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Old 02-27-2008, 12:37 PM #1
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Default Simple LASER power meter using a Peltier?

I was thinking about my tiny portable peltier cooling system for my open can, and remembered, that these peltiers actually produce current, when a temperature difference is present between it's sides.


Then i thought, what if i were to mount one side to a heatsink and paint the other black.. If i would shine a laser on the black side, the peltier would start producing a certain voltage, relative to the power of the laser.. The peltier would have to be very small (15x15mm) to "feel" this tiny difference at all, but if i would then callibrate it using a known power LD, i could calculate the power of others, compared to the known ones...


I've been thinking of using small solar cells before, but they would only be able to compare between the same wavelength, since solar cells react differently to different wavelengths.. But heat is heat, regardless of the wavelength.


This could actually work for higher power lasers, but the resolution would be questionable... Even the tinyest peltier is capable of pumping 3.9W of heat, so 200mW would be very little to it.. Still, with an exact multimeter, the readings could be usefull, at least for comparing the power of different lasers.


EDIT: At first i thought different ambient temperature would give different results, but the ambient temperature is the same on both sides anyway.. Only the difference matters, and this would be caused by the laser beam.

In fact, i'm gonna test it now.


Any thoughts?


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Old 02-27-2008, 02:11 PM #2
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Default Re: Simple LASER power meter?

It actually works!!!!

Just the temperature difference of my finger on it creates a current of 15uA or an open circuit voltage of 16.6mV...
But it starts dropping soon, because the other side of it warms up as well... This means, i have to mount it on a heatsink, to keep the other side cool..

With a heatsink, my finger creates 40mV.. Even just shining the laser on the white surface creates a voltage.. If i paint it black, this voltage is going to be much greater...


I'm amazed at the fact, that it actually works like i imagined it...


With an amplifier circuit the resolution could be greatly increased, and properly calibrated it could just be a good cheap substitute for an expensive laser power meter..
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Old 02-27-2008, 02:30 PM #3
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Default Re: Simple LASER power meter using a Peltier?

I think you'll also find that the size doesn't really matter either. *This is where I started before finding the surplus Coherent sensors. *The problem is creating a coating that is spectrally flat and tolerant of relatively high power densities. *Paint doesn't cut it. * *"The Doctor" on PL forums has documented a process for the coating and a schematic for the amplifier.
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Old 02-27-2008, 03:32 PM #4
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Default Re: Simple LASER power meter using a Peltier?

Quote:
Originally Posted by knimrod
I think you'll also find that the size doesn't really matter either. *This is where I started before finding the surplus Coherent sensors. *The problem is creating a coating that is spectrally flat and tolerant of relatively high power densities. *Paint doesn't cut it. * *"The Doctor" on PL forums has documented a process for the coating and a schematic for the amplifier.
Yes, i thought about that.. It would have to respond to all wavelengths the same..

Thanks for the link!


So thermopiles contain a peltier? What else? I need to look into it a bit more...

A peltier alone wouldn't do, since it picks up heat very slowly.. Well, it could serve for comparing, but that's it.. And it wouldn't be very exact...
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Old 02-27-2008, 03:44 PM #5
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Default Re: Simple LASER power meter using a Peltier?

Perhaps it would be doable to glue a small, thin piece of black anodized aluminium to the hot side working as a target? I'm not sure how much the anodization can take, but combined with the heat conductivity of the alu below it's likely to be more resilient than any kind of paint.

I wonder what the current/power curve of a setup like this would be like.. predictable, or something with a difficult calibration curve?
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Old 02-27-2008, 03:54 PM #6
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Default Re: Simple LASER power meter using a Peltier?

Quote:
Originally Posted by Benm
I wonder what the current/power curve of a setup like this would be like.. predictable, or something with a difficult calibration curve?
Well, i did a couple of tests, and it always responds the same.. It always shows the same value with the same laser..
But unfortunatelly, it takes very long to get to that value.. It's painfully slow, and towards the end, it gets even slower.

It would require some sort of active setup to be really usefull.


The more i think about it, the more i'd like to buy a thermopile.. But they don't ship outside of US...
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Old 02-27-2008, 04:35 PM #7
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Default Re: Simple LASER power meter using a Peltier?

Won't the TEC give another voltage if the cold side has another temperature? How will you calibrate it?
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Old 02-27-2008, 05:07 PM #8
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Default Re: Simple LASER power meter using a Peltier?

Quote:
Originally Posted by IgorT

I've been thinking of using small solar cells before, but they would only be able to compare between the same wavelength, since solar cells react differently to different wavelengths.. But heat is heat, regardless of the wavelength.


Any thoughts?

See this link, they do respond differently to different wavelengths but if you know how they respond then you dont even need to calibrate them, you can just do math to figure out what the power is no matter what the wavelength is.

The circuit to use them has the photodiode, a battery, and a resistor and that is all you need to measure power.

http://www.thorlabs.com/Thorcat/0600/0637-S01.pdf

They cost 9 dollars if you buy them directly from hamamatsu
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Old 02-27-2008, 05:21 PM #9
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Default Re: Simple LASER power meter using a Peltier?

Such a photodector wouldn't work for anything with a bit of power though..

Active Area: 13.0mm
Damage Threshold CW: 100 mW/cm2

A 1 mW laser focussed to a 1mm2 would already be on the limit. Also, the active area isn't all that big, which requires a narrow beam and good aim to get a usable reading.

Perhaps when used with a ND filter that drops 90/99/99.9 of incoming light it would be usable, but the tiny active area is still a drawback if you ask me... even the ca 5mm diameter beam produced by the standard aixiz optic would be too large to fit.
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Old 02-27-2008, 05:25 PM #10
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Default Re: Simple LASER power meter using a Peltier?

I have used the above mentioned sensor lots of times, if you have a ND2 filter and a lens the power handling and sensor size are not a problem, for 20 dollars in total cost of a power meter you cant beat it.
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Default Re: Simple LASER power meter using a Peltier?

in this ebay auction it looks like a pelter stuck on a large aluminium sink with a heater built in. and it looks like a black disk has been glued to the pelter.
http://cgi.ebay.com/SCIENTECH-LASER-...QQcmdZViewItem
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Old 03-01-2008, 08:10 PM #12
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Default Re: Simple LASER power meter using a Peltier?

Quote:
Originally Posted by woop
in this ebay auction it looks like a pelter stuck on a large aluminium sink with a heater built in. and it looks like a black disk has been glued to the pelter.
It's interesting, how black the coating on that disk looks in this picture.. Must be some special coating, to achieve a flat spectral response..

Unless it's one of those LPMs, that come with a correction tables for different wavelengths..


Anyway, i decided to go for the proper thermopile. I want a good accurate LPM, without too much guesswork.. It's not that expensive anyway, and the results will be much more consistent.
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Old 03-01-2008, 08:36 PM #13
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Default Re: Simple LASER power meter using a Peltier?

A thermopile is similar to a peltier. The difference is a peltier is a power device designed to turn energy into temperature while a thermopile is a highly sensitive device that does the opposite. And I'm afraid this has been tried MANY MANY times before, it SOMETIMES works and the results are marginally more accurate than guessing. A kenometer is the way to go.
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Old 03-01-2008, 08:45 PM #14
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Default Re: Simple LASER power meter using a Peltier?

Quote:
Originally Posted by BlueFusion
A thermopile is similar to a peltier. The difference is a peltier is a power device designed to turn energy into temperature while a thermopile is a highly sensitive device that does the opposite. And I'm afraid this has been tried MANY MANY times before, it SOMETIMES works and the results are marginally more accurate than guessing. A kenometer is the way to go.
Actually, a peltier doesn't turn electricity into heat. It just pumps heat from one side to the other in the direction of electron flow. It's a heat pump..
But it can also be used the other way around.. If you present a temperature difference between it's sides, it creates a current..

The problem is, that it is WAY too slow.. The current climbs very slowly. A thermopile is better, because it's much more sensitive, and shows the result immediatelly. So i'm building that instead. There's no point in trying a solar cell or a peltier, if i can get a proper callibrated thermopile relativelly cheaply..
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