Another Simple Laser Power Meter

Recently, I remembered that I had some very small c-mount sized TEC's that I had of course purchased for cooling c-mount LD's. Realizing that a TEC can also convert a difference in heat between its' two sides into a voltage, I began to wonder if shining a laser beam onto a TEC would cause the TEC to produce a voltage. So, I took a TEC and hooked it up to a DMM set on mV. The TEC was on a small heatsink, and to make the TEC absorb laser energy well, I carbon-blacked its' surface with an acetylene flame. Shining a 50mW red laser on the TEC produced 11.4mV. This looked interesting, so I set up a variable power LOC red labby. Measurements were taken at 10mW increments from 10mW to 100mW, 20mW increments from 100mW to 220mW. Power levels were set using a Laserbee I Deluxe. I then set up a variable output 405nm labby and was very happy to get the same readings for the same mW's with the bluray. This was very encouraging. I graphed the results in excel and was surprised by the linearity. As a last test, a 750mW IR handheld was used, and the reading when graphed was right on the line. I will have to test further with higher laser powers to determine the maximum for this TEC

My next decision was that I would try to make the absolute simplest LPM possible. To do this, I used a small analog panel meter salvaged from some junk and whited out the existing numbers on the meter face. A 0.1uf ceramic disk cap was placed across the TEC leads, and the TEC leads were soldered to the meter terminals. A labby was set at 50mW increments using the Laserbee, and a fine sharpie marker used to mark the analog meter plate. The result is a meter that reads from 0-400mW. A more sensitive panel meter, selector switch, and resistors would allow multiple ranges. If your black coating is good, the meter should be useful for different wavelengths of laser.

Of course, the main problem with a DIY laser power meter is getting it meaningfully calibrated. The only way to do this is if you have a buddy or a school lab with and LPM and a variable power labby. Despite this drawback, this is still an interesting project for anyone with some panel meters lying around and a small c-mount sized TEC from ebay. My 10-pack of small TECs came from ebay.

Time for a few pics ...

These are the TECs.



A front view of the meter.



A side view.



A closeup of the messy meter face.

Gee... almost looks similar to the way we started over a year ago...

Nice project...

Jerry

Thanks, Jerry! Your Laserbee is an excellent meter and I will be doing a review post over the holidays. My goal here was the simplest possible LPM that would work. It's a little messy, but it works. I'll probably get a more sensitive panel meter, selector switch, range resistors, and put the result in a project box. The next meter plate will be neater, perhaps in photoshop.

Very nice, I'm doing the same thing, but where'd you get the small tecs? I'm using 20x20mm, a bit too large.

How much would this cost to build? I have no idea what TEC's and meters would cost..

The TECs came from ebay. You only see small TECs on ebay occasionally. Another possible source is Chris (heruur), if he is still selling stuff. Another source for the TEC is dead T0-3 laser diodes.

The meter I used came from some junk that I took apart. Panel meters are available on ebay, and from many online electronics retailers. Ebay example ...

Analog AMP Current Panel Meter 50 μA Meters For DC on eBay.ca (item 250432652962 end time 23-Dec-09 06:12:48 EST)

Not seeing any small TECs on ebay today. Remember to search for both TEC and Peltier, they get listed under both names, usually lots under peltier.

By that meter Link...
I would assume you are detecting current not voltage..:undecided:

Jerry

A microammeter, due to its' internal resisitance, will act as a low voltage voltmeter. Analog multimeters are essentially a microammeter with a selector switch that puts resistive shunts across the meter to get current ranges, or resistors in series to get voltage ranges.
I chose to link to a 50uA panel meter because with resistors and a selector switch, a meter with several ranges could be developed. As a 10mW laser only delivers 1.7mV from the TEC, a very sensitive meter movement is needed. At 100mW, I get 24.6mV, 200mW gives 50.8mV. A 1V DC panel meter would hardly budge its' needle at these power levels.

VERY interesting info here, thanks for enlightening me

Heheh, basically my same idea, but i had to stop for lack of time ..... and cause there's apparently no way for get the right type of TECs all the ways that you need them (it's a damn pain you-know-where, to find the sizes that you need, both in ebay and from resellers :wtf

a 50 uA panel meter can be good, too, but keep in mind that, with high powers, the TEC convey much more heat than a thermopile to the opposite face, so if you want to make long time reading or use it for high power lasers, it need to have a very big heatsink, or be thermally stabilized on the "cold" face .....

thats very clever, i think i finnaly found a use for my little tec, even if it isn't terribly accurate it will still be neat

Well, I'm planning a professional looking build using similar setup.
Same model of TEC cooler used (Thnx Bill!)
But the voltage will be read by a small Microcontroller and accordingly a 4 digit number will be written on fancy LED (or LCD ) screen display.

Bill, want me to post those 10mW increment reading you PMed me?

Remember, the readings can be dependent on your coating. I coated my TEC face with carbon black. Someone who sharpies their TEC face may get slightly different results.

I've also just made a DMM to LPM adapter with another one of the little TEC's. Lets me read laser power on the DMM. So far, it seems to be working up to 1.85W of test laser power, checking against my Coherent 210.

awesome!
i am trying to find such small TECs for some time now. just looked through more than 500 ebay artcles, but the smallest i found was 20x20mm. could you post the auction you got those from? how tiny are those?
i plan to use them to cool 635nm diodes, so those are perfect. tiny and solderable! i wont need much heat-transfer-power, i hope to get away with one TEC, if not three, all around the diode.
the only other source seemed to be chris, but he never answered.

thank you for any hints!

manuel

This project is neat for comparing your different lasers - in order to get any kind of semi-accurate readings to actual mW values you'd of course need a calibrated power meter to compare to.

Eudaimonium - have you read up on electronics theory? You seemed to be pretty lost last time I talked to you about your microcontroller based LPM idea.

@krutz - I bought these over a year ago, 6x8x1.6mm, and haven't really looked too hard since for any. These came as a package of 10. I stuck them in a parts bin, and when I ran across them a few weeks ago, LPM's came to mind. Large TECs either do not generate enough voltage to be useful, or their response times are pathetically slow. Digikey have a 15x15mm, but even this is so big it will be slow. Ordering a single one from a manufacturer could cost easily $75.00USD plus shipping. It's a shame, but these things are a rare find on ebay. I can find manufacturers, but no deals. Sometimes you can get a working TEC from a dead TO-3 laser diode, Chris (heruur) had these available.