What exactly does "near TEM00" mean (aka the mode and stability thread)

[ElektroFreak]

OK.. so I've been tinkering around with DPSS lasers for some time now, but I've never before owned a system over 1W until I received a 1W 532nm system recently. At that time I decided to do a bit of research into inexpensive, high-powered DPSS systems. I have thoroughly searched this forum for more information on the beam characteristics and stability of high-powered DPSS, but there doesn't appear to be much info on that specifically.

I profiled the beam from my system (CNI MGL-H series rebranded as Wicked Lasers) using a magnifier and I noticed that the beam, while round, does not have even power distribution. There is a central, roughly circular region of maximum power surrounded by splotchy lower powered output (I would post a pic, but I'm at work currently). The inner region always maintains its shape and power level, but the outer surrounding output fluctuates considerably during warmup, but stabilizes once the system is warm. Under modulation the central region maintains power while the outer region fluctuates occasionally. At no point does the profile separate into multiple lobes indicative of higher-order operation.

After exhaustive research involving spec sheets from all of the major Chinese manufacturers, I found that most of them list all of their 532nm DPSS systems that are 1W and greater as being "near TEM00" rather than true TEM00. I would assume this is intended to describe a profile similar to what I have described, but not having any firsthand experience with DPSS lasers at this power level I can't say for sure, hence the reason I started this thread.

I would love to hear what the experts here think about this. It also seems that CNI lists their high-powered 473nm systems (200mW and higher) as "near TEM01" which seems like something that would set some members here off severely.. Maybe we are expecting too much from these systems?

Apparently, according to the vast majority of Chinese suppliers, these beam types are within spec, but I have always heard complaints about "defective" lasers that exhibit these same qualities. Is this a power level thing (it's easier and less expensive to get true, stable TEM00 operation at low power levels)?

Honestly I have never owned a Chinese DPSS that I would call truly 100% stable. They all exhibit blanking-induced power loss/mode-hopping of some sort under modulation (although some are MUCH worse than others) and I have always believed that short of a much more advanced cavity design (L-fold, ring, twisted-mode, etc..) a truly 100% stable TEM00 beam is impossible with DPSS. Experts opinions are needed here, since this conflicts with many peoples assumption that any linear-cavity DPSS laser that fluctuates or hops is defective, plain and simple. Is it not more complicated than that?

Any input would be appreciated.

[allthatwhichis]

As you saw in my review of the 300mWish 473nm from WL; it is definatly TEM01.

http://www.photonlexicon.com/forums/...ead.php?t=8921

[ElektroFreak]

I did see that, and the idea that it was intended to be that way from the factory is interesting to me.. that was part of the reason I started this thread, as well as the fact that there just isn't much detailed info on this. If these systems are intended to be this way from the factory, I really don't have too much of an issue with that..

Also, I heavily suspect that these Wicked Lasers Reference Series lasers are CNIs rebranded and dressed up, but I can't get confirmation from Wicked (probably to protect their profit margin).. Mine even has a CNI model sticker on it.

[Laser Ben]

I have always believed that short of a much more advanced cavity design (L-fold, ring, twisted-mode, etc..) a truly 100% stable TEM00 beam is impossible with DPSS. Experts opinions are needed here, since this conflicts with many peoples assumption that any linear-cavity DPSS laser that fluctuates or hops is defective, plain and simple. Is it not more complicated than that?

Exhibit 1:

[mixedgas]

Exhibit 1:

Depends, if you limit the power using a iris intracavity, or turn the pump down, just about anything DPSS with 2 mirrors will be tem00 at some point.. A lot depends on the tightness and quality of the pump spot as well.

Most low cost lasers use a non adjustable solid block resonator to save costs, so the optics are designed to have a high stability factor ie lase across a huge range of misalignment. by definition they are "non critical" cavities. Non critical with a short resonator and a high gain = messy beam.

I could easily design you a 2 mirror tem00 DPSS, just you might not be happy with the ratio of pump in to power out,or want to pay for the extra cost of resonator rods and real mirror mounts. Ie NO glued optics.

Steve

[ElektroFreak]

thanks for your input Steve.. very helpful. It seems, though, that when you add modulation into the mix things get even more messy due to the temperature fluctuations in the crystals. Is it possible to have a two-mirror DPSS setup that runs TEM00 100% of the time with full power stability regardless of modulation?

To me, the biggest issue with inexpensive lasers for laser shows is power stability under modulation. The audience will never notice the if the beam is slightly less than round or if the power distribution isn't perfectly gaussian, so for my needs this "near TEM00" is fine. It's images with dim spots and jellybeaning that I hate. I'm just very curious about this since it seems that the general consensus among laser hobbyists is that a DPSS with anything other than perfect TEM00 distribution is defective. To me, it seems like there is more to it than that.

[mixedgas]

thanks for your input Steve.. very helpful. It seems, though, that when you add modulation into the mix things get even more messy due to the temperature fluctuations in the crystals. Is it possible to have a two-mirror DPSS setup that runs TEM00 100% of the time with full power stability regardless of modulation?

Yes, if A. AO modulated, B. Bought from Coherent etc.

To me, the biggest issue with inexpensive lasers for laser shows is power stability under modulation. The audience will never notice the if the beam is slightly less than round or if the power distribution isn't perfectly gaussian, so for my needs this "near TEM00" is fine. It's images with dim spots and jellybeaning that I hate. I'm just very curious about this since it seems that the general consensus among laser hobbyists is that a DPSS with anything other than perfect TEM00 distribution is defective. To me, it seems like there is more to it than that.

I can live with a Phatt Beam, its the modulation issue that can be and needs to be fixed. TeMoo is a overated Holy Grail because it means you can have the least divergence and best spot size. You need it for science. You often dont need it for beam shows.

Steve

[weartronics]

It seems, though, that when you add modulation into the mix things get even more messy due to the temperature fluctuations in the crystals. Is it possible to have a two-mirror DPSS setup that runs TEM00 100% of the time with full power stability regardless of modulation?

No! e.g. CrystaLaser 1W 532nm, cost is ~25000 USD, it uses individual TECs for the pump diode and crystals, with both thermistors and optical feedback to close the loop. This is the highest quality example of two-mirror DPSS that I have found, but RMS power stability is still only 20% with 300 kHz modulation. It means that if you need to modulate the diode current with two-mirror DPSS, the power stability is always going to be bad.

[ElektroFreak]

Hmm. That's kind of what I thought. There's a big difference in price between that Crystalaser and a Chinese system and there's a big difference in quality too. When you really think about it, given the difficulty involved in properly stabilizing a two-mirror DPSS, the best Chinese systems are kind of a blend between affordability and stability, since you apparently can't have both.

Truthfully I haven't had anything but the most minor power stability issues in the laser I mentioned at the beginning of the thread. In that unit I noticed that the transverse power distribution was uneven. It's not multimode, there's only a single spot, but it's kind of smudgy and blotchy around the outside if you run it through a magnifier. Then I noticed that systems comparable to this one are listed as being "near TEM00" rather than true TEM00. This isn't very descriptive. Fortunately another member here gave me a good description of what "near TEM00" means, but I feel that it is still a very broad term and could encompass a wide variety of beam types.

[weartronics]

I think the highest quality projectors run the DPSS modules CW, and modulate the output using an AOM. I've only heard about this anecdotally though. I would be very interested to hear if anybody knows of a specific company/projector that is doing this.