HeNe

I scavenged this 25cm tube from some old /1990/ surplus medical equipment along with the corresponding HV driver board. I think that it was made in Bulgaria. The tube is running at 4.7mA now. the output power reaches a peak (5-6mW) at around 6.5mA after which it begins to roll off. The driver board needs 24VDC to run and consists of a pair of self-oscillating 2N3055 in push-pull arrangement feeding a pot core step-up transformer. The two ferrite halves are air-gapped to shift the oscillation frequency out of hearing band. the HV output of the transformer is fed to a voltage doubler, the output of which is again multiplied to generate the starting voltage for the tube. there are a couple of serially connected 2w ballast resistors on the board and a HV port to which goes a 50-ohm coax cable supplying power to the tube itself. There is a 56k ballast resistor right next to the anode.
one of the stock 2n3055s was blown, so i replaced it, but the thing was working even with just one of the transistors. i also added RC snubbers on the collectors to kill some suspiciously looking spikes.
I tested the transformer+rectifier part in an alternate push-pull circuit with a TL494 and a pair of mosfets and it worked neatly, power efficiency was much better, the mosfets could run comfortably w/o any heatsinking.
back to the laser now. the pale blue picture is a close up of the rear end mirror showing the bore of the tube. it leaks small amounts of red laser light. the output coupler is also shown /thumbnail 4/. The dot size measured 20cm in front of it is around 2mm. a faint ghost beam running at an angle accompanies the main beam, hence the pale dot right next to the main dot. The dot swells to 15mm in diameter 5 meters away. The dim picture of the dot 5 meters away was taken with very short exposure time. You can see that it is not TEM00. It looks octagonal...

The tube looks to be in great condition. I see that there are a few globs of potting on the tube, did you happen to remove it from a tube enclosure?

I have 2N3055 and TL494 if you need！
2N3055 datasheet pdf,2N3055 supplier,2N3055 price,2N3055 picture at UTSOURCE
TL494 datasheet pdf,TL494 supplier,TL494 price,TL494 picture at UTSOURCE

yes, the tube is rather spry, it lases as long as there is discharge down the bore. the apparatus i took it from has an electromechanical timer and it reads less than 500h of use.
it was indeed potted in a metal enclosure together with some other junk not related to it, so i had a fair bit of hard time breaking the silicone bonds w/o shattering the glass.
the power supply is fun to play with. i made a small 4-5mm spark gap in series with a string of 5 1M ballast resistors. the resulting discharge in free air is weak and quiet and ozonates the air in a small room noticeably in less than 1 hour...

One thing I love about HeNe lasers is the colours that are produced!

you're the proud owner of a multimode output tube.

cheech226 said:

you're the proud owner of a multimode output tube.

Click to expand...

...and a blown PSU.

Hey, the second to last pic is not 100% clear but I think the mode pattern is either TEM_22 or TEM_33.
If the diameter of the discharge tube is a bit larger then it's probably intended to be multimode... though higher order TEM waves are much more academic and not too practical (since most applications want constant phase electric field a gaussian flux density over the spot).

I think it's TEM23 or 33. Whoever designed it, they were probably looking for raw power in a conveniently sized tube, which is probably why they made the bore wider than required for TEM00 operation. Moreover, there is no brewster window inside and the beam is randomly polarized.
I had another tube with adjustable mirrors and a brewster window, but it didnt fire up. The tube needed 20+kV to produce faint blue discharge. My guess is that some of the seals leaked and the helium oozed out of the tube and some air got inside. It had a getter, but it'd have been no use since it had no usable gases inside. Im talking in past tense because i hammered it and collected the OC and HR mirrors. the rest of the glasswork is in the trash.

Briver487 said:

The tube looks to be in great condition. I see that there are a few globs of potting on the tube, did you happen to remove it from a tube enclosure?

Click to expand...

Yes, it was a daunting task, but i wanted to see and enjoy the discharge in the bore, and not so much the output.

nice tube! there's nothing like a HeNe... or Argon... or Krypton... or or or...

My red Melles Griot tube (designed for 633) is also capable to lase at multiple wavelengths - but only with an external OC mirror.

The most amazing line is the 612nm one. Even this line has less power (for sure) as the main 633 one it's stable for minutes. Have used an old Kr OC mirror to get multiple wavelengths out of this tube. Works also with some suitable dicro (reflects red).

Just give a try

mojo

A multi-line HeNe you say? Pics or it didn't happen.

Cyparagon said:

A multi-line HeNe you say? Pics or it didn't happen.

Click to expand...

Correct me if I'm wrong, but while there are tunable HeNes, there aren't any that are multiline. Unlike Argon and Krypton, that lase multiple wavelengths, simultaneously, HeNes are capable at other wavelengths, but only when tuned for a specific frequency by swapping out the mirror(s).