ALC60x argon and extended cavity - MORE GREEN -

Hi all,

yesterday evening I'd some fun with my ALC argon laser and a bottle of some fine red red wine ...

after looking at the nice colored laser beam (no problemo - lowest power at 4.5A) for at least half an hour some idea came into my mind:

I've heard about the extension of the laser cavity using OC mirrors of old green HeNe or Argon lasers. SO,why not playing around with these small dichroic mirrors for RGB mixing which have been shipped together with my argon laser? Technically there's no big difference - an OC is in general a dichroic mirror.

*wow* there's also no practical difference - the s?!t works fine!

And it's so simple! Just align the mirror that it bounces back the laser beam and you have an extentend laser cavity! The dichroic mirror used is one of these passing red/blue and bouncing back green light...

Under normal operation my ALC has 6 lines at 4.5A tube current. But with the cavity extension the laser has 8 lines including the green one.

And the beam "inside" the external cavity is quite impressive. It has a clearly visible greenish colored beam even if the room light is switched on...

Have fun with the pics!

- mojo -

THE CAVITY




MORE LINES


GREEN EVERYWHERE



AND FINALLY - THE INSPIRATION -

This is awesome...! But how does it work? I thought that you would need an OC that passes a 1% or so and reflects the rest... so how do you get more lines by reflecting all of the green but passing everything else? Wouldn't that reduce the number of lines? And what does lengthening the cavity do?

The dichro acts like an OC since no mirror is perfect. Notice how the intracavity beam is so much brighter then the beam after the dichro? The dichro adds gain to the other lines as well within it's reflection band.

Nice work!!!

That's awesome that you got more lines to lase with the external mirror - I'll have to try that with my argon some time

Wow! So the dicro is reflecting the green back to the tube increasing its gain? That's very cool. I was going to order a 445 diode from DTR soon and it looks like I'll be grabbing one of the optics packages that came from the same source as the diodes.

As far as I do know the gain of the 488nm line is much higher than the gain of the 514nm line. This is why the 488nm dominates on a typical ALC60x multiline laser system...

The "build" in cavity mirror/output coupler (OC) has e.g. 99.5% reflection for all lines. The gain of the green line is too low to start lasing at this wavelength because the "leakage of green" is too high at this power level (4.5 tube current). If you're now adding an additional mirror e.g. bouncing back 99.9& of the green spectrum the threshold of the 514nm is reached and the laser starts lasing on this line.

If the "green boosted" laser beam is send through a diffraction the 514nm line is clearly visible because with the mentioned 99.9% external mirror 0.01% of the green inside the cavity leaks... and the power inside the cavity is incredible high. The clearly visible green beam of the external cavitiy is like the beam of a >200mW green.

And dudes I'm not the expert in this area but the explanation sounds good :thinking:

- mojo -

mojo hit the nail right on the head with this one.

What you've got here is two laser cavities, one embedded within the other. The smaller cavity is resonating blue, the larger one is resonating green. Since the blue and green wavelengths utilize separate atomic transitions, they don't steal power from each other. Very cool.

Regarding the gain of 488 vs 514: 488nm has higher gain at lower current levels only. As you turn up the current, the 514nm gain increases faster than the 488nm gain does. At some point the 514 exceeds the 488 and 514 becomes the dominant line, all else being equal. This is not typically seen in small-frame argons because the current levels at which this happens is fairly high.

Ah that makes sense but just to verify the blue passes through the dicro and the green is reflected back towards the laser?

edit: oh hell even after rereading the OP several times I just saw it says exactly that. Really I can read.

i read about doing this with he ne's on sam's, looks like something to try when properly inspired.

N even integer wavelengths have to fit in the cavity when resonant and lasing. So when the reflection in the green gets higher then the speced optic, the existing optic can "drop out" of the picture and is effectively not there, or you get a three mirror cavity, one way or another.

Neat huh?

Steve

cheech226 said:

i read about doing this with he ne's on sam's, looks like something to try when properly inspired.

Click to expand...

What you can do this with HeNe's? I have some reds and a green. Sams site is harder to search than this one (sometimes). Do you have a link?
Thanks

cheech226 said:

i read about doing this with he ne's on sam's, looks like something to try when properly inspired.

Click to expand...

a glass of wine may be a good inspiration also some other stuff helps...

But pleaze consider the following:

• No high voltage stuff is laying around or you will get zapped
• No high power levels (a HeNe should be ok) or some stupid mirror will make your day
• No wine glasses should be placed close to your laser (may spill something :scowl

n2stuff said:

What you can do this with HeNe's? I have some reds and a green. Sams site is harder to search than this one (sometimes). Do you have a link?
Thanks

Click to expand...

No link to SAMs but a link to a german forum with some nice pictures
HeNe multiline

LSRFAQ said:

N even integer wavelengths have to fit in the cavity when resonant and lasing. So when the reflection in the green gets higher then the speced optic, the existing optic can "drop out" of the picture and is effectively not there, or you get a three mirror cavity, one way or another.

Neat huh?

Steve

Click to expand...

I was also some kind of skeptic. The experts always tell: "uha, laser cavity, mirror 0,0000000001 degreee, blabalablalalalbalabla". No, it's not always the thruth. Some stuff can be realized quite easy. But I will not try the experiment with my Argon running on higher power levels.

- mojo -

uh, just to inform you... I've shot another ALC60x head on eBay... hopefully the beast is not broken... if yes, this is one I will play around with (cleaning optics, realignment or what ever).

- mojo -

Ive done the trick with a dying 612 hene and gotten microwatts of 594.

I use it as a demo some times to students, a lot of single line argon tubes have a all lines reflector, its cheaper to only stock one optic for the rear.

Its not hard, the tricky part with a hene is getting something good enough to seperate out the faint lines that are so close together. Not a problem with the argons.

That is why the german dude has such a nice prism array. I ended up with two littrow prisms and a shot across the room to see the really faint lines.


What ticks me off is I spent a evening a few months ago trying that with the same type hene tube he is using, and nothing...

One of these days I'll have time to try it with a 30 mW external mirror job.

Even a cheap BD1 diectric all lines flat from Newport or a piece of high grade diectric mirror from One Stop Laser Shop (12$ for a square inch) can do it with the argon. Some times I can even get it at the REAR mirror. One Stop has dichros cheap, too, and no waiting for them to ship from China.

With Sam's, you use these old things called INDEXs, which are easier then searching...

Here ya go:

http://www.repairfaq.org/sam/laserhen.htm#hengowi

You do need a decent mirror mount to do this, but once you get it adjusted, you can even just set it on the desk and not even bolt it down. You know your alignment is close if the hene tube starts to "blink"

The other day I had to teach a student why her experiment was not working, she was feeding too much light back into a 15 mW hene, from a adsorbing filter, and it was disturbing the cavity and blinking at a few kilohertz. So I bolted a cheap AL mirror to the bench and proceeded to align it back into the hene, and she was amazed at the flashing.

Dont try this with hot DPSS, one, there are no other gain lines close, and you can damage the coatings.
HENEs/Small Argons, are very IMMUNE to the potntial damage.

You can get away with this on argons to two watts or more. Argon coatings are already high damage threshold, and ALC used the same coatings on the 100 mW lasers as they did on the 20 watt ones.
Besides, if you did "burn" a mirror, I have lots of them... Like 20 sets.


Steve

Sam's Laser FAQ - Helium-Neon Lasers

there ya go, had to look good to find it again

thx for the link! will give my HeNe a try...