Divergence?

Tabish said:

Anything over 1mRad is high. Aim for a divergence <1mRad.

1 mRad means the beam thickness will increase by 1mm for every meter it travels.

So if your beam starts of at 1mm thickness. After 10 meters, it will be 1.1 centimeter thick.

If it was 2mRad, it will be 2.1 cm thick after 10m.


For pointing in the night sky, it doesn't matter too much honestly.

For burning, a tighter beam is better.

Best burning is done when the the beam is focused on the target (meaning a negative divergence)

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Excuse me but what does thickness mean here? Lets say we have a divergence of 1 mrad and the dot starts at 1 cm. In 10 meters, how big would the dot be?

Thats what i may've been getting mixed up with & confused about that some guy on here said his dot with this laser at 3000m was 8ft wide!! I guess it wasn't focused, but thats very wide for a laser i thought!!
I'm not looking for 0mrad lol, just like what i see on the vid & that don't seem like too wide a beam!!
So if i can minimalize the dot size, be it across the room or 1000m away, surely the beam will be narrower if the dot's smaller?

Silvershot said:

Excuse me but what does thickness mean here? Lets say we have a divergence of 1 mrad and the dot starts at 1 cm. In 10 meters, how big would the dot be?

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I'm not sure the math used in the post you quoted is accurate. Here's a useful little calc for divergence: pseudonomen137's JScript mRad Calculator According to the calc, a beam with an initial diameter of 1cm and a divergence of 1mrad would give a spot 20mm wide at 10m

It's always a trade-off. As said above, for a beam in the sky it doesn't really matter. For burning though, the problem is that you can either have low divergence at the expense of having a larger initial beam, or a small initial beam at the expense of large divergence - never both (sorry!). So depending on what you're trying to burn and at what distance, you really just want to have optics for that particular task.

For the lasers you buy, the divergence and diameter are usually at a good balance for general use though. If you multiply them together you can get a factor which you can use to compare one laser to another. Keep in mind its hard to acurately measure the diameter of your beam without the right (expensive) tools, so things may not be as bad as they seem.

ElektroFreak said:

A divergence of 2mrad isn't terrible. With DPSS lasers, the divergence tends to be higher than other kinds of lasers, but 2mrad is hardly a flashlight. A 2cm dot at 10m is fairly typical for green DPSS, especially high-powered greens. Once again, that's hardly a flashlight. A flashlight could never maintain a divergence of 2mrad.

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Not quite (see above for a partial explanation). DPSS doesn't have quite the good beam properties you'd get out of a good HeNe or something like that, but in general DPSS actually has very good properties (not all that far from perfect for low power lasers). In contrast, check out some multi-mode diode lasers! you can easily get worse than 5mRads and 4mm when you collimate those jerks

pseudonomen137 said:

Not quite (see above for a partial explanation). DPSS doesn't have quite the good beam properties you'd get out of a good HeNe or something like that, but in general DPSS actually has very good properties (not all that far from perfect for low power lasers). In contrast, check out some multi-mode diode lasers! you can easily get worse than 5mRads and 4mm when you collimate those jerks

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I'm failing to see where the "not quite" comes in.. you pretty much said the same thing as I did. DPSS does have good beam properties in general, but the short-cavity MCA/DPM modules we see regularly in portables and pointers always give larger divergence than single-mode diode lasers are capable of because of the short cavity length. Of course typically the higher the power, the higher the divergence, especially with inexpensive Chinese DPSS. Personally I never take burning characteristics into account due to the fact that it is the least important use of hobbyist lasers IMO.

waterface said:

Thats what i may've been getting mixed up with & confused about that some guy on here said his dot with this laser at 3000m was 8ft wide!! I guess it wasn't focused, but thats very wide for a laser i thought!!

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Even if that were true, that's almost 2 miles. What do you expect for $160?

You're still spending an awful lot of time and energy worrying about a laser you haven't gotten yet. IMO.

^Never a better time to try to get an understanding of this stuff. Waterface is doing the right thing by asking here beforehand rather than becoming enraged with disappointment (as many n00bs do) later when he gets a laser that doesn't meet up to his expectations. There are a LOT of unknowns regarding lasers from the general public's perspective.. this leads to misunderstandings and unrealistic expectations.

I asked about on here prior to ordering & looked at the youtube clip in my post & it seems to burn matches & has a nice beam!! I'm not sure whether having a 'wide' beam for pointing in the sky is good afterall i want it for pointing at constellations too.. I think maybe i'm mixing up divergence with a wide final destination of the beam!! Maybe its that i can't burn at distance!!
I'd like a small dot, like the guy on the video points it at his mailbox & the dot seems no bigger than he gets when he points it at the bed!! Also, i wonder how big that dot would be if i were on the mountain he aimed it at!! 8ft diameter!! That must mean the beam would be 8ft wide too!!

Yeh, ElektroFreak, i think i know what i'm gonna get as all i've had before is a 5mW & a 1mW red that has a do that can be seen for miles away & is same size, but thats reds & isn't that bright!
Susie at o-like says that this laser i'm getting has a good beam & burns etc & if its like the one on the vid, i'll be relatively happy, as that burns & he says it looks better in reality!!
I'm still missing something in how i'm comparing dot size, beam width, focusing long & short to burn with divergence!! I guess as with most things, i just need to use it & work it out!!

Something more to think about......

OK, you point at a distant mountain at night, it's covered in snow.

You see the green dot on the mountain, everything seems great.

If that dot was still 2mm in size, do you honestly believe you could see that????

From this huge distance, the "dot" NEEDS to be frikkin huge in order for you to see it.

Here is a good experiment, Set your laser up on a picnic table at night, aim it out over a field until it "paints" a spot half a mile back.

Now go for a walk to your target.

ElektroFreak said:

I'm failing to see where the "not quite" comes in.. you pretty much said the same thing as I did. DPSS does have good beam properties in general, but the short-cavity MCA/DPM modules we see regularly in portables and pointers always give larger divergence than single-mode diode lasers are capable of because of the short cavity length. Of course typically the higher the power, the higher the divergence, especially with inexpensive Chinese DPSS. Personally I never take burning characteristics into account due to the fact that it is the least important use of hobbyist lasers IMO.

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Sorry, my disagreements were with the statement "With DPSS lasers, the divergence tends to be higher than other kinds of lasers", and the general treatment of 'divergence'. It must be stressed that divergence, by itself, does not give you any benchmark by which to compare lasers. It is only by taking both the minimum beam diameter and divergence into account that you get the right factor: beam quality.

In the grand scheme of lasers there are plenty types of lasers with better beam quality than DPSS. For what us hobbyists get to see though, DPSS is actually among the best beam quality (and thus can offer the best divergence, diameter and wavelength equal), with M^2 factors typically spec'd <1.2 (so less than 20% away from perfect). That doesn't hold a candle to a good HeNe of course, but sure beats a lot of the stuff we play with.

"With DPSS lasers, the divergence tends to be higher than some other kinds of lasers"

I trust that will clear things up..

The question I was answering was about divergence specifically and we hadn't quite made it into how it relates to BPP and thus m^2. You are correct on all counts, though.

I like how you say "spec'd <1.2". I have yet to find a cheap Chinese laser over 20mW that comes remotely close to that figure. Most hobbyists never really know what they're going to get in terms of divergence (or m^2) until they have the laser in hand and can measure it. The "specs" of cheap DPSS lasers aren't to be trusted.

photonaholic said:

Something more to think about......

OK, you point at a distant mountain at night, it's covered in snow.

You see the green dot on the mountain, everything seems great.

If that dot was still 2mm in size, do you honestly believe you could see that????

From this huge distance, the "dot" NEEDS to be frikkin huge in order for you to see it.

Here is a good experiment, Set your laser up on a picnic table at night, aim it out over a field until it "paints" a spot half a mile back.

Now go for a walk to your target.

Click to expand...

Yeh, i guess that & i understand that i wouldn't see it if it was still the size of aperture, but 8ft!!
I guess the divergence is so minimal then as on the video, he points it at his mailbox & the beam doesn't look any wider & its a 100ft away or so! & even when pointing in the air, most green lasers in review i seen on this site don't seem to get that much wider & when pointed in the night sky, the dot is probably much further away than 100ft!
But, i guess some divergence is important for a good beam & as i'm getting a focusable & as pseudonomen says, the lases we buy i guess have a happy medium for beam & burning!

Silvershot said:

Excuse me but what does thickness mean here? Lets say we have a divergence of 1 mrad and the dot starts at 1 cm. In 10 meters, how big would the dot be?

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That all depends on the Rayleigh Range. Within the Rayleigh range the beam will expand 1.4 times its initial diameter.

waterface said:

Yeh, i guess that & i understand that i wouldn't see it if it was still the size of aperture, but 8ft!!
I guess the divergence is so minimal then as on the video, he points it at his mailbox & the beam doesn't look any wider & its a 100ft away or so! & even when pointing in the air, most green lasers in review i seen on this site don't seem to get that much wider & when pointed in the night sky, the dot is probably much further away than 100ft!
But, i guess some divergence is important for a good beam & as i'm getting a focusable & as pseudonomen says, the lases we buy i guess have a happy medium for beam & burning!

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You do understand what the word <Divergence> means ? All light waves diverge.

ElektroFreak said:

"With DPSS lasers, the divergence tends to be higher than some other kinds of lasers"

I trust that will clear things up..

The question I was answering was about divergence specifically and we hadn't quite made it into how it relates to BPP and thus m^2. You are correct on all counts, though.

I like how you say "spec'd <1.2". I have yet to find a cheap Chinese laser over 20mW that comes remotely close to that figure. Most hobbyists never really know what they're going to get in terms of divergence (or m^2) until they have the laser in hand and can measure it. The "specs" of cheap DPSS lasers aren't to be trusted.

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Thanks. Sorry for being a pain in the butt... I just get irked by how divergence and diameter are commonly treated as independent properties on laser forums. Then many end up expecting you can optimize each one separately, which then leads to complaints when their laser X has higher divergence than laser Y, although laser X has a much better beam waist. Or turn the tables around and there are complaints that it can't even burn because of the larger diameter. Or even when there's a good balance, still people ask for more until they understand the inverse relationship between the two when working in air.

ElektroFreak said:

I like how you say "spec'd <1.2". I have yet to find a cheap Chinese laser over 20mW that comes remotely close to that figure. Most hobbyists never really know what they're going to get in terms of divergence (or m^2) until they have the laser in hand and can measure it. The "specs" of cheap DPSS lasers aren't to be trusted.

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hehe good catch, I see I can't get away with that one! I can't say I have a profiler or similar equipment to accurately measure beam quality, but I sure have seen plenty of cheap greenies that clearly fell short of their spec'd beam properties. I still feel from the point of view of a hobbyist though, DPSS offers some great beam specs compared to a lot of the other toys we get to play with (when gas lasers were more common for hobbyists, I wouldn't make that claim but now it seems like most hobbyist lasers are based around a semiconductor emitter at least somewhere in them). I know when I was looking into making a solid-state RGB laser projector, DPSS always offered the impressive beam specs but a set of 473/532/671nm was the expensive, 'premium' setup compared to the many multi-mode alternatives (and in the long-run I couldn't justify the cost of either! ).

steve001 said:

You do understand what the word <Divergence> means ? All light waves diverge.

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Are you telling me i do, from what i said in my last post! or asking me?