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Old 06-03-2011, 06:38 AM #1
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Default Having a Stable Beam Output

Hi everyone!

I just joined the site and it feels like home.

I am conducting an experiment where I need to be sure that the output of my laser should be constant. I will drive the diode using pic mcu.

My first guess is to split the beam to half using a crystal and measure the power of the half to predict the power of the other half and this can be done by building a DIY thermo powermeter. I don't need calibration, I just need to make sure the output is stable, if it starts to drop the feedback will cool down the laser and operate then on. Do I need to drive my laser by PWM in order to get constant-close beam output?

Seeking for guidances thank you!


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Old 06-03-2011, 02:38 PM #2
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Default Re: Having a Stable Beam Output

If accuracy/calibration is not important you can
use a Photo detector (Photo diode... Solar Cell etc)
which will respond faster than a thermal sensor...

No... you don't need to drive your laser by PWM...
All you need is a beam to measure...


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Old 06-03-2011, 03:15 PM #3
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Default Re: Having a Stable Beam Output

Hello and welcome to the forum.
I don't know much about your issue in particular, but I DO know that many laser diodes
have an inbuilt photo sensor diode, you could tap into that monitoring current it gives off.
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Old 06-03-2011, 03:59 PM #4
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Default Re: Having a Stable Beam Output

Or use an ion laser in light-mode. ROCK stable.

A simpler way would be just to leave it on for 20 minutes before you start to work with it. By then, it should be very stable without any need for feedback.
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Old 06-03-2011, 06:00 PM #5
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Default Re: Having a Stable Beam Output

I have a cheap 30mw green laser pointer but when I measure it's intensity with an LDR or CCD, it gives different values each time.Very close but not same. What I need is a stable beam, when I turn this pointer on, it's intensity level start to decrease after a period of time and there is no vendor providing argon laser in my region.
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Old 06-03-2011, 06:54 PM #6
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Default Re: Having a Stable Beam Output

Most diode lasers other then those used in pointers have built in photodiodes for APC (Automatic Power Control). Simple APC driver circuits are all over the place.

Simple PWM circuits will just wreck havoc as a light control method because of thermal issues in the diode.

We have plenty of APC circuits posted in Sam's laser FAQ and APC drivers are available quite cheap from a variety of places.

If your diode does not have a built in photodiode, its easy to add a small piece of glass, split the beam off, and use a silicon cell or photodiode for the feedback signal.

APC is the same thing the ion lasers use as "light mode"


(Note to programmers, PWM can be just evil, it is not the catch all panacea software, mcu companies, and engineering instructors want you to believe it is.)

Steve

Last edited by LSRFAQ; 06-03-2011 at 06:58 PM.
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Old 06-12-2011, 03:25 PM #7
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Default Re: Having a Stable Beam Output

y33t: My guess is that your 'cheapie' 30mw green will disappoint you for all time. Cheap-o green lasers are designed using DPSS technology (diode-pumped solid state.) While the IR laser diode may be reasonably stable, the frequency doubling Nd:YVO4 and KTP are going to vary in output because of thermal constraints of the "pen" housing; just the inability of the copper/brass material to conduct the heat and kept a constant temperature. Add to that the "AA" battery issues with constant voltage and almost anything goes with power fluctuation. I disassembled one of these beasts and ran it from a regulated lab powersupply and the output stills jumps around. A good portion of that green beam may also consist of IR since many inexpensive green lasers leave out the IR filter.
If you take Jerry's advice (I would) and use a photocell for measuring power, you may find this document a good read: "Photodetectors and solar cells" by: Khanh Kieu (12/03/2009)...
You can Google for the link at Arizona.edu.
I know that you probably want to keep your experiments at $30 since that is what you have, but if your pockets are a weebit deeper or your experiments are important, you may wish to consider a green-lab unit from a supplier such as O-Like for about $118. Lab lasers use TEC coolers to control the heat dissipation and provide good stability. I cannot promise that O-Like will reply, but in my case I have had great results with questions that I posed to them via email using their "contact us" link.
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