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Old 06-15-2010, 06:30 PM #1
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Default Underwater communications experiment (Newbie)

Hello all,

Very glad I found this site, there is a multitude of great information and very knowledgeable people here! I have a feeling this is the best place to start asking about a project I have in mind this summer.

I am an undergrad electrical engineer who managed to stumble into a research position this summer in the area of free space optics. While looking for a good hands on project to make use of this TI DSP system, I happened upon some papers regarding attempts at underwater visible light free space optical communications using lasers. This piqued my interest, and I thought it would be a worthwhile project to try to build a system of my own.

I thought a challenging goal would be transmission of data over the 50 meters of our full size pool here. Getting anything across that distance would be a big success as far as I'm concerned, but of course the higher the data rate the better. (10Mb/s maybe?)

So I started looking and found a few starting points, and hoped I could get some advice on where to go next from here!

First I looked at the absorption spectrum of water, anything above 532nm is absorbed too much to make a link feasible. Then I discovered 532nm lasers are all DPSS, and cant be modulated fast enough! I've been looking at the 405nm laser diodes, and recently the 445nm blue(seriously 1W!?). I would need 20mw or more of power to get enough optical power through to the receiver. I think it would be about ~30% absorption over 50m in clear water. A narrow beam divergence would be important as well.

Eye safety for this is important too. Universities don't like being sued, I like my vision, and I think pointing this on the receiver will supply a higher than normal chance to do something stupid.

I do have access to a very nice THOR labs laser diode driver / temperature control / power meter, but since this is going to have to be submerged at some point I will need a small driver circuit I can fit into a casing. There was a thread on water proofing a 405nm laser that is proving helpful. I am not too sure how to approach the driver though, the biasing circuit will be dependent on the diode I choose, and I need to have the high speed modulation capability (TTL) as well. I would love to just build my own, but in the interest of time an off the shelf IC solution might be better. It wouldn't have to be pretty, or very "feature rich". I could work out the appropriate currents / voltages for the diode with the bench system and hardware them in for the water version... does that even make sense?

If I am going to need a separate driver anyway, I would rather not buy the whole pointer. I'm looking for a good place to purchase the individual 405nm diode. http://media.digikey.com/pdf/Data%20...H04P21A2GE.pdf can be pulsed at ~20Mhz, which looks promising (there is some neat encoding for high power low duty cycle communications), but I can hardly afford (or ever use) 100 of them!.

For detection I saw this photodiode: S2684-254 Si photodiode, Hamamatsu which might make my filtering job easier.

I would love to get the hardware up as fast as I can, I'll need as much time as I can get to fight this DSP into being a useful transmitter!

I am quite new to this, and most hardware work in general, but I do love it and would very much like to see this thing work! Any help / suggestions / recommendations / comments are welcome. But specifically ideas on a 405nm diode / high speed modulation driver would be most appreciated!

Thank You!

P.S: NI!



Last edited by AKnightWhoSaysNi; 06-15-2010 at 06:47 PM.
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Old 06-15-2010, 07:32 PM #2
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Default Re: Underwater communications experiment (Newbie)

Welcome to the Forum...
You seem to have all your ducks in order...
Interesting underwater project...
I'll be eagerly watching for updates.

Jerry
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Old 06-15-2010, 11:44 PM #3
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Default Re: Underwater communications experiment (Newbie)

Hey AKnightWhoSaysNi,

Can I interest you in a shrubbery?

Interesting little project you have there. For a diode, (others may correct me if wrong here), but the specs you posted from digikey look similar to the PHR diode we use and they are relatively cheap these days, (less than $20. How mush less depends on where you get it), and there are a few suppliers here on the site. Check out the Buy/Sell/Trade section or Proshop. If you're in the states then HighTechDealz is a place recommended a lot here but in Europe then I can help.

TTL drivers are available pretty cheaply too. For an example try Odicforce.com. These are UK based but you'll get an idea on what you should be paying. This is a link to one of their linear TTL operated drivers - Laser Driver Board V1 - Detailed item view - OdicForce Lasers Online Shop only 2.80!

If you went for that one you may need to ask for a few other specs but it could fit your needs.

Others will no doubt have more ideas and info for you but good luck with the project and pictures please when you're done!

M
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