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 $. If connected as on the next schema, it acts as a resistor. The two resistors make a voltage divider so I can measure the light.
As the second resistor I used something like 22 kohm. The more you use here, the more sensitive it will be (the same amount of light will give you more voltage).
Note: the laser must not be focused and it must be aimed slightly away of the diode!
But that would be quite hard to measure. You would have to step the current and make notes of the current and luminosity. So I came with my Arduino . I replaced the voltmeter and ampermeter with it like this:
Note the following:
Rr gives you maximum current as Imax = 1.25/Rr
Imin = 1.25/(100+Rr) ~ 10mA
The potentiometer is there to adjust the current from Imin to the Imax.
The Arduino will see the voltage on Ra, so this must not exceed 5V. E.g. Imax=150mA, then the Ra=5/0.150 = 33 ohm.
Then the Arduino reads the luminosity of the photodiode, again as voltage on Rp. Mentioned 22 kohm should be fine.
It then simply output the measured values according the following program:
So if you log the output as csv file, you can generate fancy graph in Excel
Okay, here’s how I did it:
Regulator + arduino
Power supply:
Mysterious box :
Box opened reveals the laser diode with cooler (clamp :-D), photodiode,wires and camera (to actually see something, very useful).
And everything together:
And on-going measurement:
And finally some results:
So what I learned?
The diode has threshold current around 65mA and kink point at 320mA. I guess the operating current should be somewhere around 250mA. Comparing to other laser diodes and their datasheets I would guess this IR diode to be about 200 mW. Pretty good for a 8$ 5mW green laser pointer, no?
Any suggestions, corrections and opinions are welcome
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);
}
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: