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Registered Member #151
Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153
Hello guys. Look what I just tried. This is a 40x40 mm peltier module connected directly to a voltmeter. I made a black bolt on it with a permanent marker for better absorption. The laser is 235mW (checked) 650nm based on a DVD-RW laser diode. Reading is 9mV and it becomes stable 45s after turning the laser on. What do you think about this idea? I think that smaller peltier module (maybe 10x10mm) with a better absorption plate would work significantly better. Price of this is nothing in compare to thermopile laser power probe, even a used one, that I'm certainly not only on this forum who would love to own one
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
There was a post on about this technique, and the general consensus was that yes it works, but its slow (as you noticed) dependent on drafts etc in the room, and to a lesser degree ambient temperature. If you just want a ballpark figure for measuring mystery lasers powers it would probably work great
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
How about using some graphite clamped to the peltier as a way to up the power handling? I have some pyrolytic graphite that I could lap smooth and then stick on a peltier via some thermal grease. Seems this would be capable of some high powers (or long warm up times).
Im guessing one would need a calibrated laser to calibrate this sensor, no? Would be nice to test some of my REAL high power diodes (40watts plus). As of right now, if it catches white paper on fire then I assume its good. ))
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
For higher powers I would recommend putting a thermistor on the tec, and running it in a a control loop (for simple experiments a simple proportional control would probably work fine but a full pid loop would speed up the response considerably). You can compare the power it takes to keep the tec at a constant temperature with no load vs what it takes to keep it at the same temperature with the laser shining on it, which you could then use to extract the laser power using the specifications from the tec data sheet. Although due to various nonlinearities you really are better off dumping the tec altogether and just using a resistor mounted to the heatsink, and keeping the heatsink at a constant temperature. By looking at the difference in power fed to the resistor with the laser on and off you can accurately extract the laser power.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
... wrote ...
For higher powers I would recommend putting a thermistor on the tec, and running it in a a control loop (for simple experiments a simple proportional control would probably work fine but a full pid loop would speed up the response considerably). You can compare the power it takes to keep the tec at a constant temperature with no load vs what it takes to keep it at the same temperature with the laser shining on it, which you could then use to extract the laser power using the specifications from the tec data sheet. Although due to various nonlinearities you really are better off dumping the tec altogether and just using a resistor mounted to the heatsink, and keeping the heatsink at a constant temperature. By looking at the difference in power fed to the resistor with the laser on and off you can accurately extract the laser power.
Thats a great idea about the heatsink/resistor and PID idea Pete! I have been looking up PID controllers on ebay for temperature, and they all seem complicated to use. Are there any out there that let me simply set the desired temperature and then be on my way? They seem to have alot of programming needs and parameters to deal with. I have alot of low ohm high power resistors on heatsinks to use as warmers.
This could also double for my TEC/40watt laser diode combo if I can figure out how they work.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
By the time you go buy a real pid controller, you may as well just buy a used thermopile head on ebay, wihch will probably come with a sticker on the back that says xx volts/watt that you can plug into a precision voltmeter (or amplifier and normal voltmeter) to read laser power.
A bolometer is an even simpler device, you simple have a heatsink, a thermal interface, and a heat absorber. You shine the laser on the heat absorber, and measure the temperature drop across the thermal interface--which will be proportional to the laser power.
I should point out that there are a lot of easy ways to measure laser powers, the tricky part usually comes with you want to measure small laser powers (1mw isn't going to do a whole lot of heating no matter what you shine it at). For higher power lasers, you can do really simple tests like painting an aluminum cube black and shining your laser on it for a few seconds. You can then calculate the energy delivered by looking at the temperature rise of the cube, and using E=nC(delta)T, and divide by the time of the exposure to get power.
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Measuring laser power with a peltier module
I have alot of low ohm high power resistors on heatsinks to use as warmers.