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Old 01-09-2012, 09:06 PM #1
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Default How I measured my laser diode with Arduino

Hi everyone. First - I am new here and I am new to lasers, however I learned a lot from this project and I think it might be benefitial to some of you so I share it here.

I got a new 8$ 5 mW green laser from DX and because I'm quite curious, I decided to open it. Well, no surprise - case, 808nm diode, driver, KTP crystal, optics. I was wondering how much power that diode can handle because I simply had no clue what type it is. So I constructed a simple device to measure the laser diode.



Prerequisites:
* little electrical knowledge and equipment (multimeter, wires, stable power supply, etc..)
* knowledge about laser diodes

Theory:
Laser diode power output depend on the current flowing through it approximately like on this picture:

On the next picture, we see a kink level and COD (catastrophic optical damage).

The diode starts lasing at threshold current Ith (the function starts raising steadily on the graph).

With some stuff, I should be able to measure the Ith and the kink level.

Construction:
So I started constructing current regulator based on LM317 as this:

Because I had self-made 12V power supply which was easy to switch polarity on, I made some modifications:

The minimum would be 5V as noted on the schema. The resistor is to set the current, the diode to save the thing if I accidentally reversed polarity. The capacitor filters some peaks.
And I bought a photodiode off the local electro shop for 0.5 $. If connected as on the next schema, it acts as a resistor. The two resistors make a voltage divider so I can measure the light.


As the second resistor I used something like 22 kohm. The more you use here, the more sensitive it will be (the same amount of light will give you more voltage).
Note: the laser must not be focused and it must be aimed slightly away of the diode!
But that would be quite hard to measure. You would have to step the current and make notes of the current and luminosity. So I came with my Arduino :-). I replaced the voltmeter and ampermeter with it like this:

Note the following:
Rr gives you maximum current as Imax = 1.25/Rr
Imin = 1.25/(100+Rr) ~ 10mA
The potentiometer is there to adjust the current from Imin to the Imax.
The Arduino will see the voltage on Ra, so this must not exceed 5V. E.g. Imax=150mA, then the Ra=5/0.150 = 33 ohm.
Then the Arduino reads the luminosity of the photodiode, again as voltage on Rp. Mentioned 22 kohm should be fine.
It then simply output the measured values according the following program:
Code:
#define VOLTAGE1_PIN A0
#define VOLTAGE2_PIN A1

void setup(){
	Serial.begin(9600);
}

const double R = 33; //ohm

void loop(){
	int a1 = analogRead(VOLTAGE1_PIN);
	int a2 = analogRead(VOLTAGE2_PIN);

	double Ur = (double)a1 / 1024.0 * 5.0 * 1000.0;	//5000mV max
	double Id = Ur / R; 

	double Ufoto = (double)a2 / 1024.0 * 5.0 * 1000.0;

	Serial.print(Id);
	Serial.print(";");
	Serial.println(Ufoto);
}
So if you log the output as csv file, you can generate fancy graph in Excel :-)
Okay, here’s how I did it:
Regulator + arduino

Power supply:


Mysterious box :-):

Box opened reveals the laser diode with cooler (clamp :-D), photodiode,wires and camera (to actually see something, very useful).



And everything together:

And on-going measurement:

And finally some results:

So what I learned?
The diode has threshold current around 65mA and kink point at 320mA. I guess the operating current should be somewhere around 250mA. Comparing to other laser diodes and their datasheets I would guess this IR diode to be about 200 mW. Pretty good for a 8$ 5mW green laser pointer, no?

Any suggestions, corrections and opinions are welcome :-)



Last edited by edy; 01-10-2012 at 09:26 AM. Reason: correction
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Old 01-09-2012, 09:26 PM #2
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Default Re: How I measured my laser diode for few $

Nicely done...
A different way of making a Power/Current graph...

BTW welcome to the Forum...

There is just one problem with pic#5... The input volage
of ~4.4 VDC is too low to run the diode of its current
requirements due to the voltage loss of the reference
voltage and the LM317 fV drop of about 2.5VDC.


Jerry
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Last edited by lasersbee; 01-09-2012 at 09:29 PM. Reason: Spelling errors
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Old 01-09-2012, 10:32 PM #3
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Default Re: How I measured my laser diode for few $

Quote:
Originally Posted by lasersbee View Post
Nicely done...
A different way of making a Power/Current graph...

BTW welcome to the Forum...

There is just one problem with pic#5... The input volage
of ~4.4 VDC is too low to run the diode of its current
requirements due to the voltage loss of the reference
voltage and the LM317 fV drop of about 2.5VDC.


Jerry
Yea, you're probably right. I forgot that safety diode, so 0.7+2.5+2... ~6V should be enough then (depends on type of laser diode). I used 12V anyway because I needed 0-5V on Ra :-).
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Old 01-10-2012, 10:54 AM #4
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Default Re: How I measured my laser diode with Arduino

Btw. do you know what laser diode it is? It doesn't seem like a regular open can type I saw on pictures..




And... how do I get it out?

Last edited by edy; 01-10-2012 at 10:56 AM.
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Old 01-10-2012, 02:43 PM #5
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Default Re: How I measured my laser diode with Arduino

It looks like an ir 808nm pump diode for a green laser. The red wire is attach to the case pin. I think the case is positive. The crystal set has been broken off. You can see the glue marks at the front.

It is not easy to get it out in one piece. Most likely it will be broken in the process but 808nm diode replacement is pretty cheap on Ebay.
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Last edited by Blord; 01-10-2012 at 02:43 PM.
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Old 01-10-2012, 03:04 PM #6
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Default Re: How I measured my laser diode with Arduino

Well, thanks, lol. I soldered those two wires, so yes, red is positive. And I removed the crystal as well. I was wondering how does the diode look like out of the case. It does not look like this one, however it is quite similar:



I guess it is pressed in so I would need to press on that little piece of metal without touching the chip inside, am I right?

I will now try to collimate it with lenses from my cd burner.

Last edited by edy; 01-10-2012 at 03:06 PM.
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Old 01-19-2012, 10:05 AM #7
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Default Re: How I measured my laser diode with Arduino

I finally managed to destroy the diode.. Due to excessive mechanical stress, the negative pin started spinning and tore one of the tiny wires by the chip. So it shines , but don't lases now . I'll try to repair it, but don't give it too much hope :-)
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Old 01-20-2012, 09:13 AM #8
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Default Re: How I measured my laser diode with Arduino

Well, at least you have a nice host for something better. You can buy more powerful replacement modules.
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