Measuring power output of laser

[greybeard]

Trying to improve the meter reading of "10 watts" output on the sharplan720 meter. There are various things I have to do in overhauling the m/c, but I want to have an external method of measuring the power in the beam.
I have long distant memories of measuring ir in the physics lab at school, but need a refresh when it comes to making some sort of bolometer.
I've googled various combinations of "power measurement/ir/bolometer/simple" but seem only to get devices that need liquid helium, or indium antimonide lenses.
I'm looking for a simple design based on an aluminium block and perhaps a bead thermistor.

Any links/ideas gratefully accepted.
Many thanks
John

[KTP]

Hey, so you got it upstairs ok? It still works?

What part of the world are you in? I have a Synrad Power Wizard with a NIST traceable calibration that measures from 0.1 to 250 watts...

One way to get a pretty good indication of power is to aim the laser at a substance with a strong absorbtion of the CO2 laser wavelength (which is practically anything other than shiny metal ) and measure the heat rise over time. You could then use math to calculate the average power.

One trick I did before I got my meter was to bolt a 50 watt resistor (one of the aluminum housed gold colored ones) to the underside of a piece of black annodized aluminum (black annodized aluminum absorbs 10,600nm fairly well, and aluminum is a good conductor of heat). I then attached a thermometer (actually a thermocouple hooked to my multimeter) to the annodized aluminum block, fired the laser at the block and took note of the temp reading after a certain amount of time (like 10 seconds). I then let the block settle back to the starting temperature and powered up the resistor with a certain current (to get a certain wattage) and noted the temperature after the same amount of time (10 seconds). If the temp was higher than with the laser, I knew the laser was doing less power than was flowing through the resistor, if lower, then the laser was doing more power. I would then reduce or increase the resistor power until I got about the same reading.

You can see why I jumped on the chance to purchase the Synrad power meter when a brand new one appeared on ebay for $200 (they retail for over $500).

[greybeard]

hi KTP.
Thanks for the description of your first method.
Yes, it's safely upstairs now. I stripped of the vac and cooling system which reduced the weight to about 110lbs. Took the arm supporting tubes off so that my wife could carry the arm on its umbilical, while I went backwards up the stairs, doing a dead lift from step to step.
A few drinks later, I had recovered enough to start the re-assembly

When I re-connected the cooling system, I was aware that there might be an air lock in the pipe, and though I have done a quick test, the power meter still shows the same reading - about 5-10 watts. I'm loathe to go further until I confirm that the cooling is ok. I think I'll disconnect it again and top it up while tipping the arm up and down to check for any air bubbles.
The tube is housed in an opaque casing, so at the moment I have no method of guessing the layout of the water jacket inlet and outlet pipes.(I'm still chasing a lead on a manual.)

I'm in North Norfolk, UK. If you want to get a better idea of what an Old English Eccentric looks like, you'll find me at www.fanmaker.co.uk.

Back to the power meter. What I have in mind is to think of a way of using a much smaller heating element than a high power resistor - our local electronic suppliers have 10watt resistors, but their envelope is nearly 2" long.

[KTP]

Maybe inject a drop of blue or green food coloring into your cooling water and see if it looks like it is flowing the full length of the tube? You could try powering just the coolant pump by itself (with a little hot-wiring) and watch the flow..

Hmm, so one idea for a compact heat measurement device I had was to use a small TE (thermoelectric) device, like those found in solid state coolers. If you could keep the cold side constant, then the voltage generated by the device as you heat up the hot side would be proportional to laser power. You would need a power meter to calibrate this setup though...

[greybeard]

I like the colouring idea -I'd thought of a drop of ink.
Although I cant see the actual tube, I could see if it's carried round the system by putting it in the resevoir. It should then rapidly reappear out of the tube leading from the laser arm back to the resevoir.
This will confirm that circulation is occuring, so hopefully any air bubbles will be swept out.
I'll go with that in an hour or so....we're listening to Aida live from the Met at the moment.

[greybeard]

The drop of colouring in the water was a good idea - it's showing no circulation at all when I switch on. :frown:
The first power position is "Standby" when the vac pump comes on and I expect the cooling pump to come on at the same time, but zilch.
Tomorrow I'll start checking fuses/voltages. This is where the lack of a manual and being forced into reverse engineering is a real pain.
Where do all the manuals go ?

[KTP]

That is weird because almost all of the medical lasers I have messed with have a flow switch on the coolant line. No flow = laser doesn't power up.

Are you quite sure it isn't normal for the pump to not come on in Standby mode?

[greybeard]

I've just checked again and I'll recap the situation.
When it was bought and delivered it was only giving a beam reading of about 1/3 power.
On delivery we powered up and burnt a couple of places on a piece of wood.
After taking off the coolant pump and the vac system I moved it upstairs and reassembled.
I added more water to the resevoir as this was almost empty, and is now almost full.
I've carefully checked the wiring that I removed against both my notes and the photos of the junction boxes I took, I know the wiring is as before.
I've only switched to standby mode so far in order to check that all was well. The vac pump works and reads 30 torr on the meter.
The fan on the cooling radiator is blowing air but the water isn't circulating.
There is a warning light for cooling, but that isn't on(at least in Standby mode)
I don't want to switch to power mode for fear of damaging the optics.
So my next step is to check the supply to the pump, look for any fuses blown, and work out the cooling wiring circuit.

[greybeard]

Update to above
Ther is no voltage to the cooling pump when the laser is in Standby mode.
I've checked all the fuses - aok.
I've just turned on to standby mode again, and now have zero power reading on the meter when I press the meter button prior to setting the beam power. In fact, the needle actually moves backwards a small amount when I press the button !
The laser emission warning light comes on when I press the button, but no other warning lights.
As a check, I turned the m/c on to standby without the gas turned on, and the gas warning light came on after about four seconds, and the eht cct was turned off, ie those glass diodes(?) turned off.

Not sure what to do next in the absence of any manual. The pump needs 100v ac to operate, so I'll have to sort a supply out to check it off-line, but as there is no voltage at the terminal block, I would guess that the pump is ok.
There is at the front of the m/c a small push button switch labled S12. It would only be accessible with the covers off, so it may be a technician's over-ride or reset switch, for what I have no idea, but I'm not going to press it till I work it out.
Ho hum. :frown:

[greybeard]

...of sorts.
I checked the pump circuit back the fuse board. The fuse holder was open circuit, measured from the terminals!
A few rotations of the fuse in the holder, replaced it and all was well.
So I checked all the other holders' terminals, and one, identity as yet unknown, gave a reading of 110 ohms. So removed that fuse again, and half the holder came with it.
New fuse holder tomorrow if I can't repair this one with epoxy.

Edit
Evening - repaired fuseholder with superglue.
Pump didn't appear to be working until I tipped the arm up. Great whoosh of bubbles and water now flows, so great relief here. Topped up water to full mark, and level is steady during Standby operation
Tested m/c and it fires up.
Still only 10watts max, so next is to investigate the output beam pattern with some thermal paper(ex old fax machine).

[greybeard]

Fired at plain paper on cw mode,but for as briefly as possible, with the target about 3inches away. Instead of single dot hoped for, there is a fine main dot 1mm in diameter, with a fine streak pointing at 2 o'clock, about 2mm long.
Repeated at 12ins and got a single dot about 1/2mm wide.
Third attempt at 12ins for about 2 secs produced a random scatter of various sized dots.
I've come to the conclusion that m/c vibration transferred to the arm is a problem, so tomorrow sees the beginnings of remounting the arm on a solid base, prior to designing the power measuring add-on.

[KTP]

Hmmm, I think either your gas pressure is wrong or your optics are misaligned (likely). You could take the articulated arm off and see what type of output you get straight off the tube.

The tube itself has screws for aligning the output coupler with the rear mirror. They allow for fine adjustment because this is a quite critical alignment. Normally you "walk" the mirrors by adjusting these screws while the laser is on until you get the maximum power, but this would require some way of knowing the power out of the laser in real time and also the proper way not to kill yourself with the 10-20KV DC potential that the screws you want to adjust might be.