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Old 02-19-2011, 07:47 PM #1
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Question non lasing mode for a diode

Hi,

I am a french astronome and in order to check the quality of the mirror of my telescope i have made a bath interferometer. You can find what is it here :
Bath Interferometer - Starry Ridge

For this purpose i use a 1mW green dot laser diode module. It works well but it would be better if i could use the diode in a non-lasing mode because of some problems with diffraction...

With very cheap red dot laser diode, lowering input voltage can make the diode bright in non lasing mode but on my green one the electronic doesn't like and it doesn't really work.

Do you know how i can use a laser diode in an non-lasing mode ? Is there a way to change something on the elecronic of my diode ?

Thank you !


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Old 02-19-2011, 08:51 PM #2
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Default Re: non lasing mode for a diode

Hello and welcome to the forum!
That green laser you have is almost 99% not a simple diode but instead a "diode pumped solid state"
or DPSS laser.
It involves an infrared diode pumping a pair of nonlinear optic crystals.
Here's a picture (courtesy of Sam Goldwasser's excellent LASER Repair FAQ):


Your red laser light is produced directly by a diode.
That is why it is possible to run the diode below it's lasing threshold, where it will
emit photons spontaneously, but not stimulated emission.

The DPSS system on the other hand cannot work in this way, because it is ultimately
the crystal that produces the green laser light, not the infrared diode.
If the IR diode is operating below lasing threshold, the frequency doubling process
producing the green light simply doesn't happen at all.

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Old 02-20-2011, 09:16 AM #3
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Default Re: non lasing mode for a diode

Thank you

So i have to change. My green laser will not fulfill my wishes.
I have to choose a red laser diode module.

But i think in some kinds of red diode there are also some problems with electronique.
In a forum speaking about interferometry i had this reponse :

"Another red type is with laser diode which is in its transistor like housing,
containing also a photodiode which monitors power level via feedback
electronics. Lowering input voltage is counteracted by feedback
circuitry until it is not able to do it and abrupt changes or oscillations
are observed. Feedback electronic on a small printed circuit board,
usually has a small variable resistor which has a slot for a small screwdriver.
Its setting decides on power level kept constant by the circuitry.
I never tried to use it in attempt to reduce power to the non-lasing value,
and do not know if this is possible. "

So do you think if i buy one of these i can adjust the variable resistor with a screwdriver to adjust the diode to bright in non lasing mode ?

Or maybe you can help me to find where i can buy good laser diode module without those problems.
For my purpose, i need a diode module with :

-glass lens
-5mm beam diameter
- I think 5mW is good. (in fact 1mW effective power is enough but if i use it in a non lasing mode i think i have to choose higher power in order to have the same brightness...)
- the color is not important.

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Old 02-20-2011, 10:38 AM #4
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Question Re: non lasing mode for a diode

Or i have another idea. All the red laser diode i saw have three pin.
2 are for making the diode works and one is for a photodiode. I think the goal of the photodiode is to controle the diode.
If i buy a red diode in wich electronic is source of problems for me, maybe i have just to cut the photodiode pin ? What do you think about it ?
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Old 02-20-2011, 10:51 AM #5
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Default Re: non lasing mode for a diode

Quote:
Originally Posted by microgrit View Post
Or i have another idea. All the red laser diode i saw have three pin.
2 are for making the diode works and one is for a photodiode. I think the goal of the photodiode is to controle the diode.
If i buy a red diode in wich electronic is source of problems for me, maybe i have just to cut the photodiode pin ? What do you think about it ?
If you're aiming for brightness, then I would actually suggest a high-power red diode. These usually do around 200-350mW (depending on the type of diode and drive current).

They threshold (begin lasing) at around 60-75mA, and at 50mA, they are extremely bright, yet still well away from lasing threshold.

This may work if you're aiming for brightness only.
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Old 02-20-2011, 01:05 PM #6
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Default Re: non lasing mode for a diode

Quote:
Originally Posted by microgrit View Post
...
For this purpose i use a 1mW green dot laser diode module. It works well but it would be better if i could use the diode in a non-lasing mode because of some problems with diffraction...
Diffraction will always happen, so why take an incoherent source? Interferometers need the coherence of the light source to work.
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Old 02-21-2011, 07:34 PM #7
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Default Re: non lasing mode for a diode

Thank you for your help.

goninanbl00d : i think 200-350mW is far too much ! May be it would be expensive and dangerous for my eyes ! This afternoon, a friend of mine gave me a 5mW red diode and it was almost what i need. I just need a little more. I think i am going to buy a 10mW.

Bluefan : this interferometer only need monochromatic collimated source. There is no need to have coherent light. When using non coherent source there is no speckles and it is better.
Using a laser diode is easy and not expensive to have monochromatic collimated source but i don't need to have a coherent one.

And i have one more question for you : What about blue laser diode like this :
405nm 20mw blue laser AixiZ exclusive 12x30mm adj lens en vente sur eBay.fr (fin le 22-févr.-11 18:25:23 Paris)

Is it possible to use it in non lasing mode like red diode or is it a DPSS laser like green one ?

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Old 02-23-2011, 11:35 AM #8
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Default Re: non lasing mode for a diode

monochromatic = coherent. A speckle pattern form when the coherence length is larger than the surface features, which gives the speckles. A light source that has a short coherence length won't produce speckle pattern on average will have the same amount of scattered or diffracted light. But this short coherence length doesn't work well when you have an interferometer. It's better to take the source of the scattering and thus the speckle pattern away.
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Old 02-23-2011, 11:56 AM #9
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Default Re: non lasing mode for a diode

Quote:
Originally Posted by microgrit View Post
Thank you for your help.

goninanbl00d : i think 200-350mW is far too much ! May be it would be expensive and dangerous for my eyes ! This afternoon, a friend of mine gave me a 5mW red diode and it was almost what i need. I just need a little more. I think i am going to buy a 10mW.
A red diode capable of 200+ mW is not that expensive, in the order of $10.

Obviously it would be very dangerous to use it in lasing mode, but as long as you stay below the threshold, the power output is not that high. And you can always run it at even lower current when you need less light.

The upside of a more powerful diode is that is also as a higher lasing threshold. This allows the production of more light before it starts lasing.

One thing though: You say you need monochromatic light. When you operate a laser diode below threshold, the light is much less monochromatic compared to lasing operation. I think its a gaussian distribution about 10 nm wide (10%), which may be a problem?
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Old 02-23-2011, 08:02 PM #10
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Default Re: non lasing mode for a diode

10nm wide is not really a problem.

Do you have link to a web shop where i can buy this can of diode ?
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Old 02-23-2011, 08:26 PM #11
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Default Re: non lasing mode for a diode

Look in the tranding boards here. They are typically referred to as LCC or LOC (closed and open can respetively), or LPC-815 by serial number (same as LOC).
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Old 02-25-2011, 11:36 AM #12
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Default Re: non lasing mode for a diode

Oh yes ! This is diode coming from DVD burner. It's a cheap funnny project. I am going to use it.

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Old 02-25-2011, 04:03 PM #13
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Default Re: non lasing mode for a diode

Post some results of your project when its done - looks interesting
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