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FrozenGate by Avery

Calibration Lasers for LPMs

i wish i could troll as well as TJ no joke that's my goal if you can get to his level you are awesome i worship his trolling. because its true and he blatantly tells the truth doesnt say stupid sh!t...speaking of witch where the hell is he? he hasn't been on for a month?
 





Wolfman29 said:
EDIT: @IE: I think that if someone made it a plug-in module, it would be easily doable AND stable (with exception of the ambient temperature). But remember, the point of this ISN'T to make something that is the highest of high quality - just something stable for about a minute, so that they can be calibrated. If it was too complex, it wouldn't be worth it to not buy another LPM or get it "professionally" calibrated.
Click to expand...

One problem with this is that it may take much more than a minute to calibrate a power meter, especially if its a thermal one with resonable thermal mass. Then again, the output of a diode laser doesnt vary -that- much with temperature. If you are aiming for a few % error its not likely to be a problem.

The only thing needed would be to construct a driver thats better than most: insensitive of input voltage and reasonably low temperature drift. Something like that isnt hard to design, but you will need a host with reasonable room for the driver... its probably best to have a 10/25 turn pot on it to set the exact power.
 
Why would you need a host? Why not just use a project box for the calibration laser? A calibration laser doesn't need to be handhold :P
 
I've considered the idea too, but a $50 price point it too optimistic. What I'd do is build a temperature stabilized laser with photodiode feedback. That way it's very easy to get it very stable, my guess would be better than 1%. I may try and sell calibrated somewhere in the future (or sooner if anyone would be interested).
 
It wouldn't even need to be temperature stabilized. The photodiode feedback should be enough for <1%, shouldn't it?
 
Temperature stabilisation makes the wavelength and thus the photodiode response more stable. Noise and modehopping is also more constant, but I haven't done experiments yet. It's strange though how professional lasers are usually specified to have something like a 2% drift over a few hours, more drift than I expected, so there's probably more to it than it seems.
 
Photodiode feedback systems are often problematic, since you also need to deal with the wavelength and thermal response differences in the photodiode... not even mentioning the need for extreme mechanical stability to get a constant feedback ratio to the photodiode in the first place.

As for the drift specifications: they are maximums, a worst case situation resulting of all factors influencing the laser in the same direction. You could, for example, have a voltage reference that has a negative temperature slope, combined with a diode that has a negative efficiency/temperature slope. Add to that degradation of the laser diode and optics over time, and all those factors can cause the output to drop a percent or two over an hour even in a very good system.

Its a bit comparable to component tolerances. Resistors are often made with 5% tolerance, but if you get once batch of 100 pieces, you will likely find all 100 off them within 1% of eachother, and the average within 1% of the specified value. This doesnt exclude the possibility that there is one that is 5% off if you test a million of them from a monday morning batch ;)
 
Why not run a small switched mode laser driver off of a voltage regulator, powering a single mode PHR 405nm at ~50mws?

That diode won't get hot all at that output, the driver shouldn't dissipate much heat at all, an arcylic lens won't deteriorate so quickly at such low power, and the 405nm wavelength is happily absorbed by most any surface.

Would such a setup not be VERY consistent, even after a period of years?
 
The problem is that we wouldn't use a low 50mW Laser to calibrate
a 2000mW to 3000mW LPM...

Jerry
 
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It would indeed be NOT consistent. If you don't control the temperature that little heat will change things, and the ambient changes will throw everything even futher off. A temperature controlled system with photodiode feedback would do far better.

Resistor are usually 1% and usually make good use of that spread by the way.

I'm currently building a high end 445nm lab laser with cylindrical beam shaping optics, temperature regulation and photodiode feedback. It will take a while for it to complete, but I will report back my findings on stability.
 
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