Method of Calibrating LPMs

[piferal]

More easier, you can also buy a laser module with integrated APC control, with good stability for a little money, like this one for example

10mW Focusing Red Laser Module - Premium Quality (12mm) - Detailed item view - OdicForce Lasers Online Shop

Or this other.

20mW Focusing Red Laser Module - Premium Quality - Detailed item view - OdicForce Lasers Online Shop

 

[Wolfman29]

@HIM: I could never figure out how to get photodiodes to work. We measure the current they give off, which is proportional the amount of light they receive? Is that it? And what is the constant of proportionality?

@piferal: I like the idea of that, actually. Unfortunately, they only give up to 10% accuracy - the highest one gives between 18-20mW, which is about 10% accuracy.

 

[piferal]

Ok, but this one is a very affordable module, I think you can not ask much more for this price

Second, if once calibrated with this module, and if the response of your LPM is "linear, proporcional" (I do not know if it's the right word),
that means measuring for example a 100mW laser, if only have a maximum error of 2 mW, which I think is very acceptable.

But I guess to confirm this, you would have a several APC modules with different powers (knowing its real power to all modules) to verify and confirm whether at each power is only 2mW of possible difference.

Don't know if I explain well with my poor English :thinking:

 

[Wolfman29]

Well, the issue is, if I pointed it at my LPM and assumed it was 10mW, so I calibrated it to 10mW, but it was actually 8mW, because my LPM isn't linear and it works proportionally, something that was 100mW would be 80mW.

 

[piferal]

Yes, so I say you'd have to check with several modules of various known powers to check this out

 

[Wolfman29]

Hmm. Well, I'm not really concerned with this issue anymore because as of yesterday, I received my laser back in the mail which Dave did a reading on, so I got a calibration laser!

 

[lasersbee]

What kind of Laser is it..
What is the power and wavelength...:thinking:

Just curious...

Jerry

 

[Wolfman29]

Well, it's a DIY laser based on the LPC-815 and a CC driver putting out 424mA, which Dave tested to give me 250mW with fresh batteries, sagging to 245mW as they are drained more.

After calibration, I then went about testing my 473nm laser on the TEC to make sure that the wavelength absorption rate is near linear, and the 473nm is set to 10mW, and it was reading about 10mW or so (don't have decimal points, so I was just getting a reading of .010).

So, as far as I am concerned, my LPM is now pretty well calibrated!

 

[HIMNL9]

Uhm, ok ..... my old calibrator was another of the crazy projects i built years ago, for the units i had at my old workplace, after i've seen how much money they asked for recalibrate our Coherent unit ..... so i just had this crazy idea: "what if i make a laser unit stable enough, that can be used as reference, and that permit me to self-check my instruments for different wavelenghts ?"

"Model number 1" was simply using green and red ..... then i modified it, when PHR803 sleds becomed available at decent prices, and converted it in a 4-wavelenghts unit ..... now it's IR - red - green - BR (just cause this way i can check for different absorption properties for the reading plates).

It's not a single-beam assembly, i had no reasons for mix the beams like in a projector, it simply have the beams side by side (except for red and IR, where i used the double diode from the PHR) ..... and the beams are not focused for infinite distance, they simply project a "spot" of few millimeters at a specific given distance ..... but it worked for years without problems, for me

About the photodiodes, you need to use them in the feedback branch of the driver, just for keep the output stable, so is not so important the linearity and response curve, cause is not used as measuring sensor ..... it just regulate the current, inversely proportional to the light it take (in poor words, when the light increase, it lower the current til the light go back to the preset point, and when the light decrease, it increase the current).

There are a lot of possible circuits, for use a photodiode as feedback sensor, from the simplest ones using a pair of transistors, to the more complex ones with op-amps and temperature compensation ..... i can draw some examples if needed, but you can also found them already posted here and there in internet, probably.

 

[Wolfman29]

Well that's a need calibration unit!