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20watt laser diodes (EBAY)...Any good?

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LAN_403

EEYORE

Tue Aug 05 2008, 04:12PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Seller told me that one can not focus these lasers. Is this true? Not even focus it down a little? Would make for a great source of dangerous burning light! (Or for the intended use of laser pumping) How would one go about focusing these?
Matt

...

Tue Aug 05 2008, 06:02PM

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

It is pretty much true that you indeed cannot focus those lasers. Unless you count a 1cm long stripe focused.

You couls probably find a bit more information in my posts 'laser diode mounting fun' etc in the eletromatnetic radiation forum, but basically a laser diode 'bar' is doomed from the start as far as trying to focus is concerned.

First, you are starting out with a very large emitter (for a 20w 30% fill bar each emitter is about 1um by 150um) which will have a poor beam quality to start with (you can focus it into an almost difraction limited beam in the vertical 'fast' axis, but in the horisontal 'slow' axis you would be lucky to get a .1mm spot).

This problem is compounded by the fact that there are actually 19 of these emitters in a single bar. So even if you managed to get a decent spot size from one of the individual emitters, you have 18 more that you need to focus on the same spot. And therein lies the problem.

You can put on a fast axis microlens, which is basically a 1/2round piece of glass that sits directly on top of the bar (of course it is precision aligned a few um away from the bar but you get the idea) that will more or less colminate the bar to a 1cm by 1mm or so rectangular beam. They also make special lens ducts that you can set in front of the bar which will quite literally channel all of the light into a small spot about 300uM on a side, but the divergence on that spot is horrible and it is pretty much useless for burning things.

I would recomend getting one of the fiber coupled bars, they cost a bit more, but really are a lot nicer to work with. You should really pick up a piece of 800uM quartz fiber (you can get a 3m piece for like $50 on ebay with SMA ends which isn't too bad) to help normalise the beam a little (the port that sticks out of a FAP package is made of 19 little fibers, so it is kinda weird for focusing) and to take full advantage of the fiber coupled beam. You would also be best off getting a decent culminator, although they are expensive (even on ebay they bid up to $200!), but I have had good luck just using a damaged piece of fiber, cutting it in half, stripping off the kevlar shiething/teflon cladding and cleaving the ends, and just using it as a disposible fiber like the medical lasers. After the end gets all burned and nasty just cleave it off again and you are good to burn.

However, due to the sheer power of the bars, you can melt stuff and set small stuff on fire using the uncolminated 'beam' I use a 50w bar with a microscope slide sitting in front of it to protect the bar from smoke, and hold SOIC chips in front of the front facet ~1mm away and they will burst into flames pretty quickly.

BTW, I have 50w bars available (mounted just like that one) if you are interisted, pm me

EEYORE

Tue Aug 05 2008, 07:42PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Yea, I knew it wouldnt be good for focusing down to a tiny point, but id like to get most of the light onto the object to be burned. I really dont have a good application in mind here, just being immature. (Safety consciencious though!). Would it be worth getting one of these and attempting the crude focusing using a lens? Or, how about this. Would a 3watt laser diode (able to be focused down really well) be more "fun" than 20 watts with crude focusing?

Matt

...

Tue Aug 05 2008, 09:41PM

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

As long as you have a decent supply to run a big bar off of, I would go for it. If it comes down to it you could try colminating one of the emitters and just pretending it is a 1w single emitter

As long as you aren't trying to burn little tiny objects, your burning potential will probably stay pretty proportional to the diodes power.

Just be very careful to never let the front facet of the diode get dirty, otherwise it will self destruct in short order. The trick with laying a microscope slide in front of it works well for me (although you should angle it slightly so that you don't reflect too much power back into the diode bar). They will shatter when they get dirty, so be warned (but since you are wearing OD5+@808nm laser googles you should be fine, right....)

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