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Thread: RF CO2 laser drive frequency

  1. #1
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    Default RF CO2 laser drive frequency

    Recently acquired a small cute RF-pumped CO2 tube, it looks to be around 5W.
    Anyone know what frequency I should drive these at, and how much power they typically want?

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    Is there a matching network on the tube? The easiest way might be to hook it up to a VNA and look for when the input impedance goes to something resonable, although I would imagine that an unignited tube will probably resonate at a completely different frequency than one loaded with plasma.

    For whats it worth, the synraf j48 tubes run at 45-50MHz (thus the name), my universal laser systems CO2 tubes run at 40MHz. I am pretty sure the little domino lasers (ex http://www.photonlexicon.com/forums/...-10W-CO2-Laser ) run at 27MHZ or thereabouts

    I wonder if you can get it to lase by just setting it on a tesla coil

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    ... my last RF-CO2-lasers were sealed, air-cooled 5W-types from Eksma driven with 28Volts@1Amp and capable of PWM until 100kHz with the embedded driver ... but this was around 1996, so maybe not typical for modern types.

    Do you have an image and/or some more specs?

    Viktor

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    Picture:
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    As far as I can tell, no internal matching networks or anything, not even a coax power input. The tube is ceramic, about 1/2"x12", and has strips of copper tape on the top and bottom which are both electrically connected to one power lead. The other lead is presumably ground and is attached to the aluminum cooling shroud.
    Definitely a CO2 laser (judging by the OC color) but that's as far as I got.

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    Thats a ~27.125 Mhz tube, but your missing the matching network...

    Look at the US ISM FREQUENCY BAND CHART. 27.125 is industrial. It is however also CB Channel 14 in the US.

    When you have the network.. Your good for a Mhz either side of that. You'll need a 35 to 75 watt amp, and variable output power...

    Once you ask over on LPF for a network or amp to clone, You could find a Ham on 28 Mhz with a linear amp and a "Antenna Matcher" and SWR meter.
    My experience with those 10 years ago is they are not exactly matched to 50 ohms, but are far enough off to throw a 50 ohm transmitter into near shutdown.
    Hence the Antenna Matching Unit, aka AMU.... The impedance changes when the plasma lights, but you knew that aleady.

    Later units of that tube shipped with a two transistor oscillator amplifier module that is cloneable.

    I sold all mine and never documented it. I know a EE who has one at work, I will not be able to contact him til monday.
    Steve

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    Quote Originally Posted by mixedgas View Post
    Thats a ~27.125 Mhz tube, but your missing the matching network...

    Look at the US ISM FREQUENCY BAND CHART. 27.125 is industrial. It is however also CB Channel 14 in the US.

    When you have the network.. Your good for a Mhz either side of that. You'll need a 35 to 75 watt amp, and variable output power...

    Once you ask over on LPF for a network or amp to clone, You could find a Ham on 28 Mhz with a linear amp and a "Antenna Matcher" and SWR meter.
    My experience with those 10 years ago is they are not exactly matched to 50 ohms, but are far enough off to throw a 50 ohm transmitter into near shutdown.
    Hence the Antenna Matching Unit, aka AMU.... The impedance changes when the plasma lights, but you knew that aleady.

    Later units of that tube shipped with a two transistor oscillator amplifier module that is cloneable.

    I sold all mine and never documented it. I know a EE who has one at work, I will not be able to contact him til monday.
    Steve
    Thanks for the info Steve!
    Do you know where these tubes came from and how much power they can do? My guess of 5W is pretty arbitrary...

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    They were sold as OEM parts to Pfizer, Diamond, and several bar code printer companies.

    Never quite pinned down a maker. I did tear one one down. The optics are indium sealed, and the mix has Co, Xe, Co2, N2, and He.
    Lots of Xe from the spectrum I looked at. I sold my other head.

    My friend has one that is different and runs 8-9 microns using Co-Xe-He instead of the 9 to 10.6 Co2 band.

    The cavity is set up to be pretty much mis-alignment tolerant... Tyring to adjust the mirrors very much will crack the indium.

    There is no catalyst in the bore or anything.

    Steve

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    This is the guy that designed those:
    http://www.dpilasers.com/index.php?o...121&Itemid=173

    I still have a pair with the drivers and matching networks.

    Those tubes used a CO-CO2,He,Xe mix. No N2. Evidently they used oil base pumps to pump down the earlier tubes and the oil contamination causes a shift in the CO-CO2 balance over time. The CO-CO2 balance is critical and they don't lase if it is off. The newer Domino variety of tubes seem to be longer lived than the older (DEI etc.) ones.

    Here are some pics of the RF driver and matching circuit:
    Click image for larger version. 

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    Quote Originally Posted by mixedgas View Post
    They were sold as OEM parts to Pfizer, Diamond, and several bar code printer companies.

    Never quite pinned down a maker. I did tear one one down. The optics are indium sealed, and the mix has Co, Xe, Co2, N2, and He.
    Lots of Xe from the spectrum I looked at. I sold my other head.

    My friend has one that is different and runs 8-9 microns using Co-Xe-He instead of the 9 to 10.6 Co2 band.

    The cavity is set up to be pretty much mis-alignment tolerant... Tyring to adjust the mirrors very much will crack the indium.

    There is no catalyst in the bore or anything.

    Steve

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    I've been trying to fire up a couple of similar laser tubes here. I tried using an antenna tuner to get a decent match between the driver and the tube, but there was so much RF floating around that the fluorescent ring light on the microscope next to me was lighting up! I did manage to get about a watt from one tube but never got the other to even ionize. I'm going to try to build this matching circuit and drive the tube with 50 ohm coax. Any chance you can show a picture of the matching circuit? I'm wondering what sort of inductor is used. I'm guessing a ferrite core won't work, but .9mH is a fairly big air core coil. Also, any idea what the output power of the RF driver is?

    Quote Originally Posted by Phat Photons View Post
    This is the guy that designed those:
    http://www.dpilasers.com/index.php?o...121&Itemid=173

    I still have a pair with the drivers and matching networks.

    Those tubes used a CO-CO2,He,Xe mix. No N2. Evidently they used oil base pumps to pump down the earlier tubes and the oil contamination causes a shift in the CO-CO2 balance over time. The CO-CO2 balance is critical and they don't lase if it is off. The newer Domino variety of tubes seem to be longer lived than the older (DEI etc.) ones.

    Here are some pics of the RF driver and matching circuit:
    Click image for larger version. 

Name:	rfamp3.JPG 
Views:	47 
Size:	151.9 KB 
ID:	36194
    Click image for larger version. 

Name:	co2dei4.JPG 
Views:	27 
Size:	53.6 KB 
ID:	36195

  10. #10
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    Inductor on mine was a ferrite rod about 3 mm in diameter with a long single layer coil. The cap was one of those amelco surface mount, high current, RF caps like motorola calls for in the ap notes. A small portion of the coil was air wound for "scrunch-stretch" style tuning. My RF bank was good for at least 100 watts, but I never measured it. as my Wattmeter stops at 20.

    In fact the Pfizer medical amp was right off one of Motorola ap notes for Bipolar RF transistors. I'm sure it was overkill. They were modulating the DC to the amp output stage, more like a older AM radio transmitter. The drive from the crystal POWER oscillator was not attenuated or anything, it had to launch a full watt or more in to the amp.

    I sold all this long ago. 7 watt CB did not light it.

    The "Q" of the matching network needs to be darn high, to get the resonance that creates the high field. I think the steel rails around the tube need to be at RF ground, as well.

    Steve

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