Divergence

Very quick question: I've heard that the divergence of gas lasers are extremely good compared to diode lasers. Is this true?
For example, our science teacher played with a 1mW HeNe in front of the class, saying that it could project a dot about the same size on a wall 2-3 miles away.

Not always, in fact in my experience you can get insanely good divergence from single-mode diodes that puts gas lasers to shame. With most lasers, divergence is heavily dependent on cavity length. With HeNes, the shorter the tube, the worse the divergence. The beam quality of gas lasers is always superior to diodes, and the stability is typically superior to basic DPSS lasers.

They can have awesome divergence, but at 3 miles the dot won't be of the same size lol

as ElektroFreak said, the cavity length has a lot to do with divergence, a longer cavity will give a better divergence in gas laser ( after the TEM is considered of course ).

ElektroFreak said:

With HeNes, the shorter the tube, the worse the divergence.

Click to expand...

diachi said:

a longer cavity will give a better divergence in gas laser

Click to expand...

So which is it?

My HeNe had a divergence of ~1.5mRad but an initial beam diameter of <1mm. The aixiz lens gives diodes something <1mRad (I think) but an initial beam diameter of 4mm or so. Assuming half the diameter is twice the divergence, that would put diodes at 3mRad for a <1mm beam. In that sense, HeNes are better.

I believe that the term cavity in this case refers only to the space between the mirrors. When dealing with gas lasers, the OC mirror is the final optic. No collimating lenses are required. With diodes, since their cavities are so very small, the divergence straight out of the OC is huge. This is in keeping with the fact that divergence is dependent on cavity length and initial beam diameter. HeNes are far superior to diodes straight out of the OC, but you put the right lens in front of a diode and it acts as a beam expander (of sorts) and you get slightly wide but very collimated beam. Of course, the same applies to HeNes or any laser. If you expand the beam at aperture, it becomes possible to get very low divergence figures.

You have to compare "apples to apples"

If you are comparing lasers without any external lensing (bare diode to bare tube), then gas lasers have significantly better beam specs.

Once you shine the beam through a lens, you are actually measuring the effectiveness of the lens assembly, not the beam spec.

Peace,
dave

But even with a typical lens assembly (one plano-convex?), gas is better.

Correct. Any laser can have extremely low divergence if the proper lenses are placed at the aperture.

..on "costs" of the beamdiameter, that is!
when considering both beamdiameter and divergence, i would think that he-nes always have better specs than diodes? just like cyparagon calculated?

manuel

Sure, any time you decrease divergence you increase beam diameter.

I have a 10" hene with horrible divergence, then I have a tiny melles griot (like 4-5") with really good divergence. Go figure.


Cavity length does play a factor, but another factor in gas lasers is bore size.


The best divergence from any gas laser that I've seen is from my HeCd. The beam is about 1mm at the aperture. I haven't measured actual divergence but its pretty damn amazing, due to many factors.

The first factor is the small bore size, I think about 1mm.
The second is a very long cavity, about 1 meter.
The third (which is also very important) is that it is a single mode (tem00) beam.


Multimode gas lasers are a bit different than what you typically consider multimode on a dpss to be. Most hear the word "multimode" and think of two or more dots. This is not the case with gas lasers. A multimode gas laser will still have a nice round dot and most people would classify it (wrongly) as tem00. The reason it is not tem00 is because it is not brightest in the center, losing power towards the edges. In most cases with a multimode gas lasers you get a full field dot, meaning that it is still nice and round, but unlike tem00 the power is evenly distributed throughout the dot.

The fourth is the short wavelength.

GooeyGus said:

Multimode gas lasers are a bit different than what you typically consider multimode on a dpss to be. Most hear the word "multimode" and think of two or more dots. This is not the case with gas lasers. A multimode gas laser will still have a nice round dot and most people would classify it (wrongly) as tem00. The reason it is not tem00 is because it is not brightest in the center, losing power towards the edges. In most cases with a multimode gas lasers you get a full field dot, meaning that it is still nice and round, but unlike tem00 the power is evenly distributed throughout the dot.

Click to expand...

That's interesting. I always thought what you are describing is a "flat-top" profile.. not quite TEM00 but not multimode either. So gas lasers are incapable of producing a multi-lobed emission pattern? I would have thought they would follow the Laguerre cylindrical mode structure.

GooeyGus said:

Multimode gas lasers are a bit different than what you typically consider multimode on a dpss to be. Most hear the word "multimode" and think of two or more dots. This is not the case with gas lasers. A multimode gas laser will still have a nice round dot and most people would classify it (wrongly) as tem00. The reason it is not tem00 is because it is not brightest in the center, losing power towards the edges. In most cases with a multimode gas lasers you get a full field dot, meaning that it is still nice and round, but unlike tem00 the power is evenly distributed throughout the dot.

Click to expand...

I thought that saying "multimode" in case of gas lasers concerns rather longitudal, not transverse modes. And a question from me - can solid state lasers work at many longitudal modes like gas lasers and is it common? (For example does an Nd:YAG laser predestined only for burning work on one or many longitudal modes?)

I think all yags are multimode.