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

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 :D ) 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). :D

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 :D

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.

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...

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. :)

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 ? :tired:

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?

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.

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. :confused: 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:

...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. :wee:
Still only 10watts max, so next is to investigate the output beam pattern with some thermal paper(ex old fax machine).

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. :)

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.

The articulated section is already off. I did this to make sure I was only dealing with any problems inherrent in the tube alignment, and as I would want to use them on the xy mechanism.

I haven't mentioned yet that I had a microslad with the laser, so I have 200,300,and 400mm znse lenses. This will be caniballised for the final focusing head.

First test tomorrow is planned to be with the arm fixed to the bench to see if a burn spot is single or not, and possibly what shape, if I can control the output well enough to give me an image on the paper.
Is there a better material to use as a target to show up details of the beam shape ?
Bursting into flames does make it difficult to get any useful info. :D

A closer examination of the tube internals and alignment screws is being put off until last. I want to make sure I've tried everything else first !

On the subject of eht hazards. Apart from gold leaf electroscopes (I dont suppose your old enough to have ever used one, never mind make one), is there any other method of indicating residual charges left on power supply components after you've switched off ?
Would the eht supply have leak resistors built in to get rid of that sort of hazard from capacitors ?

No idea exactly what type of capacitors are on your HV supply. Personally, I would use a high wattage large value (maybe 100Kohm, 25 watt?) resistor to short the HV leads together for a minute or so before I played with the leads.

Now it may be that the optic adjustment screws are insulated from the HV and are meant to be turned while the laser is running (but I would use a heavily insulated screwdriver!). Maybe there is a label indicating this?

As for materials...yeah, it is bad and good that everything bursts into flame...they do make some expensive ceramic thingys that are for indicating a beam. The best thing to try is to turn the power down really low (like under a watt) and use short bursts.

In trying to set up my sharplan720 I've produced several burn patterns on paper in an attempt to check the beam alignment.
I have removed the beam arm from the machine and have it on the bench to avoid vibration, and I've removed the articulated output arm, so the beam is unaffected by other optics.

While there is a singe main spot, there are a number of secondary spots, perhaps four, each about 10% of the power of the main one.
They start about 2mm vertically above the main beam, then form a line at 2 o'clock.
There are also other lower power spots at 4 o'clock.

Could anyone suggest a particular fault that would account for this.

John

I think your mirrors are misaligned (the Output mirror the laser tube or the High reflective rear mirror - known in the biz as the OC and HR respectively).

It sorta sounds like you are getting higher order modes (maybe tem 1,2...I don't have a chart in front of me and don't remember what all the modes look like - except tem 00 of course :o )

Tweak that baby's mirrors somehow (safely) and you wil probably get a nice single burn spot with 3x the power you are now getting...

Either that or you have some seriously dirty optics :confused:

Thanks KTP.
While you were reading and posting, I've made the attached jpeg of the burn pattern.

Herewith OC and HR ends of the arm. The OC end has what appears to be only two screws holding the outer(black) casing to the inner part of the assembly.
I don't recognise anything that might be axial adjustment screws in this view.
Any ideas ?
John

Hmmm, that is interesting. The three flowing gas tubes I have all have obvious adjustment screws.

I suggest you post the two pictures on the alt.lasers newsgroup (or rather post a link to them) and see if anyone there is familiar with the adjustment procedure for this laser. There are a few guys there that are VERY knowledgable (I think one even worked at Sharplan for a few years).

Er um. Do you suggest I post a link to this thread ?
I'm in unknown territory here, as I'm still a newbie when it comes to making full use of the net :D

No, not really a link...I mean I guess you could, but then they would have to register here to see pictures..and that pisses some people off.

Put the pictures up on another site, or post them to a binaries newsgroup, or the metalworking dropbox...that is a good idea...post em to the metalworking dropbox and then post your question on alt.lasers. Google "metalworking dropbox" for the link.

KTP - I've now established a page to view any laser photos(cnc ones later, hopefully)
at www.fanmaker.co.uk/lasers.html .
I'm now staring at the tube in an effort to see inside it, and wondering if I'll have the nerve to take it to pieces.
On the subject of different modes. I pressume that in a bad set-up there will probably be several modes active at any time, so the final output is a mixture. Thus the pattern is more complicated than the 'simple' patterns that are illustrated for the different individual modes. Have I got this right ?
Otherwise, will the pattern that my burns show indicate some sort diffraction caused by crud on the oc lens ?

More photos of oc end of tube.

I've now progressed to taking the output end assembly to pieces, having identified that there is an air gap through the tube inboard of it.
(see bottom of page)
Todays pictures show the current state, but I will probably take the assembly out later today, as it appears to be only held in place by two screws.
www.fanmaker.co.uk/lasers.html

Assembly now out and inspected - ugh !
Drawn my idea of the optical path, so I must now decide where exactly the surfaces are, and do I need to remove them in order to clean them.

Yes: Rotary Solinoid

No: Heater

The "heater" is actually a (carbon coated?) temperature sensor used to measure the power output of the tube when the solinoid rotates some metal or optic in place to reflect the beam into the sensor. This also acts as a safety solinoid to prevent output when certain interlocks, etc. are not triggered.

Is the OC optic (the one on the end of the laser tube) really dirty? Can you see any adjustment screws yet?

when you refer to the oc optic I pressume you mean the output lens/mirror of the actual lasing cavity. In my case that would seem to be inboard of this rotating solenoid.
I assume this, because there is no gas seal outbord of it.
What is dirty is the last face of the outermost znse optical component which is also the half-silvered reflector for the HeNe beam.
I'll post the photo and drawing I've made here.
Both of the surfaces I've drawn are contaminated, and if there are additional faces that I haven't found yet, they too may be in the same state.
No sign of any adjustments for the main optics (HR +OC) yet.

Here's one of the "solenoid". I hadn't thought of the "heater" actualy being a sensor. That would make more sense(sorry ) than my idea.

You can safely remove the two optics - the one that reflects the hene at 90 degrees and the one that combines the hene and the CO2 laser beam. Keep the sensor (thing you called the heater) and the rotary solinoid in place. Now measure power output and see if things have improved.

I still think your output mirror (OC, output coupler) is dirty and misaligned.

Another photo of the inside of the solenoid and heat sensor. This part is beginning to make sense thanks to all the input from various sources.

The solenoid will have a 45 degree mirror firing the beam down onto the copper mesh. This scatters the beam safely onto the heat sensor to give a meter reading for the beam power. Pressing the beam output switch will rotate this mirror out of the way, unless other safety interlocks prevent it.
(Itallics = theory)

I'll try firing it up again tonight once I've got the arm clamped down. With no aiming beam I need to know exactly where to put the target. :D

Having repeated the burn test, with the HeNe alignment assembly removed, I find there is an obvious increase in power hitting the target but the pattern of multiple beams, though slightly different, is still apparent.

The beam power readings on the meter are still in the same range, being limited to about 7 watts maximum.
Varying the gas inlet needle valve does have an effect, moving the power reading up and down, but by very little.
The power control knob will take the reading down to zero, but turning it up, the increase is inconsistent, even dropping slightly during rotation.

So now I have to look at removing the solenoid assembly ?

Solenoid removed and new photo shows an end cap apparently with a znse plate(?) inside it.
Behind this is a single socket cap screw, not three as I was expecting. So still no sign of any adjustments.
To get a better view of this I think I shall have to remove the large knurled ring I've labled "power assembly ring".
This appears to hold the power assembly body in place onto the main beam tube, and should slide off, over the end cap.
Is it possible that there are only adjustments at one end, HR or OC ?
Or is this unheard of ?

Hmmm, maybe that knurled ring is actually something you rotate to adjust the OC somehow...or maybe when it is turned, three screws will be revealed in the holes and they are used for adjustment...just guessing here.

I think that ZnSe "plate" is actually the OC...not much else it could be. Is it really dirty?

I guess there *could* only be adjustments on the rear mirror (HR), but that would be a bit weird.

That ZnSe plate seems absolutely clean. Difficult to get a good view, but with a dental mirror and a magnifier, I couldn't see anything on the surface.

One other thing now puzzles me. It seems to be a very complicated procedure to set up if you have to take this "power measuring assembly"(henceforward PMA) off before each attempt at adjusting the alignment, then to reassemble it all before checking the result of your adjustment.

Access to the HR end does seem a lot easier, so perhaps I should switch my attention to that end before disturbing the OC end. Especially if, for the moment, I can't see any surface problem.
I have got a stereo microscope in the attic which I could set up to look down the into this oc end ZnSe. But I can already see a mirror surface a little way in, so I'm not going to get far inside.

Interesting idea of the holes revealing the adjustment screws - I'll have a look. I'll also mark the current positions of all the rings before moving anything. :D

Both rings have now been removed, but no sign of any adjustment screws.

I have also removed the inner end cap at the HR end, photographed it, and replaced it. What is exposed may be a plain mirror, but I'm not touching it at the moment.

A better look at the ZnSe "plate" does show some deposits on the inner face - tiny spots with secondary zones around them. One is central, and there are several others scattered around but with no pattern to their positions.
My first thought was that the central one was a pit caused by the main beam, but all the others look the same, so I think they are all deposits.

I wil try and draw them over the weekend - photography might be problematic - and compare their distribution with the burn pattern.
A set of small Fresnel lenses ?


The single visible socket screw located behind the "plate" has a wire running inwards to what appears to be the end of a 1.5 turn metal tube - earthing electrode ?
There is an air gap between the end cap and the "plate", but the plate turns with the end cap when I rotate it, so it must be sealed to it in some way.
It could be just an o-ring which is sticking them together. There is an o-ring at the HR end, inside the end cap, and which presses the mirror(if that's what it is) onto the main tube.

Is it possible to built a laser tube with no adjustment screws ? Two concave surfaces pressed against perfectly parallel faces of the tube ends ?

I can now say that the 720 has no adjustment screws. :rolleyes:

I've taken the end caps off, and can see from one end of the tube to the other. :)

As soon as I can get a better photo I'll post it, along with a photo of the crud encrusted OC ! :(

The ZnSe "plate" is just that - no curvature on either face, the inner one with a coating, so I've now got to wait for a supply of non-residue acetone.

I've just finished polishing off the crud from the OC with a slurry of talc and glycerin.
To cut a long story short, instead of removing the reflective surface along with the crud, it has now improved the output of the beam to 12 watts.

However, I'm still getting a series of hot spots instead of a single beam, but if focused with a 200 mm znse lens, it punches a neat hole through wood veneer.
Headway, but I'm not there yet.

Current problems - the gas bottle has to go back to the owner next week, so I'm still looking at alternative sources for gas mix.
Someone on alt.lasers has suggested catalytic converters as a method of controlling the purity of a recycled mix which is an interesting idea.

Hi greybeard,

Three months has passed, is the problem solved?
Or the patches of black images still there?

Hi coleong.
Thanks for the interest.
Unfortunately a major medical problem put everything on hold for a while. Since then, I'm glad to say, my wife has recovered.
However, the CO2 laser is still in its original condition, needing a new set of mirrors in the tube.
I'm also rethinking my whole approach to the idea of using this sort of set-up, rather than investing in a sealed tube.

Lots of pros and cons to be considered in both systems, but I think the only way forward with the Sharplan is to come up with a recycling gas system. Here in the UK the cost of setting up the gas bottles with a commercial supplier is prohibitive for the diy experimenter. There's a DIYlaser group on yahoo from which I picked up a lot of ideas in this direction, but many other projects and interests have kept the laser cnc at the bottom of the current list I'm afraid.
Regards
John

Hi greybeard,
It's good to hear that your wife has recovered,send my best regards to her.
Sorry that i do not know so much on the sharplan, but from my judgement the mirrors (OC and rear mirror) are not line up properly.
Thats way the power is low and there are patches when you burn the paper.
As per the gas mix,if you get it from BOC the gas used is of high purity.
If you will to mix your own you'll has problems like low laser power in the short term and damaged mirrors from contamination from the gas in the long term.