In over my head

Can anyone tell me how (if) a lab style, 532nm DPSS laser can have it's beam diameter and divergence improved? Can this be done through the optics? My understanding is that these two beam characteristics are determined by the pump diode and crystal characteristics.

The reason I'm asking is that a company has offered to "customize" one for me to provide whatever beam diameter and divergence I want (for no additional charge). It sounds to good to be true but I'm in over my head here and I'm hoping for some feedback.

They are in a hurry and being semi-new to this hobby I haven't quite all the knowledge I need to even tell them what I want (in technical terms). In fact, my hurried research has left me completely confused. I know how to describe it in simple terms: I want the tightest, smallest, truest beam over the longest distance possible to produce a very bright, daytime visible beam. I assume this means smallest divergence, smallest beam diameter and as close to TEM00 as possible????

Here are the specs of the laser they are offering me as it was "just tested"

beam diameter at aperture: 1.5mm
beam divergence at 5 meters: 1.8Mrad
beam mode: TEoo
power stability: 5% over 4 hours
output power:>=200mW

But what is TEoo? I thought that applied only to purely electrical waves? I'd like to think they mean TEM00 but I don't know.

Any feedback would be most appreciated.

Yes, beam specs are mostly dictated by the optics, and is also why people get beam expanders to better the divergence over great distances.  The pump LD and crystals are for producing coherent green light, then the optics collimate it via convex then concave.  But more sophisticated systems do have optics at the diode, and crystals, so their variations can be somewhat limited.  

TEMoo means that the crystals operate as they should, with one solid beam.  When crystals over saturate; over heat, or too cool, or become misaligned, the output power is greatly effected, and can cause the beam to split into multiple beams (which is bad). This is called "mode hopping" TEMXX is the measurement of the mode the laser is operating on.  532nm  is the most stable of all DPSS and should have little to no issues regarding these matters, depending on their cooling also dictates their duty cycle (how long it remains on).  

I hope this helped, it is very general and there are many members here with way more knowledge then me, and Im sure this thread will flood with info shortly...

Welcome to LPF btw  

Ask them how they intend to improve the beam characteristics. What exactly will they be doing or replacing. That would help and you will find out quickly if they are just doing a patch job.

The transverse mode should be listed as TEM00 as the standing waves are a function of the crystal set. You see TE00 in diode lasers.

The specs on that laser seem fine... A true TEM00 is ideal, and 1.8mrad divergence is typical, but not ideal. Most here would consider a divergence less than 1.0mrad to be good, but personally I prefer divergence less than 0.8mrad.

If the divergence on your laser is 1.8mrad, the "spot" would be 1.8 meters wide at 1kilometer.
A laser with divergence of 0.5mrad will produce a spot that is 0.5 meters in diameter at 1km.
So basically, Xmrad means the beam increases in diameter by Xmm for every meter that it travels.

Unfortunately, reducing the divergence increases the beam diameter at the aperture, and vice versa.
If you want the divergence to be low, the initial beam diameter will be a tiny bit larger, but that is still ok, because as the distance increases the dot will stay relatively small.
I would ask for 0.8mrad divergence or less.

I forgot some potentially important specs:

The head is TEC/Fan cooled.
The Polarization ratio: >=100:1
M2<= 2.0 (which I think is a measurement of how close the beam is to pure TEM00)

Of course all the specs are dependent upon the honesty of the seller.

Thanks for the info thus far. That's putting it in terms I can understand.

I will ask them how they intend to improve the beam. Thanks FC.

The M2 factor is how round the beam is. A value of 1 denotes a Gaussian (perfect) beam profile.

On a slightly different subject, I just realized that someday I'm going to want to make this laser battery powered (just the head, I don't really care about modulation circuitry or anything). I have an electronics background, and everything I know tells me it would be doable. Its what I may not know that troubles me.

BTW -this is the kind of laser I am referring to (not THE laser, but almost identical)

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAXS:IT&item=360081320451

Can anyone confirm for me that there is nothing "magical" about powering the head. Namely: As long as I give the pump diode the proper DC voltage and current, and power the TEC, I should be able to get the same performance out of it that the original driver circuit provided?

I guess I should ask this in another post somewhere but I don't want wear this out on the forum. :-[ I asked the seller but they said they couldn't guarantee the same performance if I did that and did not elaborate further.

Thanks in advance,
Tony

If the laser system is by Aixiz then they can alter your laser to the specifications you need. No need to go through the process of finding out what they are doing.

And, yes, if you provide the proper current controlled power to the pump diode it will lase. Keep in mind that you will also need to power the TEC, fans, and monitor the feedback photodiode if there is one located with the pump laser.

Thanks FC. It's not by Aixiz, it just looks very much like that one.

Do you think it may really have a feedback photodiode? If so I don't really want to get into that. I'm sure it's the best way to regulate the thing but I don't want the added complexity.

You don't have to use the photodiode but it's there to stabilize the output of the laser (if it really exists on this laser). The higher the quality the greater the likelihood of a photodiode or other stabilizing system. As the temperature of the pump diode changes the photodiode provides the feedback to alter the input current and stabilize the system.

I see, so I could probably get by with a good battery powered voltage/current regulated circuit, not drive it (the laser head) near its limits, then plug it back into its TTL driver it came with and use it that way. Does that sound feasible?

Easily but at least power the TEC and fan for extended use. You can have the driver circuit and a boost circuit for the fan and TEC. The laser head is already much larger than a pen laser so why not build a beasty battery pack for it so you can run it for a while?

Thanks again. I so very much appreciate your help. Yeah, I would power the TEC and fans too.

I just got an email back from them regarding how they would improve the beam quality and I quote:

"we have to rebuilt our collimating system"

This would then be the specs:

532nm 200mW green laser
transeverse mode:near TEMoo
beam divergence: 1Mrad at 5 meters
beam diameter at aperture: 1.5mm
output power: >=200mW
modulation;TTL or Analog
qty: 1cps

I notice the beam diameter is the same with improved divergence at 1 meter. Seems strange to me that without doing anything about the actual light generation they can improve the divergence that significantly since diameter and divergence tend to be inversely proportional. Perhaps the quality of the collimating optics will be improved or perhaps they are adding a bit of negative divergence (focusing the beam slightly) since they are claiming their divergence at 1 meter.

For some reason, I've got a feeling the beam diameter at the aperture would probably be higher than 1.5mm with the improved divergence.

I just noticed something that concerns me more: Is it typical to quote the divergence spec at 5 meters - does that even make sense? Isn't it usually quoted at something like "full angle" or "half angle" instead (whatever that means)?

I'll try to make that my last question. Thanks again.

Tony

Hodad said:

For some reason, I've got a feeling the beam diameter at the aperture would probably be higher than 1.5mm with the improved divergence, and they just aren't saying so.  

I just noticed something that concerns me more:  Is it typical to quote the divergence spec at 5 meters - does that even make sense?  Isn't it usually quoted at something like "full angle" or "half angle" instead (whatever that means)?

I'll try to make that my last question.  Thanks again.

Tony

Click to expand...

Sorry to butt in, but don't hold back your questions! Your questions are perfectly legitimate, as you will find you've come to the right place for answers. I apologize for my interventions, but you might think about getting a RPL style portable, but you wouldn't be getting true 100% duty cycle without the TEC cooling.