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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
 Posted - 05/09/2006 : 16:26:13
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A guy in the astronomy club disassembled his "astronomer's" green laser pointer last week to see how it worked. Now, this guy is not an electronics guy, nor know practically nothing about advanced physics. But his description made me curious about what he found.
He said that the laser diod itseld glowed bright red, but the (invisible) beam then reached some kind of small semi-transparent disc where the the beam became visibly green.
I know green lasers are used because they scatter and reflect much more easily in the atmosphere than red, and so a red beam would be mostly invisible from the view of the pointer, where a green beam is visible.
I've read that a non-linear medium can change the wavelength of the photon (double it double the frquency, halving the wavelength, just like radio-frequencies in non-linear components). I read an article some 10 years ago to that effect in "Illustrated Science" (I think) where they used a deep red laser, then doubled the frequency of the light to make a green or blue light.
This is what I proposed happened in that semi-transparent disc, but the others at the astronomy meeting disagreed, because the half the wavelength of bright red would put the beam in ultra violet.
Someone got a clue?
(Edited for clarity, in red above)
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Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
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Edited by - Dr. Mabuse on 05/09/2006 20:08:35
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H. Humbert
SFN Die Hard
USA
4574 Posts |
 Posted - 05/09/2006 : 16:58:02 [Permalink]
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According to this site:
quote:
The green lasers you can buy probably use DPSSFD (Diode Pumped Solid State Frequency Doubled) technology to make the green laser beam. Unless you think you can find an argon gas laser that's cheap and can be made the size of an ink pen. ;-)
Wikipedia's article on Diode pumped solid state laser says:
quote:
Diode pumped solid state (DPSS) lasers are solid-state lasers made by pumping a solid gain medium, for example, a ruby or a neodymium-doped YAG crystal, with a laser diode.
The most common DPSS laser in use is the 532 nm wavelength green laser pointer. A powerful (>200 milliwatt) 808 nm wavelength infrared laser diode pumps a neodymium doped yttrium orthvanadate (Nd:YVO4) crystal which produces 1064 nm wavelength light. This is then frequency doubled using a nonlinear optical process in a KTP (potassium titanyl phosphate) crystal, producing 532 nm light.
I have no idea what anything I just quoted means.
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"A man is his own easiest dupe, for what he wishes to be true he generally believes to be true." --Demosthenes
"The first principle is that you must not fool yourself - and you are the easiest person to fool." --Richard P. Feynman
"Face facts with dignity." --found inside a fortune cookie |
| Edited by - H. Humbert on 05/09/2006 17:00:57 |
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JohnOAS
SFN Regular
Australia
800 Posts |
Posted - 05/09/2006 : 18:12:40 [Permalink]
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quote:
Originally posted by Dr. Mabuse
A guy in the astronomy club disassembled his "astronomer's" green laser pointer last week to see how it worked. Now, this guy is not an electronics guy, nor know practically nothing about advanced physics. But his description made me curious about what he found.
He said that the laser diod itseld glowed bright red, but the (invisible) beam then reached some kind of small semi-transparent disc where the the beam became visibly green.
I know green lasers are used because they scatter and reflect much more easily in the atmosphere than red, and so a red beam would be mostly invisible from the view of the pointer, where a green beam is visible.
I've read that a non-linear medium can change the wavelength of the photon (double it). I read an article some 10 years ago to that effect in "Illustrated Science" (I think) where they used a deep red laser, then doubled the frequency of the light to make a green or blue light.
This is what I proposed happened in that semi-transparent disc, but the others at the astronomy meeting disagreed, because the half the wavelength of bright red would put the beam in ultra violet.
Someone got a clue?
Green pointers are usually 532 nm, being frequency doubled from 1064nm. The red glow you can see wouldn't be 1064, but rather a shorter wavelength light from the LED's used to pump the laser crystal, at around 800 nm. The laser medium is usually Neodymium Vanadate. (NdYVO4), the frequency doubling crystal is almost certainly KTP (KTiOPO4)
I worked with a guy whose PhD thesis was on frequency doubling with Vanadate and YAG lasers, back before they were commodity items. We used to have to rotate around the crystals to find undamaged sections we could use, as they weren't anywhere near as robust as we needed. You could easily blow a good chunk of your annual salary in one bad afternoon in the lab, and considering our salaries often came out of the same pool of money as the lab gear, it wasn't a reassuring feeling.
Edit: Goddammit, I should've known, Wikipedia already has a good article on this here. I also found a very nice illustration here, which came from a quite informative article called "Dissection of Green Laser Pointer". This internet thing certainly makes me feel redundant at times.
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John's just this guy, you know. |
| Edited by - JohnOAS on 05/09/2006 18:20:59 |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 05/09/2006 : 20:01:43 [Permalink]
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Gaddamn, this means that I was almost right to begin with. I only missed one stage...
My friends at the astronomy club were very skeptical about the idea that a non-linear medium would produce a shorter wavelenth, because photons are discreet packets of energy, and as such cannot spontaneously double it's energy. |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
Support American Troops in Iraq:
Send them unarmed civilians for target practice..
Collateralmurder. |
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JohnOAS
SFN Regular
Australia
800 Posts |
Posted - 05/09/2006 : 20:31:24 [Permalink]
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quote:
Originally posted by Dr. Mabuse
Gaddamn, this means that I was almost right to begin with. I only missed one stage...
My friends at the astronomy club were very skeptical about the idea that a non-linear medium would produce a shorter wavelenth, because photons are discreet packets of energy, and as such cannot spontaneously double it's energy.
Frequency doubling of this sort is more correctly called second harmonic generation. It does require the exciting photons to be phase matched, and to meet intensity requirements for the non-linear process to occur. Of course it's not a perfectly efficient process, so you actually end up with less that half of the photons at the higher energy (shorter wavelength). It can be quite good, better than 90% for the right conditions.
It is also possible to do sum, difference and even other harmonic species generation, if the wavelengths are right. Combinations of any of the above provide more available combinations.
You can also produce photons of shorter wavelengths in some mediums by step-wise excitation, pushing the energy levels up via 2 or more excitation transitions, to the unstable state where is decays producing a single photon at the wavelength you're interested in. The trick is finding materials where the intermediate states are stable enough to have the whole work at reasonable efficiencies, if at all. |
John's just this guy, you know. |
| Edited by - JohnOAS on 05/09/2006 20:33:16 |
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Ghost_Skeptic
SFN Regular
Canada
510 Posts |
Posted - 05/09/2006 : 23:30:21 [Permalink]
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quote:
Originally posted by H. Humbert
I have no idea what anything I just quoted means.
That has never stopped Bill Scott |
"You can lead a horse to water but you can't make him drink. / You can send a kid to college but you can't make him think." - B.B. King
History is made by stupid people - The Arrogant Worms
"The greater the ignorance the greater the dogmatism." - William Osler
"Religion is the natural home of the psychopath" - Pat Condell
"The day will come when the mystical generation of Jesus, by the supreme being as his father in the womb of a virgin, will be classed with the fable of the generation of Minerva in the brain of Jupiter" - Thomas Jefferson |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 05/10/2006 : 11:26:30 [Permalink]
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quote:
Originally posted by Ghost_Skeptic
quote:
Originally posted by H. Humbert
I have no idea what anything I just quoted means.
That has never stopped Bill Scott
Aww, man... That was a cheap shot. 
The efficiency of lasers are generally relatively low. |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
Support American Troops in Iraq:
Send them unarmed civilians for target practice..
Collateralmurder. |
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Dr. Mabuse
Septic Fiend
Sweden
9688 Posts |
Posted - 05/10/2006 : 11:29:11 [Permalink]
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quote:
Originally posted by H. Humbert
I have no idea what anything I just quoted means.
At least you're honest enough to let us know, unlink some other people.
(see, Ghost-skeptic, you can be offensive without naming anyone. We all still know who I mean... ) |
Dr. Mabuse - "When the going gets tough, the tough get Duct-tape..."
Dr. Mabuse whisper.mp3
"Equivocation is not just a job, for a creationist it's a way of life..." Dr. Mabuse
Support American Troops in Iraq:
Send them unarmed civilians for target practice..
Collateralmurder. |
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