, polarization, and direction of travel.
 

Offline Batroost

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« Reply #11 on: 10/05/2007 21:20:46 »
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The requirement for short pulses for fast kinetics experiments also produces broad bandwidths so it's not just a matter of power. The high power ones are just the most fun :-)

As an aside, when I was in my final year as an undergraduate I was using an Argon-Ion Laser with a 1 watt light output; roughly 100x blinding strength but looked fantastic. This had a 26kW water-cooled power supply, the size of a filing cabinet. Just a couple of years later you could get the same light output from a doped solid-state LASER that would fit in a shoebox and ran off batteries!
 

Offline neilep

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« Reply #12 on: 10/05/2007 21:56:47 »
About range, in about 200 metres the beam has diverged from a few millimetres to 20-40 centmetres, so its intensity is very reduced; I think you won't be able to see it after 1 km even in complete darkness.

I am surprised at such a wide divergence from a laser beam - is this a deliberate safety measure, or just a limitation of the small size of the diode?

I can vouch for this as I've owned a few of these in the past and those figures match up (albeit I can not be precisely exact...or even exactly precise !!)

I have managed to project the beam at least 100 meters  on to the side of wall.
 

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« Reply #12 on: 10/05/2007 21:56:47 »

 

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