idea for ultra fast power level setting

Electronics related to CNC

Re: idea for ultra fast power level setting

Postby dirktheeng » Sat Oct 29, 2011 2:45 pm

I put a bid in and won the isomet AOM for $450
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Sat Oct 29, 2011 2:57 pm

That's a pretty expensive guess if you are not sure of the answer. If it does need polarization, then at most it will only diffract 50% of the beam. Did you get to the datasheet and confirm Ge with 10.6um coating? The posting just said "IR", which could mean anything. When I clicked, the datasheet would not appear.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Sat Oct 29, 2011 5:07 pm

yea I got the data sheet... unless the unit is specialized somehow, it is a 10.6um unit.

I took the risk because I could part it out and make much more money than what I have in it if the parts don't work. Brand new, all the stuff that is in the package is dog gone close to 10 grand! I would expect that I could make almost all my money back with just selling the AOM. They were askigng $1000 for it and I really lowballed so it doesn't matter so much. It is worth the try.

Wikipedia answered the question for me:

Collinear transverse acoustic waves or perpendicular longitudinal waves can change the polarization. The acoustic waves induce a birefringent phase-shift, much like in a Pockels cell. The acousto-optic tunable filter, especially the dazzler, which can generate variable pulse shapes, is based on this principle[1].


This property forms the basis of the phase modulation AO's

However, I am still not sure if the Germanium crystal itself is a birefringent material and if it is, it depends on how it was installed in the unit. Anyhow, I found out that his may not matter anyhow because a birefringent material will bend the polarized light states at different angles but will not absorb any light in the process (theoretically), so even if it does separate the light by polarization, it should be possible to recombine it with a beam combiner after the AOM
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Sat Oct 29, 2011 5:40 pm

I think you are missing the point I'm trying to make. The DC tube is "randomly" polarized and "always" changing. Even if you place a linear polarizer in front of the tube and throw away 1/2 your light to make linear light for the AOM, it's randomness might make the linear output power swing around wildly. I'm wishing you luck, but I don't think it will work.

The Germanium doesn't have to be birefringent in its normal state. Just like quartz in a normal AOM for visible light, the acoustic signal sets up a standing wave in the crystal that compresses some areas and expands others to form a grating.

Polarization optics for 10.6um are not cheap. If you wanted to do it without super Ebay deals, you would end up spending more than you would to otherwise purchase a new Synrad RF laser.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Sat Oct 29, 2011 6:11 pm

Let's assume that the Germanium is not birefringent in it's normal state

In the Off state, the sound is off and there is no separation of the polarized states. The light passes through unhinderd as it came out of the laser.

Now lets assume that we turn on the AOM on and the material becomes birefringent. It will still defract both polarization states out of the normal beam path, but not at the same angle. This doesn't matter because I'm sending that light to a beam dump and not using it anyhow.

What will not work well (maybe... depending on how fast the polarization of the laser rotates or changes on it's randomness... if it is really fast then it should work ok because it will average out) is to use the AOM in an analog sense... you can vary the intensity of the sound and effectively split the light between the zeroth and primary beams. However, if you use the AOM in full on/off this doesn't matter because it will bend both fast and slow beams out of the way. Thus it will be useful as a fast switch that I can use to switch the full beam on and off at 1Mhz frequencies. At that speed, it is realy almost the same as doing an analog signal.

That said, you can take any polarized state and make it linear polarized without loosing much power. What you need is a polarizing beam splitting cube, a pi/2 phase rotator, and a beam combiner:

Put any light source through a polarizing beam splitter cube and it will be divided into horizonal and vertical polarized light... only losses are insersion losses (usually less than 2%). Then you take one of the polarized beams, put it through the 90 degree rotator so it matches the polarization state of the other beam and then recombine the 2. with the right optics, the losses are very small and you end up with a single polarized source with almost all the energy of the original light source.

The only thing you need to watch out for is that you have some microadjustability in one of the beam paths so that you can be sure that the beams combine constructively rather than destructively... but that is eas with these IR signals at 10.6microns... you can use a cheap micrometer.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Sun Nov 20, 2011 10:29 pm

Just a quick update:

I ended up getting the AOM unit from the guy on ebay for $250. Before I had him ship it, I asked for detailed photo's of everything that he had to be sure that it was all there. It turns out that it did not have the actual germanium cell... originally I was really bummed about this, but I got him to nock $200 off the price so I could go get one elsewhere and he did. It has the 24V PSU, the high frequency driver, the attinuator, and the amplifier. I am also working on a deal right now to pick up a cell for $150, so I have a total system for $400. Not bad considering that the new components sell for about $15k. This will be about as good of a system (probably better) than an RF laser. The rise/fall time fo this system is based on the speed of sound through the AOM and the beam diameter. It ammounts to sub microsecond times, which means the thing can be switched at megahertz frequencies. Basically, I will leave the laser on all the time during an engraving/cutting and pulse it at several hundred kilohertz pwm to achieve very defined power input to the material being cut/engraved. When the beam is "off" it will be deflected to a beam dump.

Also, the AOM is rated for 100W 10.6 micron lasers, so I could get a much biger laser which means I can cut faster and engrave faster... We'll see what santa brings for christmas!

Hopefully I will have everything I need here in a couple weeks
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Re: idea for ultra fast power level setting

Postby gene » Tue Dec 06, 2011 5:16 pm

I've been following this thread to see how it goes so make sure to keep updating. If it works well I'll start keeping an eye out for a good deal
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Re: idea for ultra fast power level setting

Postby frob » Tue Feb 07, 2012 2:23 pm

lasersafe1 wrote:...The DC tube is "randomly" polarized and "always" changing. Even if you place a linear polarizer in front of the tube and throw away 1/2 your light to make linear light for the AOM, it's randomness might make the linear output power swing around wildly. I'm wishing you luck, but I don't think it will work....


I'm an unabashed optimist usually, but i've learned the hard way not to pick a fight with the laws of physics. I have to agree with lasersafe1 here. But this could work if you used a laser tube without the built-in cavity optics, with say brewsters windows, or maybe just the one total reflector at one end. Then you could insert the AO cell in the cavity before the output mirror so it works as a Q-switch. I've seen a few such laser tubes advertised on Chinese manufacturer's websites. in theory they should cost less.

lately I have been doing a lot of late night reading on laser physics...
From what ive read, the root of the problem of quickly switching on & off the laser output, is the limit imposed by the excited state species lifetime in these lasers, which it seems are in the 1-100mS range, depending on a few factors, mainly gas pressure. If you're moving the beam on the workpiece at 1m/S the laser will keep lasing several mS thus several mm past the point where you cut the power to the tube.
The point is that this is a result of the physics in the laser tube design, much more than the CO2 gain medium.
With all my recent reading on this, I am pleasantly surprised to see how much laser technology, especially CO2, has progressed in the last couple of decades since the last time i was enamored with lasers.
Other types of CO2 lasers exist that have huge advantages over these sealed gas tube types which include, among other things, a much much shorter excited state lifetime and therefore the ability to be power-modulated at much quicker rates, by several orders of magnitude. No exotic optics, AO cell / Q-switch necessary.

If i didn't already have a ridiculous number of unfinished projects and new ones in the pipeline after them, id be very tempted to try to build one of the "modern" types of CO2 lasers.
One of the last papers i read describe a RF-excited TEA waveguide design that's made with only a few slabs of aluminum bar, no glass at all, and can easily get a couple hundred watts out of a 18" long cavity, and running at higher pressure means more than fast enough output response. And easier gas fill. Plus water-cooled electrodes means the the gas is conduction cooled right in the cavity so with no forced gas circulation it might be possible to run it sealed or semi-sealed at 100W or more. Seems eminently hackable. Hmmm.... anyone here own a CNC mill that can machine an 18" Al. bar in 1 pass?
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