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Old NeHe laser from an old FT-IR spectrofotometer

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LAN_403

TheMerovingian

Wed Mar 31 2010, 10:45PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I found this guy in an IR spectrometer my istitution was throwing away. It is an old fashion NeHe tube from melles-griot with failed 1680V 5mA power supply (a bulge on the epoxy is clear enough :D ). When hooked to a flyback transformer (with ballast) it gives the classical salmon coloured glow but not laser output at all (neither red or other wavelenghts), just a small faint blue uncollimated light with huge divergence (If you want to say that it can be operating in IR or UV longitudinal modes, dont do it, I already have checked). I tried at 0.3 0.5 1.0 and 10.0 mA of current ballasted with a string of 1-2W 22Kohm resistors. The power comes from a ZVS flyback at approx 4-5KV with bigger input filter inductor than normal to limit primary current and power.

It is quite disappoiting but probably the laser is dead (misaligned optics?) or probably I am an idiot and I'm not driving it properly.

If i cannot make it work I will use it as a fancy high-tech toy for my desk.

This thing probably has over 10 years of life so maybe some air has leached in.. no visible damage, except it is now working...

Any suggestions? ("Throw it on the trash bin!!" is non accepted as an answer, recycling it's the right way

)

MinorityCarrier

Thu Apr 01 2010, 12:26AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Regrettably you may have ruined the semi-silvered mirror.

These tubes need to be run on DC, correct polarity, and with current limiting.

The 'burn' power, 1650v @ 5mA is not enough to initially strike a plasma in the tube. Typical HeNe laser supply usually has a base current-limited HV supply with a three-stage CW multiplier on it. The CW voltage ignites the tube with ~8-10KV then the voltage falls back down to the base value to run the tube. If you ramped up ZVS voltage until the tube ignited with no significant current limiting, oh well.

...

Thu Apr 01 2010, 03:39AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Assuming you did get the polarity right and the mirrors are still good, the only chance it has is being out of alignment. Using an insulated stick start poking at the end mirror, and see if you can get any flashes out of it.

Out of curiosity, was the spectrometer working before you tore it apart? There are a lot of other nice parts in a FT-IR spectrometer, and they usually work fine with visible wavelengths (although the computer probably wouldn't like it too much)

TheMerovingian

Thu Apr 01 2010, 07:59AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I'm using a string of resistors to limit the current (starting from 0.3mA) and using a DC flyback rectified with a oil-insulated HDPE-Al-Foil cap (10nF)

The mirrors seem to be fixed in place with no chance to adjust them but they look normal (failt blue-green color when looking through them with unpowered laser)

The initial spike is provided by the inductance of the wires (the laser strikes only when i power up the flyback).

The power supply was blown, maybe there was a reason, the laser was the first defective part.

The spectrofotometer was not working.

IntraWinding

Thu Apr 01 2010, 12:24PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I doubt the laser had a fault that caused the psu to blow. They're very much 'solid state' with nothing much to go wrong electrically, except loss of gas.

It's different to the tubes I've used in having those pins sealed in. All the 'coaxial' tubes I've seen have connections made to the mirror mount at each end of the tube only.

The pairs of circular discs between the mirror ends and the rest of the tube which have three screws are for aligning the mirrors. Unless they've been tampered with or got roughly treated the mirror alignment should be stable, but that could be your problem. If you have another laser you could try shining it down the bore of the unpowered tube towards a sheet of paper and gently push a mirror sideways to see if you can spot the multiple reflections and if you can, tweak the mirror(s) to align them all into one spot as best you can. When the laser is running tweak for maximum output power and beam quality.

I would guess that tube needs a current of between 2mA and 6.5mA DC. If the red and black wires are original that probably gives the correct polarity. A HeNe tube has a negative resistance characteristic and is always operated with a specified series resistor. I can't remember off hand if it needs to be attached to the anode or cathode, but it's places close to minimise capacitance and hence prevent oscillation. I suspect you left it in the equipment you got the tube from.

When it comes to lasers there is just one place to go: Sam's Laser Faq. Here's the extensive HeNe section

With luck you may be able to identify the tube and get the current from that. It may be cryptically indicated on the power supply as a number between 2 and 6.5. Otherwise, I believe the output intensity peaks at the ideal current, so find it that way once it's running.

Sam Goldwasser hangs out at the Newsgroup alt.lasers

and may be able to give you more information on the tube. I've just noticed it's marked October 2000. That's a surprise to me a it looks like an old design. I think he will be interested in it because of the extra electrical connections.

Alan

TheMerovingian

Tue Apr 06 2010, 12:44PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Thank you all for the inputs, I will check it out.

IntraWinding

Tue Apr 06 2010, 12:58PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I'd be interested to hear what you discover

Alan

Sulaiman

Tue Apr 06 2010, 05:37PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Two dumb questions;
- did you check both ends for output?
- if you hold the tube up to a light can you see right through the inner tube (a few mm diameter) ?

Conundrum

Sun Apr 11 2010, 07:01PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

presumably you've checked that there is no infrared output using a webcam?

sometimes they get "soft", there is a fix involving carefully heating the laser enough to somewhat reactivate the getter.

if all else fails (mirror alignment etc) you could try this.

saw an article in "Electronics World" about this.

i'd sell it on ebay, someone could use it as a fancy mantlepiece ornament by hooking it up to a spare
surplus mini case light transformer.

-A

IntraWinding

Sun Apr 11 2010, 08:18PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

That reminds me of another fix. Apparently Helium diffuses out through the glass over the years. You can diffuse some in by putting the tube in Helium at atmospheric pressure for hours or days (can't remember which). Surprising but apparently true.

So did you ask Sam about this tube?

Alan

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