Affordable, Homebrewed Optical Networking?

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?"

Comments

Power, interference, coupling.

[Yarn]

Telecomms operates around 1550nm, because that's where fused silica fibre is the most transparent, and has low dispersion. We can see up to about 740nm.

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 :P
** 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.