Affordable, Homebrewed Optical Networking? 18
Graham Wheeler asks: "Lately I've been obscessed with grassroots community network projects, and the hardware that enables them. Most sites I have seen focus on wireless RF networking, but I have noticed a few projects revolving around free space optical transcievers. Twibright Labs' RONJA is a good example of what I'm talking about. Not being an electronics hobbyist, however, makes the various plans for building a comm laser from scratch look rather daunting. It seems to me that it would be easier to just make a lens and housing system into which would go one of the many cheaply available copper-to-fiber media converters. Then you could simply modify it so that the laser ports were optically connected to the TX and RX lens assemblies instead of the standard fiber interface. So, what factor(s) am I overlooking that would explain why nobody seems to be doing this?"
interference.. (Score:1)
Can fun to work around...
Things to ponder. (Score:3, Informative)
Re:Things to ponder. (Score:3, Informative)
Power, interference, coupling. (Score:5, Informative)
Note that in ideal conditions (perfect lens etc) light diverges at an angle related to its wavelength and the minimum point, the longer the wavelength, the faster it diverges.
The light coming out of a typical FC connector diverges at about 60 degrees and the beam is pretty crap*. Even coupling between two properly cleaved fibres with a sub micron air gap loses more than half the signal.
To get any signal out of the noise out there you'd *have* to use a lock-in amplifier. These are not trivial to make, unless you have training and/or experience. These 'lock-in' to a regular signal, you'd have to modulate the laser to this signal, and the data on top of that modulation.
What you want is a laser which outputs a TEM00** beam, a collimator with a large lens, a good external light modulator and a lock-in amp. At each end. Then you have to modulate the laser with carrier and data, from some kind of device. (Start with a serial or parallel port, nice and easy)
* Technical term. Take too long to explain
** Transverse Electromagnetic Mode 0,0: the most basic gaussian mode of a normal cavity laser. The easiest to focus, as it is a self-fourier-transform. Unfortunately the more power you want out the more likely it is you'll get power in the other, less clean modes.
My company does this (Score:3, Interesting)
The biggest problem we have is reflection. A unit sending data gets back a reflection of that data off the receiving lense. If you're just using 2 stations, it's not that big of a problem to scan the buffers for reflections, but we've found that with several units (usually all communicating with a central base-station) collision becomes a serious issue, and detecting/masking reflections is extremely difficult.
Hope this helps. Good luck on your project.
The answer to his question. (Score:1)
I'd say that it isn't impossible, but the work/yield ratio would be too high. Fiber tranceivers are designed to output into the end of a fiber, and everything in them is designed to do that as efficiently as possible. The optics are so tiny, and to get a beam in or out might require some microscope work.
Even if you did this, the odds are it would only work for a short distance. Fiber lasers only have to project onto the end of an optical fiber, which is naturally a very small area. In order to achieve a high enough power density across the end of the fiber, the lasers don't have be very powerful at all. You'd end up with a beam so tiny it would be nearly impossible to aim. And if you tried to use collimating lenses, the laser isn't powerful enough to begin with, and most of the power would probably be lost in the optics.
Most freespace lasercomm projects involve something a bit more powerful, which allows the laser to punch through smoke and fog a little better. I guess if you made the laser powerful enough, it could punch through interference such as pigeons too....
I think that off-the-shelf optical transceivers could have a use in freespace comm, but I think the most useful work would involve somehow using the tranceivers as modulators for a more powerful laser. Perhaps you could have an optical fiber running to the big laser and receiver optics, like a repeater?
Re:The answer to his question. (Score:1)
Laser diodes are very finicky components. It only takes a tiny bit of overcurrent to ruin one; this is because laser diodes are operated close to the physical limits of the material. Some of the first LEDS would lase if you ran just enough overcurrent through them, but it was next to impossible to control the current precisely enough to avoid frying them. Laser diodes have a detector integrated directly into the laser, which allows control circuitry to lock in the diode to a current level that allows it to lase but doesn't fry it.
Laser pointers became cheap and popular once the method for integrating a detector was perfected, and laser current controllers became widely available. The problem with modulating one of these is the controller; they were designed to hold the laser at a continuous level. It is possible that whipping the controllers with a 10MHz signal will keep them from being able to handle the current properly, and either fry the diode or not allow it to light at all.
This is why tranceivers have their own current controllers embedded in the modulation circuitry. Some diode lasers (not laser pointers) have modulation inputs for communication purposes. I'm not aware of what the most popular laser is for these experiments.
Needs to be as cheap as RF, not just affordable (Score:2)
I'll assume you want optical because it is cool.
The lens assemblies are specialized telescopes. You need to focus down to the tip of your fiber. This is a test of your optical skills instead of your electronic skills.
There are a gazillion tradeoffs. Each step creates some optical loss. You can buy bigger lasers, more sensitive receivers, better optics and/or optical amplifiers until the system works.