Hello, this is one of my first posts here, so i didn't know where exactly to put this tupic so I put it in general discussion. I've encountered this project: http://www.instructables.com/id/Laser-Transceiver/ . It is basicly a serial data communication using laser. There are lasers that are visible over a great distances, so I wanted to set this up so that I can communicate with my friend that lives about 2km away from me. I have three questions:
Your might want to investigate this long-running project:
It doesn't use lasers, but rather extremely bright near-IR LED arrays for the transmission end, and a focused telescopic system on the receiver end to collect the light for the photosensor(s). The project's main complexity, though, comes from the hardware on either end to convert from ethernet and back (yep - real networking, not just simple serial comms).
BTW - what you are trying to build is termed "free-space optical communications":
...and actually has a fairly long history behind it (see the above link). Such communications systems are used all over the world and have been extensively developed (and deployed commercially). You would do good to research the topic as much as possible before building anything.
You might also be interested in looking into Forrest M. Mims III's Engineer's Mini-Notebooks:
One of them, "Communications Projects" (volume 2 in the new series) - has a lot of good information on building such circuits (though for audio transmission - but you could do audio modem comms that way); again, nothing on lasers - just LEDs and IR photo-transistors and the like.
You might also want to pick up all the books in that series - they are a great resource to have on the shelf!
1. ) Are there any legal issues on using lasers for such great distances ? (at least in your country)
I don't know what the legalities are here in the USA; I would think that most companies deploying a commercial system would build so-called "eye-safe" devices (likely using a red laser, 5 milliwatts or under); for the human eye, IR doesn't trigger a blink response until long after the damage is done (which is why you should be very, very careful around IR lasers and high-lumen IR LEDs; wear proper filtering goggles - also, be very careful around green lasers; the cheap ones from china generally use a higher-power IR laser to pump a crystal that shifts the frequency to green - but still pumps out a ton of IR!).
That said, you'll probably want to review everything you can on any legalities; as far as I know, there are no FCC limitations, but there might be "public-use" limitations for the county/city/state/town you are in (or whatever it is for your country) - basically stuff to prevent bystanders from being blinded, limitations to address issues of pilots being blinded, etc.
I would say, though, if you stayed away from lasers, and used high-lumen red or near-IR LEDs, and proper optics systems, and didn't aim them toward close-by windows or such, and kept them elevated out of the line-of-sight of any pedestrians or such - you'll probably be ok. Otherwise, do your research, and spend the money to consult with a lawyer, if you have to.
2.) What laser color do you recommend ? I was thinking about IR, but during the setup, if we accidently don't aim laser exactly at receiver end, we could potentionally burn something with such powerful lasers.
What you'll likely burn is your retina (due to the lack of your eye having blink response to IR, as noted earlier). All lasers diverge (spread out) in the end - in fact, if you insist on using a laser, you'll want to find what its divergence is over distance. Cheap handheld lasers will diverge greatly even over 10 meters (you start out with a dot just a few feet away; 10 meters away, that dot has grown to a spot 5 cm in diameter). The nice thing about that divergence is that it makes it more "eye-safe"; the bad thing is that as the distance grows, so does the divergence, and so does the size of your "collector" optics.
Again, I would recommend against using lasers (or at the very least, using tight collimation for the output), and particularly IR. IR, of course, is best for this kind of work, but it can be dangerous to work with; as long as you understand the dangers involved, and take precautions, then it can be safely worked with.
3.) (Most important) How do we aim lasers at such distances ? Maybe if I have coordinates from google earth and take some super precise compass, or something ... dunno ... any ideas ?
First off, realize that to communicate over 2 km will mean that you are going to be elevated somewhat, so keep that in mind (take the usual precautions as needed).
For aiming, the easiest and cheapest way would be to use a visible light source at each end, and eyeball focus "down the barrel" of the receiver optics; this of course is very dangerous, especially if using IR (well, it would be impossible using IR) - even using red lasers, it would be dangerous.
Better might be to set up the system, get it aimed "roughly" by sighting each other (without any lasers or other light turned on) using telescopes and/or binoculars, then continue to sight-in using the optics of the system (without sensors or output turned on and in the way).
Once you have it roughly sighted, then to dial it in better will mean have some means to monitor to output of the receiver, and some means to carefully change the position of the transmitter and receiver alignment. First, you will want to mount both securely so they don't move, then do the rough sighting. Then, set up the system so that you can transmit a simple carrier wave or something from one direction to the other. You could then listen to the carrier wave (if in audible frequencies), then adjust stuff on either end until the signal is as loud as possible. Or you can use an o-scope to monitor the signal until the amplitude and waveform look as clean as possible. One person on each end will be needed; you'll probably want radios as well to stay in communication with each other, and to relay information about the quality, etc. Your adjustment system will need to have some means to make fine adjustments to the angles - keep that in mind (such outdoor tripods aren't cheap; then again, I don't know your budget - you might be able to get away with cheaper photography tripods if this isn't meant as a permanent long-term setup).
Something else to keep in mind - unless you mount the system very well, or have some means to deal with noise and such, your signal will likely drift in the wind and such, as the tripods and other equipment moves. Commercial systems have auto-aiming servo-mechanisms (which almost eliminates the need to "dial in" the position manually), that can correct for the drift and keep the signal at a high-quality level. Something to keep in mind, anything (climbing a pole or futzing with things on a roof constantly can get old quickly).
Good luck, hope this helps...