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Finn Hammer

Wed Dec 03 2008, 07:12PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

I just bought a 30mW green laser pointer, just to start off somewhere. Always wanted me a laser.
It works as it should, and when I point it into the dark, it makes a beam about 75meters long. Then it stops abruptly. This is as seen by myself.
It does reach much further, though:
I live by a fjord, and there are swans living it, sleeping together in large flocks.
The fjord is 1 km away, and I can hear the swans startle and make noises when I flash them.

Anyway, from what I`ve learned, these lasers start out with an infrared emitter, then are halved in wavelength by a crystal.

Now to the point. I see people buying Coherent lasers in the 35W`s range, coupled to a fiber.
Would it be possible to convert one of those to green also, and in that case, where do I get the crystals or whatever.
Sure would be amazing to point a 35W green into the sky on new years eve.

Cheers, Finn Hammer

Dago

Wed Dec 03 2008, 07:26PM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

You can make a whole DPSS (Diode Pumped Solid State) laser from discrete components but generally speaking its quite difficult. Alignment of the crystals and temperature regulation etc. is very critical. Also crystals for frequency doubling other than 808nm are fairly rare and expensive. Though KTP crystals for doubling 808nm at that power class (35W) will be very expensive as well (several hundred bucks to several thousand for one crystal depending on surplus or retail etc.). Crystals for tens of mW-few hundred mW output of green light are fairly well available and cheap (couple hundred bucks or so from ebay).

You also need a crystal (Nd:YVO4 for smaller pointers and Nd:YAG for larger systems) for raising the wavelength from 808nm to 1064nm at which the KTP crystal functions. In small green laser pointers these crystals and the mirrors and optical filters needed are usually in one unit which is a composite crystal where both crystal are glued together with optical glue. In larger systems they are separate because they need individual temperature regulation.

The problem with those 35W modules and similar is their wavelength which is something around ~1300nm because that wavelength gets attenuated the least in optic fibers. The crystals for frequency doubling have very narrow spectrum on where the doubling effect works feasibly (I recall even sub nm changes can change the output by tens of percents). There are other frequency doubling crystals than KTP (which is usually used for 808nm->532nm conversion) but they are very rare and hard to find info of. Some crystals have other problems like KDP crystals which are very hygroscopic and have to be hermetically sealed in dry environments to stop them getting wet and dissolving.

I contemplated on making one but getting all the stuff machined and all the components bought it would have been quite expensive (several hundred â‚¬). I recall though that you're a toolmaker and have access to metal working tools and equipment which would make it a lot easier and cheaper.

THE resource on lasers on the net is Sam's laser FAQ, it has a fairly good section on building DPSS lasers yourself (mainly focussing on building small few tens to couple hundred mW ones).

Heres the section about DPSS lasers:

Theres quite little information on homebuilt DPSS lasers probably because they're a very rare hobbyist project (but not impossible).

LutzH

Thu Dec 04 2008, 10:35PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

Building a DPSS green laser is not easy, yes do see Sams Laser FAQ. This said however I would recommend 2 options if you still want to do it:

1. If you are doing a home build get a 808nm pump laser diode which is already fiber coupled, because you will need to focus the pump light into a 100-300 micron spot which is nearly impossible with a diode bar. For big power .5-3 watts of green, then you can use a small Nd:YAG rod/slab say 2 x 10-15mm. You can side pump it with 1 or more diode bars, which is how they do it in medium power systems, and then follow it with your KTP. Direct doubling of the laser diode does not work outside of the lab because the LD output is not a discrete wavelength to match the crystal.

2. Buy a higher output green laser module, or kit from China, several people I know have had good luck with buying direct from China via ebay, I remember at least one major manufacturer from China even has an ebay store, and the quality is not bad from what I hear. One seller from China even offers a blue beam kit, this could be fun!!!

Good Luck : )

Brett Miller

Fri Dec 05 2008, 06:33AM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

I have actually been tooling up and starting to do research for my own 532nm green DPSS laser project. I am planning on taking it in discrete steps, just like how I managed to learn the significant TC topologies over the last decade. Take it slow, follow the safety learning curve, and don't skip steps!

Anyway, for anyone planning on working with DPSS, I'm told that the bottom line is patience. You'll have to learn to keep frustration at bay when being thwarted by such things as optics getting out of alignment, expensive diodes accidently getting fried, poor driver design, inadequte temperature control, etc.

I've read the DPSS section of Sam's laserfaq a doezen times, and I also recommend reading every post by Peter " ..." regarding lasers in this section of 4hv. He has been personally helpful in this. Some of the laser forums are ok, but don't expect the level of genius and expertise we've grown accustomed to at 4hv.

Just as LutzH suggested, I am planning on driving my crystals with a fiber coupled diode (which I haven't procured yet). These can be pretty expensive on ebay, but there is some variation in price. I will use this 808nm diode (probably at 1W) to pump a Vanadata/KTP bonded crystal. The fun part will be determining how to mount everything for maximum output and cooling. According to Sam's faq, and others I've talked to, this is just about the easiest way to achieve your first home brew 532nm green. Once I've seen some green from my first completely DIY laser (not one which was simply a modification of an existing module), I'll naturally want to shoot for a higher power level for my 2nd project, and on and on. At some point over 100mw or more (maybe afteer 200), things like the use of a TEC (peltier cooler/thermoelectric cooler) come into play. The game gets complicated. Eventually I might just want to get into other things (besides green, red, or violet (blu-ray)). I am hoping the prospect of obtaining a crystal of LBO and making DPSS blue 473nm is actually feasable for the amateur, or that maybe it will be in a couple years.

If companies can do this (473nm blue) in a small pointer enclosure, then why couldn't an amateur do it on the test bench in his/her lab?

Does anyone know of a website which shows a successful blue DPSS homebuild?

-Brett

...

Fri Dec 05 2008, 08:12AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I hate to break it to you Finn, but you aren't going to build a 35w 532nm laser by newyears

It is somewhat amazing how incredibly picky aligning optics can be (I am working in an optics lab, trust me on that one...), and when you start to throw high powers into the mix you not only need to keep them aligned to insane tolerances, you also need to keep everything cool, and even when you do have everything cool you get thermal lensing (literally, the material heats up where the beam is, which causes it to expand, and make a tiny lens) which goes and throws all of your alignment off.

If you wanted to start with one of those 40w 808nm laser diodes off ebay, you would need to focus the 0.8mm fiber bundle output that high power fiber coupled diodes have down into a spot about .1mm diameter on a well cooled block of nd:yag or preferable nd:yvo4 (it will be dissipating somewhere between 20-40w depending on how good your design is). If you vandate crystal doesn't have a HR@1064nm on one side you will also need a mirror between the pump and the vandate. Then you need to add the KTP crystal, and the OC mirror (which in this case would actually be coated HR@1064, and preferably HT@532nm). It would be best to use cavity mirrors that are curved slightly so that they focus down on a spot inside the KTP crystal to help the doubling process, although be careful not to blow up the crystal by focusing down too tightly. You might be able to get 10w of power out of you are lucky, but even .1w would be pretty good for a first try..

A more reasonable goal would be to try to start with a vandate/KTP combo crystal and a 1w fiber coupled laser diode, which should be able to get you 100mw without too much trouble. Just hold the fiber output from the laser up against the vandata/ktp crystal and viola :)

The only place I have found to get reasonably priced laser parts is ebay, and even then you need to watch carefully and be patient. The best place to get vandate/KTP crystals is cheep green laser pointers, they are actually good for quite a bit of power when you couple them with a better pump source

*note-some common terminology
HR - high reflector, a coating that has almost 100% reflectance (usually followed with an @ sign and the wavelength it is designed for)
HT - high transmission, a coating that has almost 0% reflectance (again, followed by an @ sign for the wavelength)
OC - output coupler, a coating that is a partial mirror, that will reflect some percentage of the light passing through it. It will also have a wavelength it is designed for
BB - broadband, appended to and of the other abbreviations to show that it isn't tuned for a single wavelength, but for everything.
gain medium - something that will amplify light passing through it when it is given an external energy source (light, electricity, chemical, etc). ex, nd:yag
cavity (or resonator) - the space between the HR and OC, which is where the lasing actions happens. You need at the very least some type of gain medium, but many lasers will have some type of nonlinear crystals, q-switches, polarizers , etc in it as well.
vandate - nd:yvo4, almost exactly the same thing as nd:yag but has a wider tolerance for the wavelength of the incoming pump beam, but poorer thermal properties
KTP, LBO, both nonlinear crystals that can double the frequency of a laser beam with sufficient power. They usually need to be inside the laser cavity
pump - the high power 808nm laser diode you are using to power your laser

Brett Miller

Sat Dec 06 2008, 03:15AM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

Thanks for that laser glossary, Peter!

-Brett

Finn Hammer

Sat Dec 06 2008, 07:10AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Thanks a lot you laser gurus!

Sam`s laserfac is next stop then. I am happy that you gave these explanations, the terminology is so funny and difficult to get into, just as with magnetics and medicine.
But it should be difficult, like Kennedy said: we chose to go to the moon, not..... but because it is difficult.
So let`s go to the moon, I mean not shine a laser at the moon, but build difficult laser, or at least give it a try.

Cheers, All

Finn Hammer

Brett Miller

Sat Dec 06 2008, 07:28AM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

Finn,

Well, by no means am I a laser guru. I consider " ..." one simply because he's been into it much longer than me and also because he not only works with lasers, but he also experiments with them for enjoyment as well. That's a pretty good recipe for success in my opinion.

I'm just starting out for the most part. It's something I always wanted to get into, but I was too busy with tesla coils. I think the things I learned about electronics and engineering when working with solid state TC's lead me down a path that eased the transition into an interest in lasers.

I usually get obsessed with something once I start and try to soak up every bit of information I can about a subject. But I firmly believe in following the proper steps when entering a field and taking on projects in a linear progression, from simple to complex. People who do this tend to be more successful when they do get to the hard stuff (like going to the moon....they didn't get there by building the Saturn V launch vehicle first...they had to amass experience -- Sputnik, explorer I, Mercury, Gemini, etc).

It's the difference between me as a kid, and now as an adult...I take the time to follow the learning curve. Of course, a master coiler like yourself knows this.

Right now I am working on an almost exact type of beginner DPSS laser that "..." described. I have almost all the parts, and when brought together, they should yield some 532 green without much trouble, so to gauge efficiency I'll need an LPM (laser power meter). Just like tesla coils, I can imagine that one of the challenges and main reasons to stay with it are to strive for greater efficiency, the prize here being brighter and more coherent green beams rather than longer sparks. Of course, safety glasses are a must when dealing with powerful IR pump diodes.

Cheers,

-Brett

LutzH

Fri Dec 12 2008, 10:25PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

Here are a few more thoughts based on one of my current non-assembled DPSS laser projects, for which I have all of the main parts. Here are some thoughts on what and why.......

DPSS laser #1 Project : 532nm at 1-3 watts output.
Pumping Medium: Nd:YAG Mini Slab 2x2x12mm about 1%Nd
Pumping Source: (1) 20-60 watt 808nm LD bar being run at a conservative 10-20 watts.
SHG Crystal: 4x4x7mm KTP Raicol Grey Tracking Resistant

For this laser at the desired output power, thermal considerations start to become critical. At 10-20 watts of pump power the cooling of the laser medium becomes a major issue. I chose Nd:YAG because it has a very high thermal conductivity almost 4-5 times that of the more efficient Vanadate host, so that I do not need exotic cooling.

The crystal will be mounted in a copper heat sink with indium foil. The end view would be the 2x2mm end of the Nd:YAG in an L shaped copper heat sink, being side pumped with the linear output of the LD Bar.

This configuration also allows for the future addition of a second LD pumping bar if needed, one pumping the side, and the other the top, both pumping in a linear manner. One LD Bar should be good for .5 to 1.5 watts of 532nm, and 2 bars for up to 2-3 watts with a total pump power of 20-40watts

By using side pumping I eliminate the need for all of the fancy and exotic collimation optics of the pump beam, which are required for end pumping. In fact it will work even without any collimation optics by mounting the LD Bar output face very close say .5mm from the side of the Nd:YAG mini slab.

The efficiency of the doubling to 532nm will be much better however if I use a simple cylinder lens to focus the LD output into the middle of the Nd:YAG crystal in a line focus. This is very easy, the only focusing lens required is essentialy a small piece of an AR coated glass rod.

I also plan to gold plate the indium foil holding the Nd:YAG, so that any pump light not absorbed by the crystal will be reflected back into the Nd:YAG slab from the heat sink, for a second pass. Thus the pump light absorbtion length will be 2 times the 2mm size of the crystal. At this 4mm length the tuning of the 808nm LD pump bar will be less critical, and compensate somewhat for the reduced absorbtion bandwidth of Nd:YAG vs Nd:Vanadate. With this approach the increased complexity of temperature tuning of the LD pump bar can be eliminated :)

When I bought my pump LD bar I chose one with the center wavelength of 806-807nm so that it will be right on at 808nm if it is a little warm.

KTP:
Here is where you do not want to be to cheap. For efficient SHG to 532nm you want at least a 5mm length of KTP. I picked up several 6-7mm long KTP crystals on ebay made in France, and Israel. The Chinese KTP is cheaper, but many reputable people report over 2 times or even greater output using the better quality KTP. Hydrothermal KTP from Litton in the US is the best, but the cost is way to high and the material from Europe and Israel is fine. For maximum efficiency you want to keep the beam waist small inside the KTP, there is a direct relationship between energy density in the KTP and conversion efficiency.

Optics:
For your mirrors the Chinese products are fine, and will save you a bundle of cash. Check out Casix on the web, they have most of what you need.

LD Bar Driver:
Sam's laser FAQ has a bunch of info on how to drive you laser diode bar, just be very careful reference ESD protection.

If you get to the 1-3 watt level you should be able to light up the other side of the Fjord easy. Feel free to contact me once you finalize your design, I have some extra components which I may be willing to let go for reasonable figures.

For real high power unless you want to spent thousands of euros, Kr arc lamp pumping is more economical especialy given how cheap new surplus Kr lamps are on ebay. My second laser project will be lamp pumped, I have a few Nd:YAG rods; A couple small ones, and one 9.2mm x155mm monster complete with a dual lamp pumping cavity good for 400W at 1064nm.

Be careful on ebay I have seen inferior Nd:Glass listed as Nd:YAG which is fraud plain and simple, but they seem to get away with it, in fact I saw two large glass rods listed as YAG last night on ebay. Nd:Glass is only good for intermittent pulse use, no CW.

Take care and I am looking forward to seeing a pic of a green beam crossing a Fjord in a future post :)

Aloha.....Lutz

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