badly collimated he-ne

Krutz · Apr 6, 2009

heya,

i got (my first) he-ne recently. fires up nicely, 3mw if i remember correctly. the beam is quite divergent, like an inch after a few meters already (hooray for mixing systems!). its in a metaltube, and i cant easily open it up to reach the tube itself. obviously, my options would be to live with it, to install a collimating lens, or try to open the thing up. did anyone ever see any similar symptoms?

will do pics if that helps!

manuel

lasersbee · Apr 6, 2009

For HeNe Lasers the longer the tube the better the divergence...

Jerry
(1700 - 58)

Event_Horizon · Apr 6, 2009

If a gas laser has bad divergence there isn't really anything you can change to fix it. You can fiddle with the mirrors on larger lasers but HeNe's rarely have external mirrors. Your only real option is to build a beam expander and attach it to the front of the laser.

Krutz · Apr 6, 2009

wow, thanks for the quick reply!

well, "bad divergence" isnt really what it is.. its more like totally out-of-focus! the "raw" beam out of a he-ne, with only the two mirrors, should be much better collimated than that! thats what i would expect, no experience here..
to me it looks like there must be some lens/expander after the tube, but inside the housing, which went wrong.

manuel

GooeyGus · Apr 6, 2009

gas lasers dont usually use any lenses, the raw output from the tube should be pretty good. Unless it's a short tube with multimode mirrors, then your beam wont be as nice because they just needed to get the most possible power out of the smallest tube they could use, in the original application.

disma · Apr 7, 2009

The tube probably has either one or both mirrors concave, this is very likely whats causing the diverged beam. A convex lens with a long focal length should correct for it. Look for an old pair of binoculars (could just use it as a beam expander.), the objective is the lens im thinking might correct it. (big lens at the front)

Krutz · Apr 7, 2009

thanks for the input!

the powersupply says: 1.3-1.8kv, 5.0mA, siemens lgn 7461

tube has no manufacturer stated, says 15mw max, made in germany (siemens? psu is made in usa), and extra sticker says "licensed by patlex corp"
the (metal-)tube is 28cm long.

measured output is 5.5mw

2mm beam at the aperture, 20mm at 4m distance.
calculated divergence is 4.5mRad.
at 4m, its slightly oval, and i can barely see (with googles) something that looks like modehopping. nothing visible when the dot is smaller, though.

there is a very faint second dot directly at the side of the original dot. same size, and less than one percent of the original brightness.

oh, and nothing rattling inside, and gentle tapping doesnt affect anything neither ;-)

to open it i would have to destroy the metaltube i guess.. perhaps i really have no other choice than putting a telescope in front of it?

manuel

daguin · Apr 7, 2009

LGN 7461 is a model number for a Lasos (Germany) PSU. Since the laser also says "made in Germany" it is also probably a Lasos. Just Google "lgn 7461." You will find Lasos' site.

The secondary (dim) spot is the result of the mirrors being very slightly out of alignment. My orange HeNe does this. I am not willing to open it up for that dim secondary dot. I MIGHT for a red HeNe, but I won't molest my orange this way

If it was "rattling" it probably wouldn't be lasing.

If you decide to "open it up" be very careful. The tube is glued in place with silicone at all of those "spots" you see on the outside of the tube. It should NOT just slide out. You will have to grind/cut the tube along its entire length to separate the tube from the housing. The glass tube is a small fraction of an inch away from the housing. It is very easy to slip deep enough to damage or break the tube.

Peace,
dave

Krutz · Apr 7, 2009

thanks for the pointers!

well, so there are no signs that i would find anything inside to easily change its divergence?
before seeing this one, i always assumed he-ne had among the best divergence inherently, just because of the long resonancelength..
i will better leave it just as it is (and definitely not risk anything because of the spot) and experiment with a telescope/widener..

what you mean, "red"? i call mine orange, thats the point of having such a thing, its miles away from the regular 650nm!

(joking, its 632nm of course, i see the difference when comparing though)

manuel

daguin · Apr 7, 2009

Krutz said:
t
what you mean, "red"? i call mine orange, thats the point of having such a thing, its miles away from the regular 650nm!
(joking, its 632nm of course, i see the difference when comparing though)
manuel

Orange HeNe's are 612nm. Red Hene's are 632nm.

Here's my second dot.

Here are the HeNe colors
red; orange; yellow; green

Peace,
dave

ElektroFreak · Apr 7, 2009

Every short HeNe I've ever owned has had crappy divergence. Some of my really short tubes have had divergence numbers of 2.0 mRad or worse..

Also, Dave, is Lasos affiliated with Siemens in any way? It's odd that Lasos and Siemens would have the same PS model number if they aren't..

GooeyGus · Apr 7, 2009

Single mode henes have very good divergence, multimode not so much. There is nothing on the tube that you'll be able to change. It's literally a glass tube with a mirror on each end which are hard-sealed to the tube.

Warske · Apr 7, 2009

GooeyGus said:
There is nothing on the tube that you'll be able to change. It's literally a glass tube with a mirror on each end which are hard-sealed to the tube.

Some of them are adjustable.

I used this one for entertaining my cat before visible laser diodes were available...

daguin · Apr 7, 2009

GooeyGus said:
Single mode henes have very good divergence, multimode not so much. There is nothing on the tube that you'll be able to change. It's literally a glass tube with a mirror on each end which are hard-sealed to the tube.

Depending on how bad it is (and if you are willingto take the chance) you can GENTLY push the end with the metal part around a bit. Sometimes you can overcome very small alignment issues this way.

I have a bare-tube yellow HeNe that needs to be held with just a slight bit of pressure on the metal end to make TEM00.

Peace,
dave

Rebel · Apr 7, 2009

Tha's a nice Ole' hughes you got there ! Where did you get it ?

daguin · Apr 7, 2009

ElektroFreak said:
Also, Dave, is Lasos affiliated with Siemens in any way? It's odd that Lasos and Siemens would have the same PS model number if they aren't..

Lasos took over both Siemens and Zeiss.

Peace,
dave

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